Seguridad Electrica 3

8/13/2019 Seguridad Electrica 3

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SAFETY PROCEDURES

AND METHODS

INTRODUCTION

The way work is performed in or around an electrical power system is just as important asthe safety equipment that is used. Proper voltage measurement can mean the differencebetween life and death. Standing in the right place during switching operations can mitigatethe effects of an electric arc or blast and proper application of safety grounds can prevent anaccidental reenergization from becoming a fatality.

This chapter summarizes many industry-accepted practices for working on or aroundener-

gized electric power circuits. The methods and techniques covered in this section should beused as guidelines. Local work rules and regulatory standards always take precedence. Notethat any and all safety procedures should be reviewed at least annually. System changes,employeereassignments, or accidents shouldbe considered excellent reasons for modificationof existing procedures or implementation of new procedures.

Safetyis the one, truly personalconcern inanelectricpower system. In the majorityofelec-trical accidents, the injured victim was the so-called last link in the chain. The use of properprocedures and/or proper safetyequipmentcould have prevented the accident. Equipment andprocedures can be provided, but only the individual employee can make the decision to usethem. Employees must be impressed with the knowledge that only they can make this final

decisiona decision which may result in life or death.Caution: This chapter presents several procedures that require technical calculations

using a variety of mathematical and engineering applications. These calculations should beperformed or evaluated only by qualified, experienced engineers or other such technicalpersonnel with the requisite knowledge and skills.

THE SIX-STEP SAFETY METHOD

Table 3.1 lists six important steps to practicing safe behavior and will serve as the founda-tion for a personnel safety philosophy. If implemented, this method will greatly enhance thesafe and efficient performance of electrical work.

Each individual is responsible for his or her own safety. The steps listed in Table 3.1 areall individual steps that can be taken by everyone who works on or around electric powercircuits and conductors.

3.1

CHAPTER 3

Copyright 2006 by The McGraw-Hill Companies, Inc. Click here for terms of use.


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3.2 CHAPTER THREE

ThinkBe Aware

Many accidents could have been prevented if the injured victim had concentrated on the

safety aspects of the job. Thinking about personal or job-related problems while workingon or near energized conductors is a one-way ticket to an accident. Always stay alert to theelectrical hazards around the work area.

Understand Your Procedures

Every company has defined safety procedures which are to be followed. Each worker shouldbe thoroughly familiar with all the safety procedures that affect his or her job. Knowledge ofthe required steps and the reasons for those steps can save a life. All employees should go

through extensive safety training.

Follow Your Procedures

In the past, some facilities have allowed the violation of safety procedures in the name ofproduction. Such actions have invariably proven to be costly in terms of human injuryand/or death. Violation of safety procedures without good cause should be a dischargeoffense. What constitutes good cause must be decided on a local basis; however, excusesof lesser significance than immediate danger to life should not be acceptable.

Use Appropriate Safety Equipment

No matter how meticulous workers are, accidents do occasionally happen. Equipment fail-ures, lightning strikes, switching surges, and other such events can cause shock, arc, or blast.Also, sometimes it becomes necessary for employees to work on or very close to energizedconductors, which increases the chance of accidental contact.

Because of these reasons, appropriate safety equipment should be used any time work-ers are exposed to the possibility of one of the three electrical hazards. Remember that noth-ing is sadder than an accident report which explains that the dead or injured worker was not

wearing safety equipment.

Ask If You Are Unsure, and Do Not Assume

Ignorance kills and injures many people each year. No one should ever get fired for askinga questionespecially if it is a safety-related question. Anyone who is uncertain about aparticular situation should be encouraged to ask questions which should then be answeredby a qualified person immediately and to the fullest extent possible.

Thinkbe aware

Understand your procedures

Follow your procedures Use appropriate safety equipment

Ask if you are unsure, and do not assume

Do not answer if you do not know

TABLE 3.1 Six-Step Safety Method


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Do Not Answer If You Do Not Know

No one should answer a question if they are not certain of the answer. Self-proclaimedexperts should keep their opinions to themselves.

PRE-JOB BRIEFINGS

Definition

A pre-job briefing (sometimes called a tailgate meeting) is a meeting which informs allworkers of the job requirements. In particular a pre-job briefing is used to alert workers to

potential safety hazards. A pre-job briefing need not be a formal gathering; however, it ismandatory that all workers involved attend, and worker attendance should be documented.

What Should Be Included?

OSHA rules require that a pre-job briefing discuss, at a minimum, the following issues:

Special precautions to be taken

Hazards associated with the job

Energy control procedures Procedures and Policies

Personal Protective Equipment

Note that the first letters of each of these bulleted items form the acronym SHEPP, whichcan be used to help remember the important issues that need to be discussed.

Pre-job briefings should be proactive meetings in which workers are informally quizzedto make certain that they fully understand the safety issues that they will face.

When Should Pre-Job Briefings Be Held?

At the beginning of each shift

At the beginning of any new job

Any time that job conditions change

When new personnel are introduced to an ongoing job

ENERGIZED OR DE-ENERGIZED?

The Fundamental Rules

All regulatory standards are quite clear in their requirements to de-energize a circuit beforeemployees work on or near it. Stated simply:

All circuits and components to which employees may be exposed should be de-energized before work begins.

SAFETY PROCEDURES AND METHODS 3.3


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3.4 CHAPTER THREE

Start

1.

2.

3.

Yes

Yes

Yes

Yes

NO

NO

NO

NO

Will this workexpose the worker toenergized part over

50 volts?

Work is de-energized.Proceed

Circuits of less than 50volts to ground maygenerally be consideredas de-energized. If the

circuit has high arcingpotential, answer #1 asYes.Will additional

hazards (see list) beintroduced by de-

energizing?

Would the de-

energization require amajor shutdown?

Does thenature of the work

require energization? (Seeexamples)

STOP!! The circuitsMUST be de-energized for

this work!!!

NONO

4.

5.

Yes

Can thework be

rescheduled to a latertime when it can be

performed de-energized?

Can the workbe performed safely

using safety-related workprocedures?

6.

STOP!!Schedule thework for later

Yes

Proceed withcaution

FIGURE 3.1 Hot-work flow chart.

A few basic points will clarify this requirement:

Production or loss of production isnever an acceptable, sole reason to work on or nearan energized circuit.

Work that can be rescheduled to be done de-energized, should be rescheduled.

De-energized troubleshooting is always preferred over energized troubleshooting.

The qualified employee doing the work, mustalways make the final decision as to whetherthe circuit is to be de-energized. Such a decision must be free of any repercussions fromsupervision and management.


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A Hot-Work Decision Tree

Figure 3.1 illustrates a method that may be used to determine the need to work on a circuitwhen it is energized. The numbers in the following explanation refer to the numbersassigned to each of the decision blocks shown in Fig. 3.1.

1. Work performed on or near circuits of less than 50 V to ground may usually be consid-ered to be de-energized work. Note that if the circuit has high arcing capability, decision1 should be answered as a Yes.

2. If de-energizing simply changes the hazard from one type to another, or if it actuallyincreases the degree of hazard. This decision should be answered Yes. Table 3.2lists the types of additional hazards that should be considered in answering thisdecision.

3. The need to keep production up is common to all industriesmanufacturing, petrochem-ical, mining, steel, aluminum, and electrical power systems. However, many employersabuse the concept that production must continue. The following points should clarifywhen production issues may be allowed to influence the decision to de-energize.

a. Shutdown of a continuous process that will add extraordinary collateral costs may bea signal to work on the circuit energized. Table 3.3 lists examples of these types ofcollateral costs.

b. Shutdown of a simple system which does not introduce the types of problems identi-fied in Table 3.3 should always be undertaken rather than allowing energized work.

4. In some cases, the very nature of the work or the equipment requires that the circuit remainenergized. Table 3.4 shows three of the most common examples of such work. Note, how-ever that this work should still be de-energized if it is possible to do it that way. For exam-ple, troubleshooting a motor starter may be faster with the circuit energized; however, if itcan be done de-energized it should be, even at the cost of a little more time.

5. If decisions 2 or 3 lead in the direction of energized work, the next decision should berescheduling. If energized work can be done de-energized on a different shift or at a latertime, it should be postponed. Many companies miss this elegantly simple alternative toexposing their personnel to hazardous electrical energy.

6. The final, and arguably the most important, decision of all is to determine whether thework can be done safely. If, in the opinion of the qualified personnel assessing the job, thework is simply too dangerous to do with the circuits energized, then it must be de-

energized.

TABLE 3.2 Examples of Additional Hazards

Interruption of life-support systems

Deactivation of emergency alarms

Shutdown of ventilation to hazardous locations

Removal of illumination from the work area

TABLE 3.3 Examples of Collateral Costs that May Justify Energized Work

Excessive restart times in continuous process systems

High product loss costs (in polyethylene process, for example, the product has to be

physically dug out of process equipment after an unscheduled outage)


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3.6 CHAPTER THREE

After the Decision Is Made

If the work must be done energized, all employees who work on or near energized conduc-tors must be qualified to do the work, must use appropriate personal protective equipment,and must use appropriate safety-related work practices.

If the circuits are to be de-energized, the steps listed in Table 3.5 must be followed. Notethat proper, safe procedures for each of the items in Table 3.5 are discussed later in this

chapter.

SAFE SWITCHING OF POWER SYSTEMS

Introduction

The most basic safety procedure is to de-energize the parts of the system to which workersmay be exposed. This procedure virtually eliminates the hazards of shock, arc, and blast. De-energizing, also called clearing, involves more than simply turning the switches off. To ensuremaximum safety, de-energizing procedures that are precise for each situation should be written.

The following sections discuss the proper safety techniques for operation of various typesof equipment and provide de-energizing and reenergizing procedures which may be used asthe basis for the development of site-specific procedures. Please note that specific proceduresmay vary depending on the application and type of equipment. Refer to manufacturers and/orlocal facility procedures for specific information. The methods given in these sections shouldbe consideredminimum requirements. These procedures assume that the device is being oper-ated when one or both sides are energized.

Caution:

Switching of electric power should only be carried out by qualified personnel who arefamiliar with the equipment and trained to recognize and avoid the safety hazards asso-ciated with that equipment.

Non-load-interrupting devicesthat is, devices which are not intended to interrupt anycurrentshould never be used to interrupt current flow.

TABLE 3.4 Examples of Work that May Require Energization of Circuits

Testing electrical circuits (to verify de-energization, for example)

Troubleshooting complex controls

Infrared scan

TABLE 3.5 Steps Required Before De-Energized Work May Commence

1. All energy control devices feeding the work area must be opened.

2. Locks and tags shall be placed on the energy control devices.

3. Voltage measurements shall be made at the point(s) of exposure to verify that the circuit is

de-energized.

4. Safety grounds (if required) shall be placed to ensure the existence of an equipotential work

zone.

5. The work area must be closely inspected by a qualified person to make certain that no

energized parts remain. This critical step is often missed.


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Remote Operation

The procedures on the following pages provide recognized safe work practices for manualoperation of the various types of gear. Note that the bestway to operate any electrical device

is doing it remotely. If the equipment has supervisory or other type of remote control, it shouldalways be operated from the remote position, with all personnel safely out of harms way.

Operating Medium-Voltage Switchgear

General Description. Figures 3.2 through 3.6 illustrate various types of medium-voltageswitchgear and the breakers used in them. In this style of gear, the circuit breaker rolls intothe switchgear on wheels as shown in Fig. 3.3 or on a sliding type of racking mechanism asshown in Fig. 3.6. The opening and closing of the breaker is performed electrically using a

front-panel-mounted, pistol-grip type of control. Turning the grip in one direction (usuallycounterclockwise) opens the breaker, and turning it in the other direction closes it.The breaker connects to the bus and the line via a set of disconnects, visible at the top

of Fig. 3.3 and on the right side of Fig. 3.6. When the breaker is open, it can be movedtoward the front of the cubicle so it disconnects from the bus and line. This action is referredto as racking the breaker. The breaker may be completely removed from the switchgear, orit may be put into two or more auxiliary positions. Racking a circuit breaker is accom-plished using removable cranks. These cranks may be of a rotary type or a lever bar that isused to walk the breaker from the cubicle.

Most switchgear have two auxiliary positions: the testposition and the disconnectedposi-

tion. In the test position, the breaker is disconnected from the bus; however, its control poweris still applied through a set of secondary disconnects. This allows technicians to operate thebreaker for maintenance purposes. In the disconnected position, the breaker is completely dis-connected; however, it is still in the switchgear.

FIGURE 3.2 Medium-voltage metal-clad switchgear. (Courtesy WestinghouseElectric.)


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3.8 CHAPTER THREE

FIGURE 3.3 Circuit breaker used in switchgear shown in Fig. 3.2. (CourtesyWestinghouse Electric.)

FIGURE 3.4 Switchgear of Fig. 3.2 with door open showing circuitbreaker cubicle. (Courtesy Westinghouse Electric.)


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FIGURE 3.5 Metal-clad, medium-voltage switchgear for vacuum-type circuit break-ers. (Courtesy General Electric.)

FIGURE 3.6 Medium-voltage, vacuum interrupter circuit breaker usedin switchgear of Fig. 3.5. (Courtesy General Electric.)


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3.10 CHAPTER THREE

For some types of switchgear, the front panel provides worker protection from shock,arc, and blast. This means that the switchgear is designed to contain arc and blast as longas the door is properly closed and latched. Such gear is called arc-resistantswitch gear.

Closed-Door Operation. Table 3.6 lists the recommended safety equipment to be usedby operators when performing both closed-door and open-door switching on medium-voltage switchgear. Note that both the primary operator and backup operator should be

wearing the recommended clothing (Fig. 3.7). The primary operator is the worker who

Switchgear cubicles

Breaker or starter

being operated

Alternative positions

of operating handle

Primary operator position(facing away from gear)

Backup operator position(facing primary operator)

Frontof gear

Rearof gear

FIGURE 3.7 Proper position for operating electrical equipment (top view).

TABLE 3.6 Recommended Minimum Safety Equipment for Operating

Metal-Clad Switchgear

Closed door (arc-resistant gear only)

Hard hatANSI type A, B, G, or E (as required by voltage level) Safety glasses with side shields

Flame-resistant work clothing (select using arc-flash calculations)

Open door or non-arc resistant gear

Hard hatANSI type A, B, E, or G

Safety glasses with side shields

Rubber gloves with leather protectors (class according to voltage level)


Note: Closed doormeans that the front safety panels are closed and latched. If

the front panels are not designed to contain blast and arc (such as with arc-resistantswitchgear), then open doorprotective equipment should be worn.


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actually manipulates the handle which opens and/or closes the circuit breaker. The backupoperators responsibility is toback up theprimary operator in the event thereisa problem.Thebackup operator may be optional in some facilities.

To operate the switchgear with a closed door, the following steps apply:

1. The primary operator stands to the side of the cubicle containing the breaker to be oper-ated. The side to which he or she stands should be determined by which side theoperating handle is on. If the handle is in the middle, the operator should stand onthe hinge or the handle side of the door depending on which side is stronger. (Referto the manufacturer.)

2. The primary operator faces away from the gear. Some workers prefer to face the gear toensure better control. This is okay but the preferred method is to face away.

3. The backup operator stands even farther from the cubicle, facing the primary operator.

4. The primary operator reaches across to the operating handle and turns it to open or closethe breaker. Note that the primary operator continues to keep his or her face turned awayfrom the gear. Some operators prefer to use a hot stick or a rope for this operation. Thiskeeps the arms as far as possible from any hazard.

5. If the breaker can be racked with the door closed, and if the breaker is to be racked away,the primary operator inserts the racking handle. In this operation, the primary operatormay have to face the breaker cubicle.

6. If lockout-tagout procedures are required, the primary operator places the necessarytags and/or locks.

Open-Door Operation. Refer to Table 3.6 for a minimum listing of safety equipment forthis operation. If the door must be open for racking the breaker away from or onto the bus,the following steps should be observed:

1. The breaker is opened as described earlier under closed-door operation.

2. The primary operator opens the cubicle door and racks the breaker to the desired position.


Operating Low-Voltage Switchgear

General Description. With some exceptions, the operation of low-voltage switchgear isvery similar to the operation of medium-voltage gear. Figures 3.8 through 3.11 illustrate var-ious types of low-voltage switchgear and the breakers used in them.

In this style of gear, the circuit breaker rolls into the switchgear on a sliding type of rack-ing mechanism as in Fig. 3.11. The circuit breaker is tripped by the release of a powerfulset of springs. Depending on the breaker the springs are released either manually and/or

electrically using a front panel button and/or control switch. The springs may be chargedmanually and/or electrically, again depending on the breaker.

The breakersmay beclosedeitherwithanother set of springs orbymeans ofa manualclos-ing handle. The springs for spring-operated breakers may be charged either manually or elec-trically. For example, the breaker shown in Fig. 3.9c is a manually operated breaker. The largegray handle is cranked several times to charge the closing spring. When the spring is fullycharged, the breaker may be closedby depressing a small, mechanicalbutton on the faceof thebreaker. Tripping the breaker is accomplished by depressing another pushbutton. Other break-ers have different means of opening and closing.



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3.12 CHAPTER THREE

Low-voltage breakers connect to the bus and the line via a set of disconnects, visible on

the right side of Fig. 3.9b. When the breaker is open, it can be moved toward the front of thecubicle so that it disconnects from the bus and line. This action is referred to as racking thebreaker. Racking the larger-size low-voltage breakers is accomplished using removablecranks. These cranks are usually of a rotary type. The breaker may be completely removedfrom the switchgear, or it may be put into two or more auxiliary positions. Smaller low-voltage breakers are racked by simply pulling or pushing them.

Like medium-voltage switchgear, most low-voltage breaker have two auxiliary positions(testanddisconnected). In the test position, the breaker is disconnected from the bus; how-ever, its control power and/or auxiliary switches are still applied through a set of secondarydisconnects. This allows technicians to operate the breaker for maintenance purposes. In the

disconnected position, the breaker is completely disconnected; however, it is still in theswitchgear.

For some types of switchgear, the front panel provides worker protection from shock,arc, and blast. This means that the switchgear is designed to contain arc and blast as longas the door is properly closed and latched. Such gear is called arc-resistant.

Closed-Door Operation. The closed-door operation of low-voltage breakers is virtuallyidentical to the closed-door operation of medium-voltage circuit breakers. Table 3.6 lists therecommended safety equipment to be used by operators when performing both closed-door

FIGURE 3.8 Low-voltage metal-clad switchgear.(Courtesy General Electric.)


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FIGURE 3.9 Low-voltage circuit breaker used in the switchgear shown inFig. 3.8. (Courtesy General Electric.)

(a) (b)

(c)

FIGURE 3.10 Low-voltage metal-enclosed switchgear. (Type R switchgear manufactured by ElectricalApparatus Division of Siemens Energy and Automation, Inc.)

3.13


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3.14 CHAPTER THREE

FIGURE 3.11 Low-voltage power circuit breaker racked completely awayfrom the bus. (Type RL circuit breaker manufactured by Electrical ApparatusDivision of Siemens Energy and Automation, Inc.)

and open-door switching on low-voltage switchgear. Note that both the primary operator andbackup operator should be wearing the recommended clothing (Fig. 3.7).

The primary operator is the worker who actually manipulates the handle which opens and/orcloses the circuit breaker. The backup operators responsibility is to back up the primary opera-tor in the event there is a problem. The backup operator may be optional in some facilities.

To operate the switchgear with a closed door, the following steps apply:

1. If the breaker requires manual spring charging, the primary operator may face thebreaker to obtain the necessary leverage on the cranking handle.

2. After the springs are charged, the primary operator stands to the side of the cubicle con-taining the breaker to be operated. The side to which he or she stands should be deter-mined by which side the operating handle is on. If the handle is in the middle, theoperator should stand on the hinge or the handle side of the door depending on whichside is stronger. (Refer to the manufacturer.)

3. The primary operator faces away from the gear. (preferred)



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5. The primary operator reaches across to the operating buttons or handle and operates themtoopen or close the breaker. Note that the primary operator continues tokeephis or her faceturned away from the gear. Some operators prefer to use a hot stick or a rope for this oper-ation. This keeps the arms as far as possible from any hazard.

6. If the breaker can be racked with the door closed, and if the breaker is to be racked in orout, the primary operator inserts the racking handle and turns it. Note that breakerswhich are racked manually cannot be racked with the door closed. In this operation theprimary operator may have to face the breaker cubicle.


Open-Door Operation. Refer to Table 3.6 for a minimum listing of safety equipment forthis operation. If the door must be open for racking the breaker, the following steps should

be observed:

1. The breaker is opened as described earlier under closed-door operation.

2. The primary operator opens the cubicle door and racks the breaker to the desired position.


Operating Molded-Case Breakers and Panelboards

General Description. Molded-case circuit breakers are designed with a case that com-pletely contains the arc and blast of the interrupted current, as shown in Fig. 3.12. Suchbreakers are permanently mounted in individual enclosures or panelboards (Fig. 3.13)along with many other such breakers.

Molded-case breakers have a three-position operating handleopen, closed, and tripped.When the operator opens the breaker, he or she does so by moving the operating handle to theopen position. Likewise the close operation is accomplished by moving the handle to the closeposition.


FIGURE 3.12 Various molded-case circuit breakers. (CourtesyWestinghouse Electric.)


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3.16 CHAPTER THREE

When the breaker trips via its internal automatic protective devices, the handle movesto the tripped position. The trip position is normally an intermediate position between the

full-closed and full-open positions. After a trip operation, the breaker cannot be operateduntil the handle is moved forcefully to the open position. This action resets the internal trip-ping mechanism and reengages the manual operating mechanism.

Operation. Caution: Circuit breakers should not be used for the routine energizing andde-energizing of circuits unless they are manufactured and marked for such purpose. Theymay be used for occasional or unusual disconnect service.

Table 3.7 lists the minimum recommended safety equipment to be worn when operat-ing molded-case circuit breakers, and Fig. 3.14 illustrates the proper position for the oper-ation. Notice that a backup operator is not required for this procedure; however, secondary

assistance is always a good practice. The procedure can be summarized as follows:

1. The operator stands to the side of the breaker and/or panel, facing the panel. The oper-ator may stand to either side, depending on the physical layout of the area.

2. The operator grasps the handle with the hand closest to the breaker.

3. The operator turns his or her head away from the breaker and then firmly moves theoperating handle to the desired position.

4. If locks or tags are required, they are placed on the breaker using the types of equipmentas described in Chap. 2.

FIGURE 3.13 Panelboards equipped with molded-case circuit breakers. (Courtesy McGraw-Hill, Inc.)

TABLE 3.7 Recommended Minimum Safety Equipment for Operating Molded-

Case Circuit Breakers




Hand protectionleather or flame-resistant gloves (do not need to be insulating)


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Operating Enclosed Switches and Disconnects

General Description. Figure 3.15 shows several basic types of enclosed switches. Thesedevices are used to connect and/or disconnect circuits. Such devices may be load interrupt-ing or nonload interrupting. If they are nonload interrupting, they must not be operatedwhen current is flowing in the circuit. If you are uncertain as to whether the switches are loadinterrupting or not, look on the nameplate or check with the manufacturer. The presence ofarc interrupters, such as those in Fig. 3.15d, is a good indication that the device is intendedto interrupt load current.

These switches are operated by moving the handle. In some units, the handle is boltedor locked in place to prevent inadvertent operation. Enclosed switches have a mechanicalinterlock that prevents the case from being opened until the handle is in the open position.

Qualified personnel may temporarily defeat the interlock if needed for maintenance or trou-bleshooting purposes. Such switches should not be operated with the door open when loadcurrent if flowing. Be very cautious about opening medium-voltage switches.

Operation. The basic operating procedure for such switches is similar to the proceduregiven for molded-case circuit breakers. Caution: Switches should not be used to interrupt loadcurrent unless they are intended for that purpose. Refer to the manufacturers information.

Table 3.8 lists the minimum recommended safety equipment to be worn when operatingenclosed switches, and Fig. 3.14 illustrates the proper position for the operation. Notice thata backup operator is not required for this procedure; however, secondary assistance is always

a good practice. The procedure can be summarized as follows:

1. The operator stands to the side of the switch and/or panel, facing the panel. The opera-tor may stand to either side depending on the physical layout of the area.

2. The operator grasps the handle with the hand closest to the switch.

3. The operator turns his or her head away from the switch and then firmly moves the oper-ating handle to the desired position.

4. If locks or tags are required, they are placed on the switch using the types of equipmentdescribed in Chap. 2.


FIGURE 3.14 Proper position for operating molded-case circuitbreakers, enclosed switches, and motor controls (top view).

Panel, switch, or control

Operator

Note: Operator turns his/her head when operatingthe circuit breaker.

Front

ofPanel

TABLE 3.8 Recommended Minimum Safety Equipment For Operating

Enclosed Switches and Disconnects

Hard hatANSI type A, B, E, or G (as required by voltage level)





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Operating Open-Air Disconnects

General Description. Open-air disconnects may be manually operated ormechanism-operated. Mechanism types (Fig. 3.16) are normally installed as overhead devicesin medium-voltage substations or on pole lines. The switch has an operating handle close to theground which is used to open or close the contacts. At ground level, a metal platform is oftenprovided for the operator to stand on. This platform is bonded to the switch mechanism and tothe ground grid or ground rod. The operator stands on the grid; thus the operators hands andfeet are at the same electric potential. Note that the operation of some switches is accomplishedby moving the handle in the horizontal plane, while others are moved in a vertical direction.

3.18 CHAPTER THREE

(c) (d)

FIGURE 3.15 Various types of low- and medium-voltage, enclosed disconnect switches.(Courtesy General Electric, Crouse-Hinds Co., and Eaton Corporation, Cutler HammerProducts.)

(a) (b)


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Manually operated switches are operated by physically pulling on the blade mechanism.In some cases, such as the one shown in Fig. 3.17, the switch blade is a fuse. The manual oper-ation is accomplished by using a hot stick. Manually operated switches may be located over-

head in outdoor installations, or they may be mounted indoors inside metal-clad switchgear.

Operation. Recommended protective clothing depends upon the type and location of theswitch. Table 3.9 lists the minimum recommended safety equipment for operating over-head, mechanism-operated switches such as those shown in Fig. 3.16. Figure 3.18 showsthe correct operating position for operating such a switch. Caution: Not all open-airswitches are designed to interrupt load current. Do not use a nonload interrupting switchto interrupt load current.

The basic operating procedure can be summarized as follows:

1. The operator stands on the metal platform (if available).2. He or she grasps the operating handle firmly with both hands and moves it rapidly and

firmly in the open or close direction as required.

3. If locks or tags are required, they are placed on the mechanism using the types of equip-ment described in Chap. 2.

Table 3.10 lists the recommended equipment for operation of manually operated open-air switches. Note that the use of flash suits is dependent on the location of the switch. Ifthe switch is on an overhead construction that is outdoors, the worker may opt to wearflame-resistant clothing instead of a flash suit. Because the operator must use a hot stick to

operate this type of switch, he or she must stand directly in front of the switch.The general operating procedure is as follows:

1. Stand in front of the switch.

2. Carefully insert the hot stick probe into the switch ring.

3. Look away from the switch and pull it open with a swift, firm motion.

4. Since one side of the switch may be hot, locks and tags are not always applied directlyto the switch. If the switch is in an indoor, metal-clad enclosure, the lock and tag maybe applied to the door of the gear.

FIGURE 3.16 Mechanism-operated, three-phase, open-air switch. (CourtesyAlan Mark Franks.)


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3.20 CHAPTER THREE

Operating Motor Starters

General Description. With some exceptions, the operation of motor starters is very sim-ilar to the operation of low- and medium-voltage gear. Figure 3.19 shows a single motorstarter in a cabinet suitable for mounting on a wall.

In the motor control center, the starter mounts on a mechanism specially designed for thepurpose. In either type of construction, the motor is stopped and started by depressing theappropriate button. The starter also has a fused disconnect or a molded-case circuit breakerthat is used to disconnect the motor and its circuitry from the power supply.


Overhead, Mechanism-Operated Switches





FIGURE 3.17 Open-air, fused disconnect switch.(Courtesy General Electric.)


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Bonding

Operatingrod

Operatorhandle

Direction of rotation

Metal platformPole ground

FIGURE 3.18 Correct position for operating an overhead, mechanism-operated switch.

Motor starters used in motor control centers connect to the bus and the line via a set ofdisconnects. When the starter is open, it can be moved toward the front of the cubicle sothat it disconnects from the bus and line. This action is referred to as racking. Rackingstarters is usually accomplished manually. The starter may be completely removed from themotor control center. Note that large medium-voltage contactors may be operated like

medium-voltage circuit breakers.For many types of motor control centers, the front panel provides worker protection from

shock, arc, and blast. This means the motor control center is designed to contain arc and blastas long as the door is properly closed and latched.

Closed-Door Operation. The closed-door operation of motor starters is virtually identicalto the closed-door operation of low-voltage circuit breakers. Table 3.6 lists the recommended


Manually-Operated, Open-Air Disconnect Switches





Hot stick of proper length with proper fittings


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3.22 CHAPTER THREE

FIGURE 3.19 Combination motor starter in individual metal-clad cabinet.

safety equipment to be used by operators when performing both closed-door and open-doorswitching on motor starters. Note that both the primary operator and backup operator shouldbewearing the recommended clothing (Fig. 3.7). The primary operator is the workerwho actu-ally manipulates the handle that opens and/or closes the motor starter. The backup operatorsresponsibility is to back up the primary operator in the event there is a problem. The backup

operator may be optional in some facilities.To operate the starter with a closed door the following steps apply:

1. Depress the stop button to stop the motor.

2. After the motor is stopped, the primary operator stands to the side of the cubicle contain-ing the starter to be operated. The side to which he or she stands should be determined bywhich side the operating handle is on. If the handle is in the middle, the operator shouldstand on the hinge side or the handle side of the door depending on which side is stronger.(Refer to the manufacturer.)

3. The primary operator faces away from the gear.Note: If the operating handle of the dis-connect has a very tight operating mechanism, the primary operator may face the motorstarter to obtain the necessary leverage on the cranking handle.


5. The primary operator reaches across to the operating handleand operates it toopen or closethe starter disconnect. Note that the primary operator continues to keep his or her faceturned away from the gear. Some operators prefer to use a hot stick or a rope for this oper-ation. This keeps the arms as far as possible from any hazard.


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6. If the starter can be racked with the door closed (an unusual configuration), and if thestarter is to be racked in or out, the primary operator inserts the racking handle and turnsit. Note that starters which are racked manually cannot be racked with the door closed.In this operation, the primary operator may have to face the breaker cubicle.


Open-Door Operation. Refer to Table 3.6 for a minimum listing of safety equipment forthis operation. If the door must be open for racking the starter, the following steps shouldbe observed:

1. The motor is stopped as described earlier under closed-door operation.

2. The primary operator opens the cubicle door and racks the starter to the desired position.


ENERGY CONTROL PROGRAMS

An energy control program is a procedure for the proper control of hazardous energysources. It should include a listing of company-approved steps for the proper and safe ener-gizing and de-energizing of energy isolation devices as well as general company policystatements on policies with respect to preferred methods of operation. Energy control pro-grams fall into two categoriesgeneral and specific.

General Energy Control Programs

Overview. A general energy program is one that is inherently generic in nature. Its stepsare broad-based and designed in such a way that the program can be used as a procedure fora wide variety of equipment types. General energy control programs should be used only

when the equipment being isolated meets all the following criteria:

The equipment can be disabled, so it has no potential for the release of stored or residualenergy after shutdown.

The equipment is supplied from a single energy source which can be readily identifiedand isolated.

The equipment is completely de-energized and deactivated by the isolation and lock-ing out procedure.

No employees are allowed to work on or near the equipment until it has been tagged and

locked. (See the tagout-lockout section later in this chapter.) A single lockout device will achieve a locked-out condition.

The isolating circuit breakers or switches are under the exclusive control of the employee(s)who placed the lock and tag.

De-energizing and working on the equipment does not create a hazard for other employees.

There have been no accidents involving unexpected activation or reenergization of theequipment during previous servicing.



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Specific Energy Control Programs

When a part of the system or piece of equipment does not meet all the criteria laid out inthe overview to this section, a specific energy control program should be written. Although

the procedures will vary depending on the specifics of the installation, at a minimum theprogram should include the following information:

The description of the system and/or equipment that will be de-energized.

Any controls, such as motor starter pushbuttons, that exist on the equipment.

The voltages and short circuit capacities of the parts of the system which will be de-energized.

The circuit breakers, switches, or contactors which are used to de-energize the system.

The steps that must be used to de-energize the system. The steps should include:

1. The methods and order of operation of the circuit breakers, switches, and so on.2. Any special requirements for the lockout-tagout procedure.3. Special notifications and safety requirements.

Reenergizing requirements and procedures.

Basic Energy Control Rules

The safest and securest method to protect personnel from the electrical hazard is to de-energize the conductors which they must work on or near. De-energization is the preferredmethod.

If conductors cannot be de-energized, safety equipment and safety-related work prac-tices must be used to protect personnel exposed to the energized conductors.

Before personnel are allowed to work on or near any exposed, de-energized conductors,the circuit breakers and/or disconnect switches must be locked and tagged to prevent theirinadvertent operation.

All personnel should be instructed to never operate or attempt to operate any circuitbreaker and/or disconnect switch when it is tagged and/or locked.

Only authorized, qualified, and trained personnel should be allowed to operate electricequipment.

Locks and tags should be removed only by the personnel that placed them. Two excep-tions may apply under the following situations:

1. If the worker who placed the lock and tag is not available, his or her lock and tag maybe removed by a qualified person who is authorized to perform such an action. Thisprocedure is often called bypassing control as the person who removes the lock and tagis, in fact, bypassing the authority (control) of the person whose tag is being removed.

2. Some facilities may authorize the concept of a group lock. A group lock is placed by anauthorized shift worker, such as the shift operator, and may be removed by anotherauthorized shift worker. This activity should not be used to prevent any employee from

placing his or her tag and lock on energy-isolating devices that may feed conductorswhich they must work on or near.

De-energizing Equipment. The general energy program for de-energizing equipmentshould include the following steps:

1. Before beginning the process, carefully identify the voltage levels and short circuitcapabilities of the portion of the system which will be de-energized. This informationserves to establish the level of the hazard to all personnel.

3.24 CHAPTER THREE


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2. Notify all employees who will be affected by the de-energization that the system is tobe de-energized.

3. Perform necessary checks and inspections to ensure that de-energizing the equipment

will not introduce additional safety hazards, for example, de-energizing safety-relatedventilation systems.

4. Shut down all processes being fed by the electric system which is to be de-energized.

5. Open the appropriate circuit breaker and/or switch.

6. Rack the circuit breaker away from the bus if it is of the type that can be manipulatedin this manner.

7. Release stored energy from springs, hydraulic systems, or pneumatics.

8. Discharge and ground any capacitors located in the de-energized portions of the system.

9. Apply tags and/or locks.

10. Attempt to operate the breaker and/or switch to make certain that the locks are pre-venting operation. If a motor starter is involved, press the start button to make certainthe motor will not start.

11. Measure the voltage on the conductors to which employees at the point where they willbe exposed.

12. Notify personnel that the system is safely de-energized, locked, and tagged.

Reenergizing Equipment. Reenergization of some systems is more hazardous than de-

energization. While the equipment has been out of service, personnel have grown used to itsde-energized voltage status. In addition, tools and/or other equipment may have been inad-vertently left on or near exposed conductors.

Because of these factors, the same type of rigorous steps should be followed duringreenergization.

1. All personnel should be notified that the system is to be reenergized and warned to stayclear of circuits and equipment.

2. A trained, qualified person should conduct all tests and visual inspections necessary to ver-ify that all tools, electric jumpers, shorts, grounds, and other such devices have been

removed and that the circuits are ready to be reenergized.

3. Close and secure all cabinet doors and other safety-related enclosures.

4. Because the tests and inspections may have required some time, the personnel warningsshould be repeated.

5. Locks and tags should be removed by the personnel that placed them.

6. If breakers were racked into disconnected positions, they should be racked in the con-nected position.

7. Make final checks and tests, and issue final warnings to all personnel.

8. Reenergize the system by closing and reconnecting breakers and switches. These opera-tions are normally carried out in the reverse order of how they were opened.

Procedures Involving More than One Person. When more than one person is required tolock and tag equipment, each person will place his or her own personal lock and tag on thecircuit breakers and/or switches. The placement of multiple locks and tags on equipment isoften called ganging. Since few circuit breakers or switches have the ability to accept mul-tiple locks and tags, this procedure can take one of two common approaches.



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1. A multiple-lock hasp may be applied to the breaker or switch. Such hasps will accept upto six locks. If more than six locks are required, multiple-lock hasps may be cascaded.See Fig. 2.55 for examples of such hasps.

2. A lockbox may be used. In such an operation, the lock is applied to the breaker or switchand the key is then placed inside the lockbox. The lockbox is then secured by the use ofa multiple-lock hasp. This approach is used when the presence of many locks on theswitch or breaker might cause operational problems.

After the work has been completed, each employee removes his or her lock from thelockbox. The key for the lock is retrieved and the lock can them be removed.

LOCKOUT-TAGOUT

Definition and Description

Tags are used to identify equipment that has been removed from service for maintenance orother purposes. They are uniquely designed and have clear warnings printed on them instruct-ing personnel not to operate the equipment. Locks are applied to de-energized equipment toprevent accidental or unauthorized operation. Locks and tags are normally applied together;however, some special circumstances may require the use of a tag without a lock and/or a lockwithout a tag. See Chap. 2 for a detailed description of the construction of safety locks and tags.

Employers should develop a written specification which clearly defines the lockout-

tagout rules for the facility. This specification should be kept on file and reviewed periodi-cally to ensure that it is kept up to date. The following sections define the key elements thatshould be included in the specification.

When to Use Locks and Tags

Locks and tags should be applied to open circuit breakers, switches, or contactors whenever per-sonnel will be exposed to the conductors which are normally fed by those devices. The applica-tion of the tags will warn and inform other employees that the equipment is not available for

service, who applied the tag, and why the tag was applied. The lock will prevent the operationof the breaker, switch, or contactor so that the circuit cannot be accidently reenergized.

Minor inspections, adjustments, measurements, and other such servicing activitieswhich are routine, repetitive, and integral to the use of the equipment do not require theplacement of locks and tags unless one of the following conditions exist:

1. Guard, insulation, or other safety devices are required to be removed or bypassed.

2. An employee is required to place his or her body into close proximity with an exposed,energized electric conductor.

Locks and tags do not need to be used on plug and cord connected equipment as long asthe cord and plug stay under the exclusive control of the employee who is exposed to theelectrical hazard.

Locks without Tags or Tags without Locks

Tags may be used without locks under both of the following conditions:

1. The interrupting device is not designed to accept a lock.

3.26 CHAPTER THREE


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2. An extra means of isolation is employed to provide one additional level of protection. Suchan extra procedure might take the form of an additional open point such as removing a fuseor disconnecting a wire or the placement of safety grounds to provide an equipotentialwork area.

Locks may be used without tags under both of the following conditions:

1. The de-energization is limited to only one circuit or piece of equipment.

2. The lockout lasts only as long as the employee who places the lock is on site and in theimmediate area.

Rules for Using Locks and Tags

All electric equipment with the capability to be reenergized and harm employees shall besafely isolated by means of a lock and tag during maintenance, repair, or modification ofthe equipment.

When two or more crafts must both have access to the equipment, authorized employeesfrom both crafts shall place locks and tags on behalf of the members of their craft. This prac-tice is referred to asganging. This should not be construed to limit the right of any employeeto place his or her individual lock and tag on the equipment.

Responsibilities of Employees

Employees who are authorized to place locks and tags have certain responsibilities whichthey must exercise when placing those tags.

The system must be surveyed to ensure that all sources of power to the system have beenidentified.

All the isolating equipment (circuit breakers, switches, etc.) must be identified and cor-

related with the portions of the system to which they apply. The voltage level and short-circuit magnitude for each part of the system to be de-

energized must be determined. This step helps to assess the hazard to individuals whowill be exposed to the de-energized system parts.

All personnel who will be affected by the outage must be notified. This includes employ-ees who may be served by the electric power or who may work on or around the equip-ment which will be affected by the outage.

The employee(s) who place the locks and tags must maintain knowledge and control ofthe equipment to which they have affixed their locks and tags.

When locks and tags are removed, authorized, qualified employees must perform cer-tain tasks, including the following:

1. Notify all affected personnel the system is going to be reenergized.

2. If appropriate, in a gang lock situation, make certain that all authorized employees areprepared to remove their locks and tags.

3. Inspect and/or test all parts of the system to make certain that they are ready to bereenergized.



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3.28 CHAPTER THREE

Note: See the Energy Control Program section discussed earlier in this chapter for moreinformation about these requirements.

Sequence

The following steps should be followed when shutting down an energized electric system:

1. Motors and other operational equipment should be shut down using normal or emer-gency procedures as required. During the shutdown process personnel safety must bethe prime consideration.

2. All isolating equipment (circuit breakers, switches, and/or contactors) should be opened.

3. Isolating equipment that is capable of being racked out should be racked to the discon-nected position.

4. Stored energy, such as closing/tripping springs, hydraulic pressure, pneumatic pressure,or other such mechanisms should be discharged and released.

5. Discharge and ground capacitors.

Lock and Tag Application

Isolating equipment is capable of being locked out if either of the following conditions is met:

The equipment has an attachment which is an integral part of the equipment, throughwhich the locking device may be passed in such a way as to prevent operation of the iso-lating equipment.

The lock can be attached in some other way to prevent operation without dismantling,rebuilding, or replacing the energy-isolating equipment. This might apply to a breakerwhich cannot be locked open but can be locked in the racked or disconnected position.

Locks and tags should be applied to all isolating equipment which is capable of beinglocked. Locks and tags should never be applied to selector switches, pushbuttons, or othersuch control devices as the sole means of control.

Isolation Verification

After the locks and tags have been applied, the following steps should be employed to ver-ify that the de-energization was successful:

1. Make certain that all employees are clear of the de-energized conductors.

2. Attempt to reenergize the system by operating the circuit breaker control handles, push-ing the switch to close it, or whatever other procedure is appropriate.

3. Using proper procedures and test equipment, measure the voltage on conductors at thepoint(s) where employee exposure will take place. The voltage should be zero.

Removal of Locks and Tags

Normal Removal. When the work is finished and an employee is ready to remove his orher locks and tags, the following general approaches should be used:

The work area should be inspected to ensure that nonessential items have been removedand all components are operationally intact.


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The work area should be inspected to ensure that all employees have been safely removedor positioned.

Remove any specialized equipment such as safety grounds or spring tension blocks.

Notify all affected employees that locks and tags are to be removed. Locks and tags should be removed by the personnel that placed them.

Control Bypass. If the employee who placed the locks and tags is absent and not avail-able to remove them, and if the locks and tags absolutely must be removed, another autho-rized employee should assume control of the equipment and remove the absent employeeslocks and tags. The following steps must be employed:

1. The employee who is assuming control must make an assessment of the situation and

determine that a genuine, critical need exists to remove the absent employees locks andtags. Examples of such critical needs include:

a. Immediate operation and/or production requires the equipment to resume operationand avoid general shutdown.

b. Personnel safety requires restoration of power to the de-energized systems.c. Equipment must be temporarily returned to service to allow testing or other such

evaluation.2. The employee assuming control must make every reasonable attempt to contact the

absent employee to obtain his or her assessment of the equipment.3. The employee assuming control must contact other employees who may have knowl-

edge of the availability of the equipment and the advisability of removing the absentemployees locks and tags.

4. The employee assuming control must develop and document a formal conclusion withregard to the decision to remove the absent employees locks and tags.

5. The employee assuming control must follow the normal removal steps to remove thelocks and tags and either reinstall his or her own locks and tags or reenergize thesystem.

6. The area where the absent employees locks and tags were removed should be postedwith large, easy-to-read signs indicating that the locks and tags have been removed.

7. The employee assuming control must contact the absent employee immediately upon his

or her return. The returning employee must be told that his or her locks and tags havebeen removed and completely updated as to the status of the equipment. The absentemployees shall remove the signs posted in step 6.

Figure 3.20 shows a form which may be used to document the control bypass procedure.In this figure the employee who is assuming control is referred to as the authorized con-troller. The absent employee is referred to as the authorized employee. Notice that each oneof the major steps is documented on this form.

Temporary Removal. Locks and tags may be temporarily removed by the employee whoplaced them. When doing so, the same steps used in normal removal should be observed.

Safety Ground Application

Safety grounds should be applied as an additional safety measure when equipment isremoved from service. The only exception to this is when the nature of the work pre-cludes the use of safety grounds. Procedures like insulation measurement cannot be



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3.30 CHAPTER THREE

FIGURE 3.20 Lockout-tagout control bypass form.


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performed when safety grounds are applied; therefore, they may not be used, or must betemporarily disconnected for the measurement process.

Control Transfer

Lockout-tagout control may be transferred from one employee to another as long as bothemployees are present. The following steps should be used:

1. The employee relinquishing control follows all steps involved in the normal removal oflocks and tags.

2. The employee assuming control follows all steps involved in the normal application oflocks and tags.

Nonemployees and Contractors

Nonemployees such as contractors should be required to use a lockout-tagout program

which provides the same or greater protection as that afforded by the facilitys procedure.Contractors should be required to submit their procedure for review and approval. No workshould be allowed until the contractors lockout-tagout program has been reviewed andapproved.

All employees should be familiarized with the contractors procedure and requiredto comply with it. Copies of the respective standards should be made available for bothcontractors and employees. Likewise all contractors must be familiar with the facilitysprocedure and must comply with it.

If the contractor is working on machines or equipment that have not yet been turned overto the facility, the contractor shall be required to lock and tag that equipment if it might

operate and endanger employees. The contractor must guarantee the safety of the lockedand tagged installation. As an alternative to this procedure, the contractor should berequired to allow gang lockout and tagout with employees and contractor personnel.

Lockout-Tagout Training

All employees should be trained in the use, application, and removal of locks and tags. SeeChap. 13 for more information on general employee safety training.

FIGURE 3.20 (Continued)


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3.32 CHAPTER THREE

Procedural Reviews

The entire lockout-tagout procedure should be reviewed at intervals no longer than 1 year.A review report should be issued which identifies the portions of the procedure which were

reviewed, changes which were considered, and changes which were implemented.

VOLTAGE-MEASUREMENT TECHNIQUES

Purpose

No circuit should ever be presumed dead until it has been measured using reliable,

prechecked test equipment. Good safety practice and current regulatory standards requirethat circuits be certified de-energized by measurement as the last definitive step in the lockout-tagout procedure.

Instrument Selection

Voltage-measuring instruments should be selected for the category, voltage level, the appli-cation location, short-circuit capacity, sensitivity requirements, and the circuit loadingrequirements of the circuit which is to be measured.

During voltage measurement, the instrument may be subjected to voltage transients.This is especially critical in low-voltage (below 1000 V) applications. When selecting alow-voltage-measuring instrument, be certain to select the correct category of instrumentas defined in IEC 61010. See Chap. 2 for a description of the categories.

Voltage Level. The instrument must be capable of withstanding the voltage of the circuitwhich is to be measured. Use of underrated instruments, even though the circuit is dead, isa violation of good safety practice.

Application Location. Some instruments are designed for outdoor use only and shouldnot be used in metal-clad switchgear. Always check the manufacturers information andverify that the instrument is designed for the location in which it is being used.

Internal Short-Circuit Protection. Industrial-grade, safety-rated voltage-measuring instru-ments are equipped with internal fusing and/or high-resistance elements which will limit theshort-circuit current in the event the instrument fails internally. Be certain the instrument beingused has internal protection with an interrupting/limiting rating that is at least equal to theshort-circuit capacity of the circuit that is being measured.

Sensitivity Requirements. The instrument must be capable of measuring the lowest nor-mal voltage that might be present in the circuit which is being measured. If the instrumenthas two ranges (high and low, for example), be certain to set it on the correct lowest rangewhich applies to the voltage being measured.

Circuit Loading. A real voltmeter can be represented by an ideal meter in parallel with aresistance. The resistance represents the amount of loading the meter places on the circuit(Fig. 3.21). In the diagramRS represents that resistance of the system being measured and


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RM is the internal resistance of the voltmeter. The voltmeter will read a voltage given by theformula:

(3.1)

If the meter internal resistance is high compared to system resistance, the voltage acrossthe meter will be very close to the actual system voltage. If the meter resistance is very lowcompared to the system resistance, the meter will read a lower voltage than actually existsbecause most of the voltage drop will be across the system resistance instead of the meterresistance. In this second situation, the meter is loading the circuit.

If a very low resistance meter is used, the meter may not read a potentially lethal staticor inductively coupled voltage. This situation is unacceptable since an inductively coupledor static voltage can be lethal. The meter that is selected for a safety-related voltage mea-surement must have an internal resistance that is high enough to avoid this problem.

Instrument Condition

Before each use, an instrument must be closely inspected to ensure it is in proper working

order and that insulation systems have not been damaged. The case physical condition, probeexposure, lead insulation, fusing, and the operability of the instrument should be verified.

Case Physical Condition. The case must be free of breaks, cracks, or other damage thatcould create a safety hazard or misoperation. Broken instruments should be taken out ofservice until they can be repaired or replaced.

Probe Exposure. Modern instrument probes have spring-loaded sleeves which cover theprobe until forced back. Check to make certain that only the minimum amount of proberequired to do the job is exposed.

V V R

R RMM

M S

= +


V VM

RS

RM

Circuit being measured Voltage measuring instrument

FIGURE 3.21 Equivalent circuit of voltage measurement.


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3.34 CHAPTER THREE

Lead Insulation Quality. Carefully inspect the lead insulation to make certain it is notdamaged in any way.

Fusing. Accessible fuses should be checked to be certain that they are correctly installed

and have not been replaced by incorrect units.

Operability. Before each usage (at the beginning of each shift, for example), the instru-ment should be checked to make certain that it is operable. Do not substitute this check forthe instrument checks required in the three-step process.

Three-Step Measurement Process

Step 1Test the Instrument. Immediately before each measurement, the instrumentshould be checked on a source which is known to be hot. This step confirms that the instru-ment is working before the actual circuit verification is made.

The preferred method is to use an actual power system conductor of the same voltage asthat being verified. Finding such a circuit is easier when low-voltage measurements arebeing made; however, even then a hot circuit may not be available. Some manufacturerssupply a device which creates a voltage that is sufficient for testing the instrument.

Some instruments intended for measurement of medium voltages have a low-voltageswitch setting on them which allows them to be checked on low voltage. This is not a preferredmethod since a problem with the switch could give erroheous, dangerous false readings.

Step 2Measure the Circuit Being Verified. After the instrument is tested, the workerthen measures the circuit being verified to make certain it is de-energized. The actual wiresthat should be measured and the techniques to be used are discussed later in this chapter.

Step 3Retest the Instrument. After the circuit has been verified, the instrument shouldbe rechecked as it was in step 1. This ensures that the instrument was operable both beforeand after the measurement, thus affirming that a zero measurement is zero and not causedby an inoperable instrument.

What to Measure

As a general rule of thumb, all normally energized conductors should be measured toground and to each other. The readings to ground should be made whether the system isgrounded or not. Note that all readings should be made as close to the point of exposure aspossible.

The procedures given in the following paragraphs will apply only to contact type instru-ments. Proximity instruments measure the electrostatic field set up around an energizedconductor rather than the actual voltage between two conductors. If the proximity indica-

tor shows an unexpected voltage, the circuit should be rechecked, possibly with a contacttester.

Single-Phase Systems. The hot wire of single-phase systems should be measured bothagain neutral and ground. Figure 3.22 shows the points which should be measured.

Two-Phase Systems. A voltage measurement should be taken between the two hotphases, between each hot phase and neutral (one at a time), and between each hot phase andground. Figure 3.23 shows the measurement locations for a typical two-phase system.


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Three-Phase Systems. Measure between each of the hot wires, two at a time, betweeneach hot wire and neutral, and between each hot wire and ground. Figure 3.24 shows themeasurement locations for a typical three-phase system.

Using Voltage Transformer Secondaries for Voltage Measurements. Many medium-voltage power systems are equipped with step-down voltage transformers which are used

for metering or telemetering purposes. Because of the lower voltage on the secondary, someworkers wish to measure the secondary winding voltage to verify that the primaries are de-energized. Such transformers may be used for safety-related voltage measurements underthe following conditions:

1. The transformers can be visually traced and are known to be connected to the systemwhere exposure will occur.

2. The transformers are located close to the part of the system where exposure will occur.


Hot wire (H)

Neutral wire (N)

Ground wire (G)

Measurement to be madeat location where exposure

will occur.Measurements: 1. H to N 2. H to G

FIGURE 3.22 Measurement points for a single-phase system (one hot wire).

1. H1 H22. H1 N3. H2 N4. H1 G5. H2 G

Hot wire (H1)

Neutral or ground (N or G)

Hot wire (H2)


will occur.

Measurements:

FIGURE 3.23 Measurement points for a single-phase system (two hot wires).


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3. The transformer secondaries must be measured both before and after the circuit is de-energized. This verifies that the transformer fuses are not blown, resulting in an erro-

neous zero voltage reading.

4. A safety ground is applied to the primary circuit of the transformer after the measure-ment but before personnel contact occurs.

5. All other safety-related techniques should be used as described in the other parts of thischapter.

Caution: This procedure is considered a secondary, nonpreferred method. Do not use thisapproach unless absolutely necessary.

Using Panel Voltmeters for Voltage Measurements. Panel voltmeters may be used as ageneral indication of system energization; however, they should not be used for safety-related voltage measurements.

How to Measure

Preparation. Table 3.11 identifies the steps which should precede the actual measure-ment procedure. The area should be cleared of unnecessary personnel. This will preventtheir exposure to arc or blast in the event that a problem occurs. The person making the

3.36 CHAPTER THREE

A phase (A)

B phase (B)

Neutral ground (N or G)


will occur.

Measurements:

C phase (C)

1. A B2. B C3. C A4. A N

5. B N6. C N7. A G8. B G9. C G

FIGURE 3.24 Measurement points for a three-phase system.

Clear area of unnecessary personnel.

Wear appropriate safety equipment.

Expose conductors to be measured.

Position measuring instrument.

TABLE 3.11 Voltage Measurement Preparations


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measurement should wear and use appropriate safety equipment. (Safety equipment infor-mation will be covered later in this chapter.)

After the safety equipment is on, the panels, doors, or other access means should be

opened to expose the conductors that are to be measured. The measuring instrument is thencarefully positioned. When making medium-voltage measurements with hot sticks, be certainthat the hot stick is not contacting your body. The instrument should be securely positionedso that it does not fall to the floor if the leads are inadvertently overextended.

Safety Equipment. Table 3.12 lists the minimum recommended safety equipment to beworn when making low-voltage measurements. Table 3.13 lists equipment for medium-voltage measurements. The flash suit is identified as optional for outdoor, open-air mea-surements. Although many line personnel do not wear flash suits when performingoverhead work, the use of these uniforms is strongly recommended.

Measurement. After all preparations are made and the safety equipment is put on, themeasuring instrument should be applied to the conductors. If a measurement to ground isbeing made, one lead should be connected to the ground first and then the phase connec-tion made. When measuring between two hot wires, the order of connection is unimpor-tant. If a contact instrument is being used, each lead should be carefully placed on theappropriate conductor. The meter or readout is then observed to see if the circuit is hot. Ifa proximity instrument is being used, it should be moved gradually toward the conductoruntil it indicates or until the conductor is touched.

PLACEMENT OF SAFETY GROUNDS

Safety Grounding Principles

Safety grounds are conductors which are temporarily applied to de-energized systemconductors. They are used to provide a safe zone for personnel working on or around

TABLE 3.12 Recommended Minimum Safety Equipment for Making Low-

Voltage Safety Measurements


Safety glasses with side shields Flame-resistant work clothing (select using arc-flash calculations)

Class 00 or higher rubber gloves with leather protectors

Class 00 or higher rubber sleeves (if the measurement requires proximity to

energized conductors)

TABLE 3.13 Recommended Minimum Safety Equipment for Making Medium- andHigh-Voltage Safety Measurements

Hard hatANSI type B, E



Class 1 or higher rubber gloves with leather protectors (class according to voltage level)

Class 00 or higher rubber sleeves, if the measurement requires proximity to energized

conductors (class according to voltage level)


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3.38 CHAPTER THREE

de-energized conductors and to ensure that an accidental reenergization will not cause injury.Power system components should be considered to be energized until safety grounds are inplace. Safety grounds should never be placed until the conductors where they are to beinstalled have been measured and verified to be de-energized.

See Chap. 2 for a detailed description of the design and description of safety grounds.Caution: Safety grounds must be properly designed and sized for the conductors and shortcircuit capacity to which they may be subjected. Refer to Chap. 2 and/or manufacturersinformation for specifics.

In the event that the system is accidently reenergized, enormous magnetic forces areexerted on the safety ground wires. The forces can cause the grounds to whip violently andcause injury to personnel. To minimize this effect the safety grounds should be as short as pos-sible, and the conductors should be restrained.

Safety Grounding Location

Equipotential Grounding. Safety grounds should be applied in such a way that a zone ofequal potential is formed in the work area. This equipotential zone is formed when fault cur-rent is bypassed around the work area by metallic conductors. Figure 3.25 shows the properlocation of safety grounds for three different work situations. In each of these situations, theworker is bypassed by the low-resistance metallic conductors of the safety ground.

Assume the worker contacts the center phase in Fig. 3.25b. With a fault current capac-ity of 10,000 A, safety ground resistance (Rj) of 0.001 , and worker resistance (Rw) of500, the worker will receive only about 20 mA of current flow.

Figure 3.26a throughdshows two nonpreferred or incorrect locations for the applica-tion of safety grounds. In these diagrams, the workers body is in parallel with the seriescombination of the jumper resistance (Rj) and the ground resistance (Rg). The placement ofgrounds in this fashion greatly increases the voltage drop across the workers body in theevent the circuit is reenergized. Such placements should not be used.

Single-Point versus Two-Point Grounding. Single-point grounding is the placement ofonly one safety ground set. In this procedure, the safety grounds are placed as close to thepoint of work as is possible. When possible, the grounds are placed between the worker and

the source of electric energy.Two-point grounding is the placement of two safety ground sets. They are usually

placed on opposite sides of the work area; that is, one set is placed upstream and one setis placed downstream from the workers.

In general, more safety ground sets are better than fewer; however, the safety groundsystem must provide a zone of equal potential as described in the previous section.

Application of Safety Grounds

Safety Equipment. Table 3.14 lists the minimum recommended safety equipment thatshould be worn and used when applying safety grounds. Full shock, arc, and blast protec-tive clothing is required. No matter how carefully the work area is prepared, the possibilitystill exists for error and/or inadvertent reenergization during the application process.

Notice also that hot sticks are recommended for the actual application process. The useof hot sticks distances the worker from the point of contact and, therefore, minimizes thepossibility of injury caused by arc or blast.


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Procedure. The actual application procedure will be different for each specific applica-

tion. The following general steps are recommended.

1. Thoroughly inspect the safety ground set which is to be used. Points to check includea. Insulation qualityb. Condition of conductorsc. Condition of clampsd. Condition of ferrulee. Condition of cable-to-ferrule connection


Rj

RjRj

G

RgTower ground

(b)Metal tower

FIGURE 3.25 Application of safety grounds to provide a zone of equal potential.


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3.40 CHAPTER THREE

RjRw

Rg

Rj Rj

(d)Electrical equivalent


(c) Wood pole

Static wire

Grounding bararound pole G

G


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(b)

FIGURE 3.26 Two incorrect, nonpreferred safety groundconfigurations.

(a)

TABLE 3.14 Recommended Minimum Safety Equipment for Application of Safety Grounds

Hard hatANSI type B, E


Flame-resistant work clothing with full head protection (select using arc-flash calculations)

Class 0 or higher rubber gloves with leather protectors (class according to voltage level)

Hot stick with suitable fittings for application of safety ground clamps*

*Hot sticks may not be usable in cramped switchgear locations. Workers should exercise best safetyjudgement in such situations.

3.41


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3.42 CHAPTER THREE

Rw

Rj

Rdg

(d)


(c)


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2. Identify the point at which each ground clamp will be connected to the system. Be cer-tain to select points which minimize the amount of slack in the safety grounds. This willminimize the whipping action in the event the system is reenergized.

3. Put on required safety equipment.

4. Measure the system voltage to make certain that the system to be grounded is de-energized. (See the previous sections on voltage measurement.)

5. Make certain that all unnecessary personnel have been cleared from the area.6. Apply the ground end of the safety ground sets first. (These points are labeled with the

letter G in Fig. 3.25.)7. Connect the phase-end safety ground clamp to the hot stick.8. Firmly contact the de-energized conductor with the phase end of the safety ground.9. Tighten the grounding clamp firmly. Remember the amount of resistance in the clamp

connection can make the difference between a safe connection and a hazardous one.10. Repeat steps 6 through 8 for each of the phases to be grounded.11. Record the placement of each safety ground by identification number. (See Control of

Safety Grounds in the next section.)

Figure 3.27 shows a worker applying safety grounds to a de-energized system.

The Equipotential Zone

When a proper equipotential zone is established, there will be no lethal potential differenceswhich the worker can reach in the work area. Figure 3.25 illustrates configurations that can

be used to accomplish this end.


FIGURE 3.27 Worker applying safety grounds. (CourtesyMcGraw-Hill, Inc.)


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Note however, that some situations make the establishment of such a zone difficult orimpossible. Consider, for example, the employee who must stand on the earth when he orshe is working. Since the earth has a relatively high resistivity, the workers feet will be at adifferent potential from that of the metallic elements that he must contact.

There are two approaches that can be used in such circumstances:

1. A metal platform can be laid on the ground and bonded to the grounded metal of theelectrical system. Figure 3.18 is an example of this approach. In this situation, as longas the worker stays on the metal pad, he or she will remain in an equipotential zone.

2. The worker can be insulated from the earth or other high resistivity conductors. Rubbermats, gloves, or blankets can be placed so that the worker is completely insulated fromelectrical contact.

Of course, the best approach is always to establish the equipotential zone; however,

these two work-around approaches may be used when absolutely necessary.

Removal of Safety Grounds

Safety Equipment. The removal of safety grounds is no less hazardous than applyingthem. All the safety equipment listed in Table 3.14 should be worn. Safety grounds shouldbe removed using hot sticks.

Procedure. Safety grounds should be removed as follows:

1. Put on all required safety equipment.

2. Remove each of the phase connections one at a time.

3. Remove the ground connection.

4. Check the ground off as being removed.

Remember that when safety grounds are not present, the system should be considered to beenergized.

Control of Safety Grounds

Safety grounds must be removed before the power system is reenergized. They must alsobe inspected periodically and before each use. The following sections describe two meth-ods of controlling the safety ground sets.

Inventory Method. Each safety ground set should be identified with its own unique serialnumber. The serial number should be etched or impressed on a metal tag which is perma-nently attached to the safety ground set.

A safety officer should be appointed to control the inventory of safety ground sets. This

person will control the use of the safety grounds and is responsible for keeping lists ofwhere the grounds are applied during an outage.

As each safety ground is applied, the safety officer notes its placement on a placement con-trol sheet. As each ground is removed, the officer notes its removal on the sheet. No reener-gization is allowed until the safety officer is satisfied that all safety grounds have beenremoved. Figure 3.28 is a typical safety ground placement control sheet.

The sheet shown in Fig. 3.28 has the minimum required information. Other columnssuchas where the ground set is installed, who installed it, and who removed itmay be addedas needed.

3.44 CHAPTER THREE


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Visual Method. Some small facilities may not be able to justify the complexity of a controlsystem such as that described in the previous section. A visual control system may suffice forfacilities which have only one or two safety grounding sets. In such a system, a brightly col-ored rope is permanently attached to each grounding set. Nylon ski rope is ideal for this appli-cation. The length of the rope should be determined by the applications; however, 3 meters(m) (10 ft) is a good starting point.

At the remote end of the rope, attach a brightly colored warning sign stating Groundsare Applied. After the safety grounds are attached to the system, the rope should be

FIGURE 3.28 Safet

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