Arc Flash Studies

Background

Arc flash is the ball of fire that explodes from an electrical short circuit. The unfortunate aspect of arc flash is that many times, there is a person standing in its path. This explosion includes a ball of fire and molten metal as well as a pressure force or blast.

This overview discusses the ball of fire only. Although the pressure blast can be enormous, enough to knock a person across the room, it is another topic and is not discussed here. Arc flash temperatures can easily reach 7500 to 8500 degrees C.

These temperatures can be reached by a fault in several seconds if not several cycles. The heat generated by the high current flow may melt or vaporize the material and create an arc. This arc-flash creates a brilliant flash, intense heat, and a fast moving pressure wave that propels the arcing products.

Some of the effects of an arcing fault include:

Extreme Heat, Pressure Waves, and Sound Waves

Molten Metal, Shrapnel and Vapor

Intense Light

Arc flash is related to the available fault current and total clearing time of the over-current protective device during a fault. It is not necessarily linear, as lower fault currents can sometimes result in a breaker or fuse taking longer to clear, thus extending the arc duration and thereby raising the arc flash energy. To perform an accurate arc-flash hazard analysis a realistic value for the three-phase bolted fault and the total clearing time for the affected over-current protective device must be known.

Arc flash is measured in thermal energy units of calories per centimetre squared (cal/cm2) and for arc flash analysis is referred to as the INCIDENT ENERGY of the circuit. 1.2 cal/cm2 of thermal energy on a person’s skin for a short period of time generally produces a second degree burn. A second degree burn, although painful, is considered curable. This amount of energy can be compared to holding your hand several inches above a disposable lighter. The intent of an arc flash hazard analysis is first to determine the amount of personal protective equipment (PPE) required by the worker to limit any burn to a second degree burn and second, to determine the safe distance away from energized equipment for unprotected persons.

Types of Faults

In order to understand the hazards associated with an arc flash incident, it’s important to understand the difference between an “arcing” short circuit and a “bolted” short circuit.  A bolted short circuit occurs when the normal circuit current by-passes the load through a very low impedance path resulting in current flow that can be hundreds or thousands of times the normal load current. In this case, assuming all equipment remains intact, the fault energy is contained within the conductors and equipment and the power of the fault is dissipated throughout the circuit from the source to the short. All equipment needs to have adequate interrupting ratings to safely contain and clear the high fault currents associated with bolted faults.

In contrast, an arcing fault is the flow of current through a higher impedance medium, typically the air, between phase conductors or between phase conductors and neutral or ground. Arcing fault currents can be extremely high in current magnitude approaching the bolted short-circuit current but are typically between 38% and 89% of the bolted fault. The inverse characteristics of typical over-current protective devices generally results in substantially longer clearing times for an arcing fault due to the lower fault values.

The amount of energy released during an arcing fault depends upon the voltage, the current, and the duration of the arc. The arc duration is dependent on the arcing fault current magnitude and the protective device settings. Due to its nature, the magnitude of an arcing fault is subject to many variables and therefore is difficult to perfectly predict.

Using sound judgments and assumptions, it is reasonable to think of the arcing fault as a range of possible currents that result in a minimum and maximum arcing fault current.

 Benefits of Performing an Arc-Flash Hazard Analysis

In addition to reducing or preventing injury to workers, the additional benefits associated with performing an arc flash hazard analysis can include most of the following:

Provides workers with the best possible PPE.

Insurance premiums can be reduced.

Brings electrical system documentation up to date by providing current and accurate one-line diagrams.

System reliability can be enhanced with a proper protective device coordination study to insure device closest to the fault opens in the least amount of time.

Over duty equipment can be identified from an accurate system wide short circuit analysis.

Since the system is typically modelled on software, it will be easy to make future changes or upgrades with minimal expense or effort.

Most importantly, there will be fewer injured worker when an analysis is performed and recommended procedures are followed.

Costs of Not Performing an Arc-Flash Hazard Analysis

Costs due to not performing an arc flash hazard analysis can range from minor costs associated with fines, to millions of pounds for lifelong medical cost and can include any of the following:

Cost of non-compliance fines

Cost associated with lost productivity

Increased equipment repair costs

Medical expenses for injured workers

Legal costs

Most importantly, loss of life, there is no price for this.

Prior To Beginning an Arc-Flash Hazard Analysis

Prior to beginning an arc flash hazard analysis, the following questions should be answered to help in the assessment:

Are the over-current protective devices set to trip in the fastest possible time?

Is the plant having unexplained outages?

Has the plant been expanded and/or added new electrical equipment?

Is the equipment rated to safely clear available fault current?

What can be done to mitigate excessively high fault currents or long tripping times?

Has equipment been properly tested and maintained to insure proper operation?

NOTE: Proper maintenance and coordination of protective devices is vital when doing an arc flash hazard analysis. All the studies in the world are useless if the equipment does not function as expected or designed.

When used, PPE represents the last line of defence against injury. The protection is not intended to prevent all injuries but to mitigate the impact of an arc flash upon the individual, should an incident occur. In many cases, the use of PPE has saved lives or prevented injury. The calculations represented herein will provide a level of protection that is balanced between the calculated estimated incident energy exposure and the work activity being performed. It is important to realize that too much PPE can also be a hazard. Workers can be protected for more incident energy that is available but may not be able to perform their intended duties due to heat stress, poor visibility, and limited body movement. At all times, professional judgment must always be used in the selection of adequate PPE.

The information presented is not intended to imply that workers be allowed to perform work on exposed energized equipment or circuit parts. It must be emphasized that the industry-recommended way to minimize electrical injuries and fatalities is to ensure that equipment is de-energized and in an electrically safe work condition. But even the act of creating an electrically safe work condition, subjects the worker to potential hazards, which if they occur, require PPE for protection against arc-flash burns.

Obviously, the best way to prevent an arc-flash hazard is to totally de-energize the equipment. It would be great if we could turn off the country everyday so electrical workers could go to work. However, even if you can totally de-energize the equipment you need to open devices upstream somewhere. It is best if this can be done remotely, but if it cannot be, workers must be trained and know the proper arc flash protection required for the given task.