CSE Arc Flash Primer

7/27/2019 CSE Arc Flash Primer

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Electrical equipment not properlymaintained, which can cause overheating, looseconnections, and overloading of conductors. It is recommended that electrical equipment bemaintained, cleaned, and infrared-scanned once every 12 months.

Accidental touching of live exposed parts by dropped tools, which can cause sparks andinitiate a fire in electrical equipment

Reducing the hazard

Arc flash is a very serious matter and can cause severe injuries such as loss of hearing, loss ofsight, and burns requiring many years of skin grafting and rehabilitation.

Injuries also can happen while workers are wearing their PPE equipment, especially when thework area has not been cleared of all obstacles. Workers can fall backwards hitting their head againstobjects causing neck, back, or other permanent injuries. These injuries also can happen to workerswho are several feet away from the explosion.

As a result of an arc flash, equipment can be damaged including primary switch gear,transformers, and low voltage distribution equipment. Financial impact can be considerable: lostproduction from downtime and potential litigation costs. Also, a companys reputation may be affectedby such incidences through the loss of the ISO ratings and poor safety records.

To reduce the potential of an arc flash, the following must be considered: proper design;preventative maintenance on electrical equipment; established goals and objectives; andperformance and maintenance of short circuit study, arc flash analysis, and coordination study.

Proper design

Prior to release of the 2002 edition of NEC, many engineers and designers were not aware of arcflash and related OSHA regulations. Arc flash mitigation consulting was not offered to theircustomers, nor were the needed calculations performed. On more than one occasion, I asked mymentor: Why dont we perform arc flash calculations and specify the settings for the breakers or typeof fuses on the drawings?

It is a good engineering practice to perform all necessary design in compliance with the NEC(including sections 110.16 Flash Protection and 240.12 Electrical System Coordination), the authorityhaving jurisdiction, OSHA regulations (29-CFR, Part 1910) and customer safety standards. A designis not complete and may be rejected by the authorities having jurisdiction if short circuit studies, arcflash analysis, and coordination study are not performed and documented properly, even though thedrawings were properly sealed by a professional engineer. The complete design will include allbreaker settings, all fuse types, and coordination between all the protective devices. Relying on thecontractor to perform engineering design is a failure on the engineers part. Lack of properengineering studies may allow the contractor to install the least expensive (not necessarilyappropriate) type of protective devices. Later on, the customer will likely be forced to pay the highcost to correct the deficient protective devices in order to meet the safety requirements at the facility.

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Preventative maintenance

Maintenance of the electrical equipment, including protective devices, is critical to maintainproduction and prevent hazardous incidences. At a minimum, maintenance should include infraredscan, oil samples, and cleaning and ventilation of electrical spaces. After infrared scan is completed,the engineer, electrician, or maintenance staff should replace the defective parts, tighten loose boltsand lugs, clean contacts, and clean coils and bushings. Compare the oil sample results to IEEEstandards and manufacturer requirements. Keep a log of the results for the life of the equipment.Maintenance, in my opinion, should be performed every six months, depending on the area in whichthe electrical equipment is located. Refer to NFPA 70B Recommended Practice for ElectricalEquipment Maintenance.

Goals and ob jectives

The goals and objectives of arc flash calculations are as follows:

Protection of workers from potential harm

Compliance with codes, local authorities, and OSHA

Improved production efficiency

Stability of insurance costs

Training and education of electricians and maintenance staff of the potential hazards

Reduction of arc flash related accidents

Perform and update short circuit calculations, arc flash analysis and coordination study

To allow the engineer to perform the arc flash study the following should be completed:

Gather as much information about the installed or proposed protective devices, cable size andlength, transformer impedance, voltages and X/R ratio. Contact the local utility and request the shortcircuit contribution. Also make sure to obtain the types of equipment at the plant (motors, lighting,resistive loads, etc).

Develop and maintain updates to the electrical one-line diagram as accurately as possibleincluding all the above information.

All electrical equipment should be tagged properly.

Input the collected data into appropriate analysis software. Save and run the calculationsoften. It is easier to trouble shoot later on.

IEEE 1584 does not require the calculations to be completed for systems less than 240 Vwhen fed from 125 kVA transformer or less. However, in my experience, safety departments of manycompanies require the calculations to be completed down to 120-V receptacles.




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Example

The partial one line diagram in Figure 1 shows a 12,470-V power source, 3,000-kVA transformer,and 4, 000-amp switchgear feeding a 250-hp compressor. The first calculation indicates that category

3 (8.4cal/cm2) exists at the SWBD-1. This means a higher degree of PPE equipment should be usedto work on this energized equipment. This will be uncomfortable for the electrician due to heat, sweat,and the need for big gloves to handle tools.

Figure 1 - Partial one-line diagram

To reduce a higher category to a lower one, try to adjust the trip settings on the main breaker. If

category 0 or 1 cannot be achieved with existing protective devices, then the installation of newerprotective devices may be needed. This could mean additional capital and downtime.

De-energizing electrical equipment is the safest way to perform maintenance. However, reducinghigher categories to lower categories is very important to achieve a safer environment introubleshooting control cabinets and machinery.




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It is best that the initial design of the electrical system includes the short circuit calculations, arcflash analysis, and coordination study between protective devices. In the above example, thecoordination study must be completed to calibrate the settings on both the main breaker for SWBD-1and the breaker for the compressor. In case of a fault, properly designed and coordinated devices willallow the compressor breaker to trip first, enhancing system safety.

After these calculations are generated, the arc flash and shock hazard tags must be printed andplaced on the specified equipment. See Figure 2 and 3. The flash hazard boundary, flash hazard,and the category type should be listed on the tag. Many maintenance and safety personnel havebeen requesting that the limited, restricted and prohibited approach be listed also.

Figure 2 - Typical Tag




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Figure 3 - Protection boundaries of electrical system

For more information, refer to the following standards and product manufacturers:

OSHA Standards 29-CFR, Part 1910, Occupational Safety and Health Standards1910 sub partS (electrical), Standard number 1910.333, specifically addresses Standards for Work Practices andreferences NFPA 70E.

The National Fire Protection Association (NFPA) Standard 70The National ElectricalCode (NEC) contains requirements for warning labels.

NFPA 70Eprovides guidance on implementing appropriate work practices that are required tosafeguard workers from injury while working on or near exposed electrical conductors or circuit partsthat could become energized.

The Institute of Electronics and Electrical Engineers (IEEE) 1584Guide to Performing Arc-

Flash Hazard Calculations.

Electrical systems analysis software:

- Easy-Power

- SKM

Definitions

1.Arc flash hazard: A dangerous condition associated with the release of energy caused by anelectric arc.

2.Arcing fau lt cu rrent : A fault current flowing through electrical arc plasma, also called arc faultcurrent and arc current.

3.Available fault cu rrent : The electrical current that can be provided by serving utility andfacility-owned electrical generating devices and large electric motors, considering the amount ofimpedance in the current path.

4. Bolted fault current: A short circuit or electrical contact between two conductors at different




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potentials in which the impedance or resistance between the conductors is essentially zero.

5.Circuit: A conductor or system of conductors through which an electric current is intended toflow.

6.Electrical hazard: A dangerous condition in which inadvertent or unintentional contact orequipment failure can result in shock, arc-flash burn, thermal burn, or blast.

7.Electric Shock: Physical stimulation that occurs when electrical current passes through thebody.

8.Energized: Electrically connected to or having a source of voltage.

9.Exposed (live parts): It is applied to parts that are not suitably guarded, isolated, or insulated.

10. Fault current: A current that flows from one conductor to ground or to another conductor dueto an abnormal connection between the two.

11. Flash hazard analysis: A method to determine the risk of personal injury as a result ofexposure to incident energy from an electrical arc flash.

12. Flash-protection boundary: An approach limit is a distance from live parts that are un-insulated or exposed within which a person could receive a second degree burn.

13. Incident energy: The amount of energy impressed on a surface, a certain distance from thesource, generated during an electrical arc event. Incident energy is measured in joules per centimetersquared.

14. Shock hazard: A dangerous condition associated with the possible release of energy causedby contact or approach to live parts.

15.Arc B last:The explosive result of an arcing fault. As current begins passing through ionizedair, large volumes of ionized gases, along with metal from the vaporized conductors, are rapidly

expelled, creating such hazards as intense heat, thermoacoustic shock wave, molten metal,shrapnel, blinding light, toxic smoke and contact with energized components.

16. Current lim iting fuse: A UL Listed, current-limiting fuse must clear a short circuit current inless than one half cycle. By isolating a faulted circuit before the fault current has sufficient time toreach its maximum value, a current-limiting fuse tremendously limits the total electrical energydelivered to the fault, reducing both the magnitude and duration of a fault current.

17. Short circuit: An electrical malfunction where current takes the path of least resistance toground, Current flow is excessive from low resistance resulting in a blown fuse.

18. Interrupting capacity:The interrupting capacity is the maximum value of current that acontact assembly is required to successfully interrupt at a specified voltage for a limited number of

operations under specified conditions.

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2008, Reed Business Information, a division of Reed Elsevier Inc. All Rights Reserved.

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CSE Arc Flash Primer