An arc flash primer

Historically, arc flash analysis was not a priority for the operation and maintenance of electrical systems—at least not until the 2002 edition of the National Electrical Code, which mandated arc flash analysis. Companies are beginning to allocate the capital necessary to perform this important work.

05/20/2008


Historically, arc flash analysis has not been a priority for the operation and maintenance of electrical systems. Since the issue of the 2002 edition of the National Electrical Code, which mandates arc flash analysis, companies are beginning to allocate the capital necessary to perform this important work.

Persons who perform short circuit calculations, arc flash hazard analysis, or coordination studies should understand the following terminology: arc blast, arc flash hazard, short circuit, interrupting capacity, bolted fault current, current limiting fuses, safe work condition, flash protection boundary, limited approach boundary, restricted approach boundary, prohibited approach boundary, incident energy, PPE categories, and others listed in OSHA standards 29 CFR part 1910, NFPA 70E, and IEEE 1584. Refer to “Definitions” at the end of this article for clarification.

What is an Arcing fault?

The primary cause of arc fault is human error. Arcing is the flow of current through the atmosphere when two-phase conductors touch, or between phase conductor and neutral, or between phase conductor and ground. This can release arc fault occurs for many reasons:

Reducing the hazard

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

Injuries also can happen while workers are wearing their PPE equipment, especially when the work area has not been cleared of all obstacles. Workers can fall backwards hitting their head against objects causing neck, back, or other permanent injuries. These injuries also can happen to workers who 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: lost production from downtime and potential litigation costs. Also, a company’s reputation may be affected by 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; and performance 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 arc flash and related OSHA regulations. Arc flash mitigation consulting was not offered to their customers, nor were the needed calculations performed. On more than one occasion, I asked my mentor: “Why don’t we perform arc flash calculations and specify the settings for the breakers or type of 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 authority having jurisdiction, OSHA regulations (29-CFR, Part 1910) and customer safety standards, in lieu of all breaker settings, all fuse types, and coordination between all the protective devices. Relying on the contractor to perform engineering design is a failure on the engineer’s part. Lack of proper engineering studies may allow the contractor to install the least expensive (not necessarily appropriate) type of protective devices. Later on, the customer will likely be forced to pay the high cost to correct the deficient protective devices in order to meet the safety requirements at the facility.

Preventative maintenance

Maintenance of the electrical equipment, including protective devices, is critical to maintain production and prevent hazardous incidences. At a minimum, maintenance should include infrared scan, 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 bolts and lugs, clean contacts, and clean coils and bushings. Compare the oil sample results to IEEE standards 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 which the electrical equipment is located. Refer to NFPA 70B Recommended Practice for Electrical Equipment Maintenance.

Goals and objectives

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

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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:

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Example

The partial one line diagram exists at the SWBD-1. This means a higher degree of PPE equipment should be used to 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 newer protective devices may be needed. This could mean additional capital and downtime.

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

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

After these calculations are generated, the arc flash and shock hazard tags must be printed and placed on the specified equipment.

Figure 2 - Typical Tag (Right)

Figure 3 - Protection boundaries of electrical system

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

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Definitions

  1. Arc flash hazard: A dangerous condition associated with the release of energy caused by an electric arc.
  2. Arcing fault current: A fault current flowing through electrical arc plasma, also called arc fault current and arc current.
  3. Available fault current: The electrical current that can be provided by serving utility and facility-owned electrical generating devices and large electric motors, considering the amount of impedance in the current path.
  4. Bolted fault current: A short circuit or electrical contact between two conductors at different 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 to flow.
  6. Electrical hazard: A dangerous condition in which inadvertent or unintentional contact or equipment failure can result in shock, arc-flash burn, thermal burn, or blast.
  7. Electric Shock: Physical stimulation that occurs when electrical current passes through the body.
  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 due to an abnormal connection between the two.
  11. Flash hazard analysis A method to determine the risk of personal injury as a result of exposure 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 the source, generated during an electrical arc event. Incident energy is measured in joules per centimeter squared.
  14. Shock hazard: A dangerous condition associated with the possible release of energy caused by contact or approach to live parts.
  15. Arc Blast: The explosive result of an arcing fault. As current begins passing through ionized air, 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 limiting fuse: A UL Listed, current-limiting fuse must clear a short circuit current in less than one half cycle. By isolating a faulted circuit before the fault current has sufficient time to reach its maximum value, a current-limiting fuse tremendously limits the total electrical energy delivered 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 to ground, 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 a contact assembly is required to successfully interrupt at a specified voltage for a limited number of operations under specified conditions.


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