Arc Flash Hazards
What is Arc Flash?
An Arc Flash is a short circuit through the air when insulation or isolation between electrical conductors is breached or can no longer withstand the applied voltage.
In an arc flash incident an enormous amount of concentrated radiant energy explodes out from the electrical equipment creating pressure waves that can damage a person’s hearing, a high-intensity flash that can damage their eyesight and a superheated ball of gas that can severely burn a workers body and melt metal. The pressure wave can send loose material like pieces of damaged equipment, tools and other objects flying through the air.
The best way to prevent Arc Flash from occurring is to de-energize the system before beginning any work and always verify that the energy is controlled.
Examples:
- Arc flash incidents typically occur in applications above 120 volts and can occur when electrical equipment is being serviced or inspected.
- Some incidents occur when a worker is removing a cover or trim from a piece of equipment.
What are the two types of faults in an electrical system?
A Bolted Fault consists of a low impedance short circuit between two separate phases or between phase-to-ground. Because of the low impedance path, the short circuit current present is very high compared to an “Arc Fault” in a similar system.
A typical rule of thumb is that the Bolted Fault current a system is capable of sustaining is roughly twice that of an Arc Fault in a similar system. Traditionally electrical equipment was designed to withstand this very high Bolted Fault current. However, a Bolted Fault rarely results in the devastating explosion associated with an Arc Fault and as such electrical equipment that has only been type tested for resistance to Bolted Faults may not maintain their integrity under Arc Fault conditions.
An Arc Fault is a phase-to-phase or phase-to-ground short circuit through air which is caused by a reduction in the insulating clearances to such an extent that the potential differences applied can no longer be withstood by the system. Such reductions can be as a result of dropped tools, or other elements that may be accidentally left behind that could compromise the distance between energized components.
An Arc Fault condition results in an Arc Flash. An Arc Flash consists of a plasma arc between shorted components which once established has virtually unlimited current carrying capacity. In addition to the plasma arc itself an explosion occurs which results in a huge amount of radiant energy (heat), a high-pressure blast wave and noise levels that can deafen.
What is a Flash Protection Boundary
As per NFPA 70E and the NEC the flash protection boundaries define the safe working distances in which any trades person can operate. An approach limit at a distance from exposed live parts within which a person could receive a second-degree burn if an electric arc flash where to occur. There are three such boundaries:
- Prohibited Approach Boundary
- Restricted Approach Boundary
- Limited Approach Boundary
The danger zone for Arc Flash conditions is different for different types of equipment and is established in part by the voltage of the system. Typically, the higher the voltage the larger the danger zone identified as the Flash Protection Boundary. To make it easier for facilities to determine these danger zones for each piece of equipment IEEE 1584 provides definitive calculation steps in support of NFPA 70E. It outlines a method for calculating the anticipated incident energy (amount of heat in cal/cm2), so the facility owner can make an informed decision about the level of personal protective equipment that those who work on the equipment must wear.
