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PR5V0X Insthimdntation Planning Guide

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September 1981 Publ, No. PN4:003

AC and DC Power & Grounding

INTRODUCTION

Tills installation planning note provides system-level recommendations and guidelines for the ac and dc power and ground wiring in your PROVOX instru­mentation system. Specific instructions for wiring and grounding of indivi­dual products are given In the installation planning notes for each product. Proper wiring and grounding are of prime importance for operator safety, signal Integrity, and electrical protection of the instrumentation system.

Note

All power and ground wiring practices must conform to the current National Electric Code and to current state and local codes and regulations, as recommended in the latest National Electrical Code Handbook from the National Fire Protection Association (NFPA 81).

Syotum ac power problems can arise from two sources: (1) the commercial power network and (2) the plant distribution system. Generally, the only cure for commercial power problems Is a backup power source for the critical portions of the process. However, plant distribution problems can be minimized by following the recommendations in this installation planning note to the max­imum extent possible. Major topics to be discussed are:

o AC, DC, and Signal Grounding Requirements o Input AC Power Transformation o AC Power Wiring and Distribution o DC Power Wiring and Distribution o Power Alarm Wiring o System Grounds o Ground Wiring o Common Crounding Between PROVOX Instrumentation and Other Peripheral

Devices and Systems o Earth Ground Systems o Ground System Testing o Lightning Protection

©Fisher Controls Company 1981; All Rights Reserved

/ ' ^ /

AC, DC, AND SIGNAL GROUNDING RKquiKEMENTS li v i j 5

Certain basic c r i t er ia , e s tab l i shed as requirements for the ac power, s igna l , n n d ^ l r supply c o ^ n grounds, are l i s t e d below. Ai l PROVOX instrumentation syntcas oust confora to these requirements.

AC power ground systems must exh ib i t :

o A dedicated, pr iva te ground for the PROVOX instrumentation system

alone. . . . . o Less than 5-vol t , peak-to-peak, measurement between the ac neutra l

and grounding conductor a t t he i r points of connection to the console or computer.

o Consoles and computers that are isolated from the conduits through which their interconnecting ac power is routed.

o Consoles or computers and their associated peripherals that are connected to the same power distribution and ground system.

o A cabinet connection to only one local circuit-breaker panel. o An earth ground connection that is made in accordance with local,

state, and national electrical codes. o An isolated ground that, at a minimum, satisfies safety require­

ments.

DC power ground systems must exhibit:

o A dedicated, private ground for the PROVOX instrumentation system

alone, o Resistance measurements of 3 ohms or less from the cabinet laminated

bus bar terminal lugs to the earth ground. o A potential difference of no more than 200 millivolts dc between the

signal common (SC) and power supply common (PSC) at all points within the system, with all units powered.

o A redundant power supply that is connected to the same local ground point as the main power supplies that it Is backing up.

o A common connection between the PSC and SC circuits only at a point where no additional power supply return currents flow beyond that point (i.e., between that point and ground); the connection point can be, at the local ground point (LGP) or master ground point (MGP).

o A single-point ground for (1) all cabinets using common analog signals and (2) all cabinets sharing a redundant power supply (the data highway is isolated and does not require a single-point

ground).

INPUT AC POWER TRANSFORMATION

Commercial ac'power utilities normally provide power that meets the voltage and frequency requirements of the PROVOX instrumentation system. However, plant distribution networks may drop up to 5 percent or more of the Input ac power between the service entrance point to the plant and the final power connection to the various portions of the instrumentation system. Furthcr-

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more, starting transients from large motors and other loads tied to the dis­tribution system can cause additional momentary line-voltage reductions as well as possible waveshape distortions.

To minimize these problems, the PROVOX Instrumentation system must be provided with a separate ac power transformation system that is Isolated from all other loads; each building or site containing PROVOX instrumentation should have a separate power transformer. This is particularly important for the instru­mentation system control center, which generally contains the operator console with Its computer and mass storage devices. The power transformer primary should be supplied from the highest line voltage available from the commercial source, and only PROVOX equipment should be connected to the secondary. The . powor transformer should have sufficient capacity to handle Initial surge currents (lasting about ten cycles with overcurrent inrush as shown in figure I), and still regulate Its output voltage within the equipment tolerances for the steady-state line-to-neutral nominal-rated voltage, as measured at the input power side of the system when the system is energized (see table 1). Wiring from the power transformer to the PROVOX Product power terminals should be of sufficient size to maintain these specified tolerances at the input terminals when all equipment Is energized. The wiring must also conform to local, state, and national electrical codes to ensure that it can handle the current load safely without overheating. The ac voltage and frequency re­quirements and tolerances for each PROVOX product can be found In the appli­cable product bulletin,

AC POWER WIRING AND DISTRIBUTION

The ac power wiring for a PROVOX instrumentation system is, for the most part, the responsibility of the user, but standards are available for this wiring as exemplified by the recommendations of this section. Furthermore, all local, state, and national electrical codes and regulations for such wiring must be followed. Therefore, the recommendations and wiring diagrams of this section are necessarily typical examples rather than specific requirements. Wiring for your instrumentation system will probably be similar but not an exact duplicate of these examples.

Two types of power distribution systems can be used to supply ac power to the PROVOX instrumentation system equipment. The first type, a System-Level Power Distribution System, should be used for buildings containing both system cabinets and consoles or computers. Tho second type, a Remote-Area Power Distribution System, should be used for buildings containing only system cabinets or only consoles and computers. This system could also be used in large buildings, helping to avoid problems associated with routing an isolated power system over long distances to various areas within the building.

Regardless of the distribution system used, an isolation transformer is re­quired to isolate power supplied to the PROVOX instrumentation system equip­ment from the power supplied to nil other functions in a controlled process

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€1 Table ,. VoU.g* - » * ~ * f * 4 — " M

Voltage

Range (V)

108 to 125 216 to 250

108 to 125 216 to 250

86 to 113 102 to 132 172 to 226 204 to 264

Frequency Rahge' x

(Hz)

58 to 62 48 to 52

** 59.8 to 60.1** 49.9 to 50.1

58 to 62

86 to 113 172 to 226 187 to 242 204 to 264

's„lU-P>«.e or rtree-phaae powr reared.

**

•nll t-phase-or three-phase power t « H — ipj.it puaoo frequency range i s W n I * . « * « * • » « » . t ^ ' I ' S Video T . « . » U . » - c d

0

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*

system. The power from the isolated distribution system should be connected

only to the PROVOX equipment, and a separate distribution system iBi.r*tfuifred

for each building containing PROVOX equipment.

The PROVOX system cabinets and consoles require single-phase power, whereas

the computers require either split-phase or three-phase power. If a multi­

phase distribution system is required, exercise care to balance the load

between phases at each power panel, minimizing any voltage differentials

between the ac neutral and the grounding conductors.

System-Level Power Distribution

Typical system-level power distribution systems are shown in figures 2 through /». Input ac power is supplied through an isolation transformer with the PROVOX ac ground point established at or near the transformer. The ac circuit conductors are routed through the main power panel (containing the main dis­connect switch) into the main power circuit breaker panel.

In the main power circuit breaker panel, the neutral conductor is bonded to a bus bar that is electrically isolated from the grounding conductor and from the circuit breaker panel. The grounding conductor is connected to a separate bus bar that may be bonded to the circuit breaker panel and connected exter­nally to the PROVOX ac ground point.

The ac circuit conductors are then routed to the local-area circuit breaker panels, with each panel dedicated to either system cabinets or to consoles and computers, as shown in figures 3 and 4. Inside the local-area circuit breaker panel, the neutral and grounding conductors are bonded to separate bus bavs that are isolated from the panel and from each other. From the bus bars In the main circuit breaker panel to the point of final connection at the PROVOX equipment, all ac circuit conductors are electrically isolated from their conduits and circuit breaker panels.

All ac power for the system cabinets is routed either from a local-area cir­cuit breaker panel as shown in figure 3 or from a remote-area circuit breaker panel as shown in figure 5, and is connected to the cabinet power distribution panel assembly for internal routing from there. Each power distribution panel assembly contains up to three separate ac connections as shown in figure 8. One twiatlock receptacle is dedicated to the primary power supply of a cab­inet, and the second twistlock receptacle is dedicated to the redundant power supply, if supply redundancy is selected. The third connection, a duplex receptacle, is used for the cooling fans within the cabinet. Each twistlock receptacle in a nominal 100- or 115-volt ac power distribution panel assembly must ba supplied from a separate 20-amp circuit breaker (15 ampB for the 200-, 220-, or 240-volt ac sources). Figure 1 shows the typical inrush current of the power supplies, which should be used In selectLng the

riate circuit breaker. The duplex receptacle may be connected in paral--..j „fi„„i<. receptacles or It may use a separate circuit ffsS^^^^5^*- """M'1,"tt

breaker.

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PN4:003 Kl <J 7 i 9

All ac conductors from the c i r c u i t breaker panels to the receptacles In a power d i s t r i bu t i on panel assembly must be provided by the user and are termin­ated d i r e c t l y a t each recep tac le . Standard factory-assembly methods provide the f lexible conduit for ac wiring from the cabinet entry to the power d i s ­t r ibu t ion panel assembly. All ac power supplied to a single cabinet grouping (up to six system cabinets) must come from the same c i rcu i t breaker panel.

£ojnso_le_s_and_Com£u_te£s

All ac power for the PROVOX console or computer equipment i s routed e i the r from a local -area c i r c u i t breaker panel as shown in figure 4 or from a remote-area c i r c u i t breaker panel as shown in f igure 6. The specified ac power requirements must be met by the power supplied at the point of connection to the console or computer equipment (these requirements are l i s t e d in t ab le 1) . There should be no more than a 5-volt peak-to-peak d i f fe ren t i a l between the neu t ra l conductor and the grounding conductor, measured at the point of con­nection for a fully powered system.

Power Is supplied to a l l components of the PROVOX consoles from a u t i l i t y power s t r i p located inside each console bay as shown in figure 9. All u t i l i t y power s t r i p s in a console grouping must receive power from the same c i r c u i t breaker panel . Each nominal 115-volt ac u t i l i t y power s t r i p i s supplied single-phase power from a separate 20-amp c i r c u i t breaker (10 amps for 230 vo l t s a c ) . The connection to the console i s made inside the u t i l i t y power s t r i p to three wires ( p i g t a i l s l i c e s ) .

Power i s supplied to a l l components of the PROVOX computers from power s t r i p s located in the computer cabinet bays. These power s t r i p s are supplied from « computer power d i s t r i bu t ion u n i t . For a 120-volt, 60-hertz ac s ingle bay computer, the computer power d i s t r i b u t i o n un i t must be supplied with s p l i t -phase 120/240 vol t ac or three-phase 120/208 volt ac power; for a multibay computer, three-phase 120/208 vol t ac power must be supplied. Each phase conductor must be supplied through a separate 20-amp c i r cu i t breaker (part of a ganged c i r c u i t breaker) . For a 230-volt , 50-hert!! single bay computer, the computer power d i s t r i b u t i o n uni t must be supplied from a 20-amp, 230-volt , s ingle-phase branch c i r c u i t . For a multibay computer, the computer power d i s t r i b u t i o n uni t must be supplied from a 30-amp, 230-volt, single-phase branch c i r c u i t . The connection to the computer power d i s t r ibu t ion uni t i s made to a terminal block mounted Inside the u n i t .

Console and £°HPilte.r_Peripheral £qul£ment

All peripheral equipment are supplied from the u t i l i t y power s t r i p s ins ide the console or computer cabinet or from remote i so la ted ground receptacles (shown in figures 4 and 6 and deta i led in figure 7 ) . Each peripheral un i t must receive I t s ac power from the same c i r c u i t breaker panel as the common e l e c ­t ron ics uni t with which i t In te r faces . The loads Imposed on the u t i l i t y power s t r i p s by the per ipherals and the common e l ec t ron i c s unit should be balanced between the u t i l i t y power s t r i p s as much as poss ib le .

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PN4:003

00720 Remote-Area Power Distribution

T„n<rAl remote-area oower distribution systems are shown in figures 5 and 6. Input c pier is sup

PpTied through an isolation transformer with the PROVOX ac input m. yvwc, AO ^ ° .. th(J transformer. The ac circuit con-

K o l s ^ ^ ^ ^ ^ P.n.1 «* its associated « u power disconnect switch to the remote-area circuit breaker panel.

In the remote-area circuit breaker panel, the neutral conductor is bonded to a bus bar that Is electrically isolated from the grounding conductor and from the circuit breaker panel. The grounding conductor is bonded to a separate bus bar thlt mayle bonded to the circuit breaker panel and connected exter-n«llv to the PROVOX ac ground. From the bus bars In the circuit breaker panel to L point of connecaon at the PROVOX equipment, all ac circuit conductors Ire electrically isolated from their conduits. Distribution to the system cabinetsor tc> the consoles and computers are then routed as described above for the system-level power distribution system.

DC POWER WIRING AND DISTRIBUTION

All system cabinets containing controller, multiplexer, or communications files are powered by a 24-volt dc power distribution system. When the files are installed In a PROVOX system cabinet, their dc power is supplied from a laminated bus bar located on the left side of the cabinet. Power to this bus can be obtained from the PROVOX instrumentation system standard dc power supplies or from a user-supplied source within the processing plant. The dc power supplies mount at the bottom of a cabinet and are provided with ac power through the power distribution panel. Two dc power supplies can be used for redundancy in a single cabinet with both supplies connected to be load sharing through the power distribution panel. Details for wiring the dc power sup­plies can be found in the appropriate power supply and power distribution panel installation planning notes. For additional redundancy and power outage protection, a uaer-supplicd dc power source (either batteries or dc power supplies) can be connected to the dc power distribution system of a cabinet.

Although the bus voltage is nominally +24 volt8 dc, the different voltages available from batteriOB and power supplies plus the varying voltage drops that occur in the connecting wiring could cause the voltage at the equipment to be significantly higher or lower. Be sure that the voltage at equipment terminal connections is within the tolerance specified for each individual product (see the appropriate product bulletin for these specifications).

System Cabinet DC Power Requirements

The dc power for a system cabinet is generated by a PROVOX power supply unit mounted in a cabinet. The output of a power Bupply ia routed through the power distribution panel for load sharing before being connected to the lam­inated bus bar In the cabinet.

Figure 10 shows the typical terminations for the power aupply, power distrib­ution panel, and bus bar. Figure 11, cabinets I through 3, shows a typical

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, . L i« t. n,f»» nrimarv supplies, and one redundant supply, system with three cnblnctfl, three P ™ 5 J ™ « ' w l t | |

,t h e thrce primary supplies

The redundant supply 1B normally loadi .hating w i t t i d J t u r b £ c e at the

so all four supplies are in use. There wtLL lteB 'or the redundant .sup.ly.

bus bar with a failure oE one of '** P ' ^ f ^ S ^ 1 Ground Point-<*L0PJ as The redundant supply must be connected to the same uoc the primary supplies that it is backing up.

. , . J„ Moure 11 cabinets 4 and 5. This method can An alternate method is ̂ . / " ^ ^ ^ V t a " r e q u i r e a total input current of bo used when two or *™«t£?\£h£*M together and receive power from T X V X ' A^edbuUndanatr8po

Cwer supply is also shown connected to the power distribution panel.

\\r. Power Distribution

M l dc power to the card files located in the system cabinets is obtained from the cabinet laminated bus bar. The card file terminations of typical products are shown in figure 10 and detailed in the Installation planning note for each product. Each product requires +24 volts dc and power supply common (PSC), with some products additionally requiring signal common (SC), The SC terminal is to be used only by some card files as shown in figure 10 and should not be used by othet devices unless the power and signal returns are isolated. The card file terminations are made to the laminated bus bar by using a minimum of 12 AWG (2.1 mm diameter) stranded wire with a red color-coded wire for +24V, black for PSC, and white for SC.

The SC and PSC should be maintained separate until they are connected to the local Ground Point (LGP) or Master Ground Point (MGP), discussed In later sections on ground wiring. The SC and PSC can be connected together at a point (prior to PR0V0X dc ground) where there is no power supply or battery return current flow to ground. Care should be taken to make sure that a voltage measurement between the PSC and SC is always less than 200 millivolts dc at each card file in a powered system.

Field Transmitter Power

Transmitters should not be powered directly from the +24V bus bar but should be provided from the fused terminations that were designed exclusively for powering the transmitters on the individual card files.

Battery Backup

Battery backup requirements can be met by connecting a user-supplied battery stack to the battery terminations on the power distribution panel, as shown in figures 12 and 13. Any time the Inputs to the power distribution panel fall below +23 volts dc, the battery stack is automatically switched on line. When the lino power is back to normal, the battery stack will be switched off line, letting the power supplies again provide power to the system.

It should be noted that separate battery stacks are required for cabinets 1 through 3 and for cabinets 4 and 5 in figure 12, as they are on separate LGPs. A separate battery a tack is required for each I.GP, and each connection to the

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PtU:003

battery stack must be separately fused to protect I t s output. The fuse «hu«ld be sized for a maximum continuous load of 35 amps aa required by thfe^bus^Bar in the system cabinet.

The memory sections of the common electronics unit of consoles and computers are supplied with and protected by battery backup. If the ac power to a console or computer should fa i l , the contents of the memory will remain valid for one hour. The console or computer will cease operation, but will retain the operating system, programs, and configuration information as long as the memory i s valid. Upon restoration of ac power to the console or computer operation will resume from the point where i t was interrupted (if the memory i s s t i l l valid), If the memory Is no longer valid, the operating system programs, and configuration must be reloaded.

User-Supplied +24 Volt I)C Feeder Line

A user-designed dc power system, to be used with the system cabinet equipment, must meet the input voltage requirements of that equipment. The PROVOX in­strumentation system is designed to operate from 21 to 28 volts dc at the bus bar. Any user-supplied dc power system must be capable of powering up under ful l load conditions.

Figure 14 shows system cabinets powered by a user-supplied +24-volt dc feeder l ine . Each power input to the bus bar must be fused to protect both system cabinet equipment and the plant +24-volt dc line. The bus bar must be fused for a maximum continuous current of 35 amps. This system requires a separate LGP for SC and PSC. A separate 24-volt return is required for each PSC LGP as shown. The SC LGP and the PSC LGP must remain isolated until they are con­nected at the MGP. There should be no current flow from the MGP to the PROVOX dc ground.

POWER ALARM WIRING

The PROVOX instrumentation system cabinets are equipped with alarm c i rcu i t s incorporated into several of the various units within the cabinets. Standard factory-assembled wiring connects these several alarms in each cabinet Into a series circuit that terminates as dry contacts In the power distribution panel. The dry contacts can then be connected to your alarm system. The alarm circuit la an integral part of the power converter card used In the following units:

o Type CP6201 Controller Card File Unit

o Type DH6001 Local Traffic Director Unit

o Type DH6002 Network Traffic Director Unit >

o Type DH6003 Data Concentrator Unit

o Type DN6001 Multiplexer Control Unit

-9 -

The controller card f i le , l o ca l traffic director, or network traffic mr~* can have two power converter cards installed in a single card f i le" . h . ^ ^ ' two alarm circui t relays in a file can also be connected in se r ies ' F ^ W * more, the power distribution panel contains a low-voltage or min I'nJr , failure detection circuit that can deenergize the alam re?ay T h e T „ J " T ?

provides a dry contact closure indication of abnormal c o L i U o n f t o T ? nal alarm annunciator that is wired to your specifications a n e * t e r ~

tlTS^'J^™ C l r C U i t S f ° r 3 8 l ° 8 l e c a b l n e t a r e c°n"«cted in series 'for the typical wiring as shown in figure 10, each individual circuit or I,™ ,™ bination of circuits can be connected to produce an a l a r , 1 X ^ 7 U To

SYSTEM GROUNDS

Poor or faulty grounds are among the most common causes of e lec t ron ic system problems. The ground system network for the PROVOX instrumentation system i s a very c r i t i ca l consideration since th is network reflects the accuracy of the entire control system. Thus, the extra time and effort spent in laying out a good ground system will be rewarded by easier startup and more reliable oper­ation. The PROVOX Instrumentation system uses three separate grounds:

I. PROVOX AC Ground—This is the single-point ground for a l l PROVOX ac connections, providing the safety ground for all ac-powered devices as well as the ground reference for the consoles and computers. The ac ground must conform to a l l local, s ta te , and national electrical code requirements for a ground system. Furthermore, in cases of plant ground or ground-grid systems, i t is recommended that a l l welding devices, motors, e t c . be grounded at different points on the grid than the points used for the I'ROVOX equipment connections. In case of grounding for a lightning protection device, the ground must not be connected to the PROVOX ac ground; by most code requirements, the lightning ground must be a minimum of at least 6 feet (1.8 meters) from any electrode of any other ground system.

!. PROVOX DC Ground—This is an isolated, single-point ground that provides u reference for the dc power and analog signals of the system cabinet equipment. The design goal for the PROVOX dc ground Is to provide a ground of such quality thjat i t measures I ohm or less to true earth. There are several methods that can be used to obtain a high quality earth ground system, and these methods will vary depending upon the soil type and moisture content at the individual location (earth ground systems are discussed in a later paragraph). Once the appropriate ground system is designed and Installed, the system should be tested periodically to assure that the quality of the ground system is maintained. Ground systems testing is discussed in a l a te r paragraph,

After Install ing the ground system, a megohmmeter test should show a maximum impedance of 3 ohms between the PROVOX dc ground and true earth. If necessary, the ground system should be improved until the 3-ohm imped­ance requirement is achieved.

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3. Cabinet Ground—The cabinet ground is a separate ground from the PROVOX dc ground and cannot be connected to the same point. However, it can be located at the same point as the PROVOX ac ground, plant ground, or even a point other than these two grounds (such as the building steel). The cabinet ground provides protection to both equipment and personnel from accidental shock hazards. It also provides a direct drain line for any electromagnetic interference (EMI) to which the components of the cabinet may be subjected. This ground must meet all code requirements for a ground system. The cabinet grounds are connected directly to the cab­inet, usually at one of the four mounting studs on the bottom corners of a system cabinet.

Standard factory-assembly methods connect the signal common (SC) and power supply common (PSC) ground wiring from the various card file assemblies of a system cabinet to the SC and PSC terminal lugs on the laminated bus bar of a cabinet. However, unless specifically selected, no further ground wiring is done within a cabinet, but muot be supplied and connected by the user.

o Signal Common (SC)—This is the voltage reference for all single-ended signals in the system. The signal common is connected to the laminated bus bar at the base of the cabinet.

o Power Supply Common (PSC)—This Is the power return for all +24-volt dc power connections in the system. The PSC must be isolated from the SC connections until they are joined at a designated ground point. If they are not isolated, the high currents that flow In the PSC circuits could cause several hundred millivolts of difference in apparent signal values by adding a voltage into the SC circuit. The PSC is connected to the laminated bus bar at the base of the cabinet.

CAUTION

If the PSC or SC c i rcui t s are not properly connected, property damage can result because of the excessive voltage differences on the two grounds; this voltage difference cannot exceed a maximum of 200 millivolts dc at each and every card f i l e . He sure that the commonB are properly instal led before energizing the system for test or operation.

'A system grounding assembly to aid in producing a single-point termination within a cabinet is available as an optional selection that can be installed ei ther at the factory or in the field after the infltrumentatlon oystem deliv­ery. The typical cabinet grounding aaaerably is shown In figure 16, Both versions of the grounding assembly [ I . e . , locnl ground point (I.CP) or master ground point (MGP) assembly] mount onto isolation brackets at the bottom front of a cabinet. 'A local ground point assembly will handle up to six cabinets in either the in-line or back-to-back configuration, for connections to the SC and I'SC lugs on the laminated bus bar, thus keeping the two circuits separated unti l a common point is reached. The PSC connections are made to the left half, and the SC connections are made to the right half of the U3P, as shown

11

PN4:003

007.. 5

in figure 11 through 14. The LGP assembly has one lug on the left half that is available for a battery backup return line connection ^ • " 1«8 *» [)* 4 IJI * »j » MOD Those two lugs accept wire sizes of AWG I/O

middle for connection to a MGP. These two i"8» Li«««.«. <. .^n^Hnn ... to 4/0 (8.3 to 11.7 mm). For a single grouping of cabinets « fonnectloncan be made directly from the LGP assembly to theJROVOX dcsingle-point earth ground. For more than one cabinet grouping, an MGP assembly should be us d t o

i . . 4««0 touother before being connected to the connect the several cabinet groupings togecner O C U L C 6

single-point earth ground.

GROUND WIRING

j i j .„- u..r.niv common for the PROVOX instrumentation The signal common and power supply commun ».«•. -«-„-„*<„« n «

. , . i . 4 J ,,„ oonnrate wires to their respective tie system should each be carried on separate WAH-O •• r points. The wiring siZe recommended for use with these grounI points is AWG No. 8 (3.3 nun diameter) with their lengths being as i } " " " ^ ^ ' * * SC and PSC wiring should be insulated to avoid unintentional ground connec­tions that can occur if the wires touch the netal cabinet or each other. The SC and PSC circuits from each cabinet should be joined at a single point. A local ground point can be established for each group of bln« 1th the SC and Pic circuits for each cabinet connected to that.point The left side of the local ground point assembly bar should be reserved for the PSC leads and the right side reserved for SC leads; the leads should not be Intermixed. A single £re should then be run from the local ground point assemblyt. master* ground point assembly located in one of the cab inets of ^grouping. A single wire is then run from each master ground point assembly to the PROVOX dc single-point earth ground. In some situations, a single-point ground may not be practical or feasible. For example, cabinets that are connected only by the data highway need not be returned to the same single-point ground. However, the same ground point should be used in the following cases:

o The cabinets are located in the same area.

o The cabinets are bolted together to form one contlglous assembly or

unit.

o The cabinets share the same single-ended signal. This will occur if one transmitter is connected to two points in separate cabinets or if an output from a controller or multiplexer is used as a remote set point for a unit in another cabinet.

If redundant or backup power supplies are being used, a backup power supply is required for each local-ground point area being powered, and a separate return line must be used for each LGP of the system. Although separate areas can be tied together at a master ground point assembly, power supply return currents between the separate areas should be minimized. The return currents must not be run through'the single-point earth ground. Regardless of whether a singlc-or multinle-ooint ground approach is chosen, plant grounds for the PROVOX Jnawuwnutlw ayato. ahouWnot be used to ground non-PROVOX instrumentation

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PN4;003

0V...O

system equipment, especially high-current devices such as motor contactors and power transformers.

Figure 10 shows the ground and power connections for cabinets with PROVOX power supplies; typical arrangements are also shown in figures 11 through 14. In either case, the redundant supply and the primary supplies that it supports must be connected to the same LGP assembly.

A power supply system can be supported by user-supplied battery or separate dc power source to protect against commercial ac power failure, A separate battery stack is required for each LGP. Figure 12 Illustrates the connections for a battery backup to the power supplies, using separate battery stacks for each local ground point. If this arrangement Is impractical, a common battery stack can be used, but the ground system may need two sets of local ground points—one for the signal common and one for the power supply coatnon cir­cuits. Figure 13 illustrates the connections for a battery backup to the power supplies when one battery stack Is supplying power for the system.

COMMON GROUNDING BETWEEN PROVOX INSTRUMENTATION AND OTHER PERIPHERAL DEVICES AND SYSTEMS

Other peripheral devices and systems for the PROVOX instrumentation system can include such equipment as an associated high-speed printer, a mass storage device such as a disk system, a secondary computer, or even the control con­sole Itself. Although physically separated by several feet, a comason ground­ing system should exist. For any area of the instrumentation system where more than one grouping of equipment exists, the common grounding system must be designed so that it does not create excessive current loops. All three ground system types are necessary (i.e., PROVOX ac, PROVOX dc, and cabinet ground, as described above) for a particular local area. Regardless of the ground types used, the interconnecting wiring must be of sufficient size to safely and adequately handle the currents involved. Figure 15 shows a typical ground connection between the PROVOX system cabinet and a Fisher ac Series 100 instrumentation unit.

Remote Peripherals

When perlpherlals must be powered from other than console or computer utility power strips, they will be powered from isolated ground receptacles. An isolated ground receptacle, as shown in figure 7, must be installed or con­structed In such a way that the ground terminal is electrically Isolated from the conduit. The ground terminal must be isolated from any contact with a ground or grounding system through its mounting hardware.

Shield Grounding System

A shield grounding system is used in the system cabinets to provide a common termination point for shielded field wiring. Field wiring Is described in detail in the signal wiring and data highway guidelines installation planning note.

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The shield ground point (SGP) is a copper bus bar that can be mounted in the horizontal cable trays located in the system cabinet. The SGP provides grounding for the signal cable shields. The SGP has connections for up to 45 separate shield terminations plus a similar connection for grounding the SGP to the RETMA rails of the system cabinet.

00727 Signal cable shields must be grounded to properly shield the signal wiring. In general, the cable shield should be terminated only at the power source end. When the source is either +24 volts or the current output section of controller or multiplexer units, the cable shield should be terminated in the system cabinet. The SGP is the grounding bar that should be used to terminate all signal cable shields that are to be terminated inside the system cabinet.

Consoles and Computers

The PROVOX consoles and computers are grounded only from the ground bus as shown in figures 4 and 6. The conduit carrying the circuit conductors is electrically isolated from the console or computer cabinet. It is further recommended that the grounding conductor cable to consoles and computers be made up of a large number of small conductors [such as No. 8 AWG composed of 168 strands of No. 30 AWG (0.25 mm diameter)], If a multiconductor cable is being used to supply power to the point of connection, then a second cable should be connected in parallel with the grounding conductor in the multi-conductor cable, and this second cable should be of the type mentioned above and must be routed with the multiconductor cable.

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§ EARTH GROUNDS

There are basically two types of earth ground systems that are used as a single-point plant ground, These are (1) earth ground rods and (2) watered chemically treated earth grounds. With either type of earth ground, geograph­ical separation of the various local areas of the instrumentation system may require that more than one earth ground be installed. The wiring to the single-point ground should be as short and direct as possible.

Installing an Earth Ground Rod System

A checkerboard pattern of interconnected ground stakes as shown in figure 17 produces a very good earth ground rod system. The stakes should be driven into the earth deeply enough to reach the water table of the locality. An alternative ground rod system makes use of a star-shaped grid pattern of conductors radiating from a single point and covered by moist earth. Resist­ance for an earth ground rod system varies, depending upon the soil type and the moisture content of water table depth. The following dimensions and values provide a general estimate of the requirements to produce a good ground with a rod system:

o Twenty 0,5-inch (12.7 mm) rods spaced as shown in figure 17 and driven to a depth of 8 feet (2.4 meters) will produce a resistance of about 4 ohms in a dry, gravelly area. However, the resistance value will drop rapidly if the moisture content is high,

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o An eight-conductor star configuration, made from solid or standard wire conductors with 1-inch (25 mm) diameter and 150-foot (45.7 meters) length wi l l produce a resistance of about 1 ohm in the same dry gravel as above. Again, a high moisture content will drop the resistance value.

Uii/28 Resistance measurements for the above two ground rod systems are made in accordance with the instructions for ground systems testing in this note.

Installing a Watered, Chemically Treated Earth Ground System

If soil conditions make installation of a ground rod system impractical, an effective alternative is the watered, chemicalLy treated system. Each ground rod in the system must be treated in the same manner. If more than one ground rod (electrode) is needed for the system, arrange the additional electrodes in a grid pattern as shown in figure 17. Use the following procedure to install each electrode:

1. Drill a 10-inch (25 cm) minimum diameter hole to a depth of at least 4 feet (1.2 m) below the top of the water retention soil strata (usually clay) or to a minimum depth of at least 15 feet (4.6 m), if the clay layer is located deeper than 15 feet. Figure 18 shows the dimensions for the chemical ground system.

2. Fill the hole with 6 inches (.15 cm) of mixed chemicals as shown in figure 18.

3. The chemical mix recommended is 75-percent hydrated gypsum (hydrous calcium sulfate), 20-percent bentonite (an absorptive and colloidal clay), and 5-percent sodium sulfate.

4. Hang and center the electrode and the steel watering pipe in the well.

5. Fill the well with chemicals to 6 inches (15 cm) above the top of the electrode.

6. Pour about 20 to 30 gallons (76 to 114 liters) of water through the steel watering pipe, and allow the chemicals to settle (approxi­mately 12 to 24 hours).

7. Refill the well with chemicals to 6 inches (15 cm) above the top of the electrodes.

8. Backfill the well with earth to 3 feet (1 m) below the surface.

9. Install the concrete pipe well and the galvanized steel conduits for connections to the control room, other local area ground systems, and other rods in the grid system, as required. Then, backfill around the well, covering the various conduits.

PN4:003 ] 10. F i l l the concrete pipe well with earth to the bottom of the well

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12. I n s t a l l the perforated s t e e l cover over the concrete pipe^weU.

GROUND SYSTEMS TESTING

Ail ground systems should be e l e c t r i c a l l y tested for res i s tance upon i n s t a l l a ­t ion and then tested at two-month in t e rva l s thereaf ter for the f i r s t year. After the f i r s t year, the system res is tance can be checked a t six-month i n t e r -vnls. The res is tance between the grounding points in the instrumentation system and the earth ground should be kept as low as possible to avoid in ­ducing offset voltages as a r e s u l t of the excessive currents in the various ground loops. The maximum permissible res is tance depends upon the current in the ground wire and the pa r t i cu l a r process to lerances . For example, a 10-m i l l i v o l t drop in the s ignal common lead can introduce an offset (or e r ro r ) of 0.25 percent of span in a 1- to 5-volt analog s igna l .

The effectiveness of the ear th ground rods used for a s ingle-point processing plant ground system can be conveniently measured with a megohmmeter equipped with an a l te rna t ing current genera tor . Figure 19 i l l u s t r a t e s a typical t es t configuration and the t e s t lead connections for t e s t ing a ground system. The aux i l i a ry electrodes ( labeled P2 and C2) should be driven to a depth compar­able with that of the - ground system under t e s t . When a known current (as indicated on the ammeter of the megohmmeter) i s passed through the earth ground under t e s t and aux i l i a ry electrode C2, the voltmeter indicates the po ten t i a l difference es tab l i shed between the earth ground under tes t and the other auxi l iary electrode (P2). The res i s tance R to ear th of the earth ground system under t e s t is the r a t i o of the poten t ia l difference V to the current I ( i . e . , voltage/current or R - V / I ) . Resistance for an instrumentation system ground should be 1̂ ohm or l e s s . If th i s res is tance i s not obtained, the ground system should be reworked u n t i l that value, or l e s s , i s obtained.

LIGHTNING PROTECTION

In an area where damage from e l e c t r i c a l storms could occur i t i s r0™mn,Q„,^ that a l ightning protect ion system be in s t a l l ed to protec \ o , / qu L T personnel . This protect ion should include protect ion for the buildine T. power d i s t r bution system, and any cables that run outs ide to the f i e l d e r to

i 2 T o f Z T * ' , B u ^ n V h l e l d e d < ^ l e s in conduit can e l i m i L e h majo -i ty of the t r a n s i e n t s , unless the area experiences a d i r ec t s t r i k e to cround Any ground system used with l ightning protect ion devices must be i s o l l t l T t o L

he ? i r r n l n r d Wi5h P R 0 V 0 V q U l p , n ° n t ' Any ^ound system I t \ I n p t ? the l ightning ground system is required to be separated by a minimum of 6 feet (1.8 meters) from any e lec t rode that i s part of the l ightning ground sys te^!

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intni'is io safety barriers an; designed to assure safe voltages and currents for instruments mounted in hazardous-rated atmospheres; Each barrier serves a:; the interlace between the field-mounted instrument and its associated control lor or instrument. The barriers .ire grounded to .i bus bar, and tbf bus bar is connected to earth ground. The strap or cable providing this satety ground must be rugged enough to prevent accidental physical damage while providing ,i resistance of less titan 1 ohm between the harrier mounting stud and the earth ground point. Field instruments in intrinsically safe systems can he installed and operated in their working locations without danger to personnel or plant.

Groundinjg Practices

Three complete grounding systems are used with the PROVOX instrumentation system: (1) PROVOX ac ground, (2) PROVOX dc ground, and (3) a cabinet shield or safety ground. These three ground systems must remain separated until they are connected at the singJje-p_ojjjt jilant earth ground or at an appropriate intermediate point. The laminated bus bar in the PROVOX system cabinets provides signal common (SC) and power supply common (PSC) connections for each card file; it also aids in keeping the three ground systems separate. A shield ground point, as shown in figure 1, is a copper bus bar that can be mounted in the horizontal cable trays located in the system cabinet. The shield ground point is provided for grounding the signal cable shields to the cabinet and has connections for up to 45 separate shield terminations plus a similar connection to the RETMA rail of the system cabinet. For additional information on grounding practices, see the documents listed in table I.

In addition to the SC and PSC connections, provide all cabinets, consoles, and exposed metal surfaces that may become energized due to a system fault with a good earth ground for safety, Use separate, insulated ground wires for all power supply and signal common connections. Safety ground wires need not be insulated. For additional ground installation and test measurement informa­tion, see the related documents listed in table I,

DATA HIGHWAY GUI D R U N K S

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The PROVOX data highway, which provides the communications between the various devices in a distributed process control system, uses a multidrop connection as opposed to a star or radial, connection. The multidrop connection method requires planning and routing of the highway in a si?rial manner. The total communications connections for .i PROVOX i n s t rumen tnt ion system with reihindanrv is shown in figure A. Knell lorn I area o| the system can have up to .10 local devices (UV; that are cont ro I I oil hy a mc '".>:!.! I.TPs p l u s up t o s i x i i e twor l . d o v i i e : . (ND) < ,:u be >, w.wk t r a f f i c d i r e c l o r (N'TD). AM i n t e r l a c e I M I I M I I i

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