Demonstration of the power of an arc flash. Photo courtesy of EWB Engineering, LLC.
Electrocution is the obvious danger faced by anyone working on or near live electrical equipment, and it is clearly important to understand shock hazards and wear appropriate protection. However, most electrical accidents are not the result of direct electric shocks. A particularly hazardous type of shorting fault - an arc fault - occurs when the insulation or air separation between high voltage conductors is compromised. Under these conditions, a plasma arc - an “arc flash” - may form between the conductors, unleashing a potentially explosive release of thermal energy.
What is an arc flash?
An electrical system can be subject to two types of shorting faults:
A bolted fault is everyone’s idea of a short circuit, such as energizing the circuit with a ground set in place. A bolted fault results in a very high current; it is a low impedance short because of the solid connection. Bolted faults behave predictably and so conductors can be rated to withstand the overcurrent for the time required for an interrupt device to operate. Bolted faults rarely result in an explosion.
Older switchgear that holds a fault rating will usually be rated for its ability to withstand this high current for a particular time period. A bolted, fault-rated piece of equipment will usually have a BIL (Basic Impulse Level) highlighted on the casing itself in the form of a fault current for a set duration; for example, 100 kA for 5 seconds.
The second - and far more destructive - fault is an arc fault. An arc fault occurs when the insulation (or, more specifically, the air separation) between electrical conductors is no longer sufficient to withstand their potential difference. This can occur for many reasons. A dropped tool or any other conductive element (even rust), introduced between or near energized components may compromise the insulating clearances. Often, incidents occur when a worker mistakenly fails to ensure that equipment has been properly de-energized. Incidents can even occur when a worker is simply removing a cover from a piece of equipment. A significant proportion of arc faults occur simply due to some form of equipment failure and are not limited to human interaction alone.
In contrast to the low impedance required for a bolted fault, an arc fault is a high impedance short because the discharge occurs through air. The current flow is therefore “comparatively” low but the explosive effects are much more destructive and potentially lethal. Unlike a bolted fault, it is difficult to predict exactly how much energy will be released by an arc fault. In particular, it is difficult to predict the duration of an arc fault because this depends on many factors, feedback mechanisms, and the response of the over current protection devices.
How to protect yourself
The first step in protecting yourself is to understand more about arc fault itself - the makeup of an arc flash and the consequences of an arc flash. What, exactly, happens when an arc flash occurs?
You can reduce the potential dangers of an arc flash by following the relevant safety guidelines and using appropriate personal protective equipment (PPE). Some standards define a procedure for arc flash hazard analysis as well as shock hazard analysis. You’ll want to know the flash protection boundary (FPB), the minimum “safe” distance from energized equipment that has a potential for an arc fault. Warning signs and “live work” permits can help keep you safe.
For much more information, including specifics about how an electrical thermographer can deal with arc flash hazards and how IR Windows can help, read the full white paper »
NFPA 70: National Electrical Code® 2011 edition »
NFPA 70E. Standard for Electrical Safety in the Workplace® »
IEEE Standard. 1584-2002. IEEE Guide for Performing Arc-Flash Hazard Calculations »
CSA Z462-08. Standard on Workplace Electrical Safety, Canadian Standards Association »
“Protecting Miners from Electrical Arcing Injury” James C. Cawley, P.E., and Gerald T. Homce, P.E.; NIOSHTIC-2 No. 20032718, National Institute for Occupational Safety and Health »
EWB Engineering, LLC (Electrical Engineers), www.ewbengineering.com »