Arc Flash, Electrical Faults protection

An arc fault happens when electric current flows through air gaps between conductors. Arc flashes often occur when racking in a breaking, performing switching, Insulation failure, accidents caused by touching a test probe to the wrong surface or slipped (non-insulated) tools. In its most basic form, an arc is made up of four elements; Thermal Energy (heat), Acoustical Energy (sound), Presssue Wave and Debris. Each of these elements can cause serious injury or death to a person.

 

Arc Flashes

In other words, arc flash is caused by uncontrolled conduction of electrical current from phase to ground, phase to neutral, and/or phase to phase accompanied by ionization of the surrounding air. Because of the expansive vaporization of conductive metal, a line-to-line or line-to-ground arcing fault can escalate into a three phase arcing fault in less than a 1/1000 of a second. The heat energy and intense light at the point of the arc is called arc flash. These massive releases of energy are unpredictable and uncontrollable.

While arc flashes are commonly thought to release out the front of the switchboard they originated in, in some cases they have been known to release rearwards and blow the back off the housing; there is no standard arc flash incident.

Arc flash Electrical Burn Image Electrical burn of the hand and arm HV Line Burn Image Arm with third degree burn from HV line

Short circuits and arc faults are extremely dangerous and potentially fatal to personnel. The product of arc fault current and voltage concentrated in one place, results in enormous energy released in several forms.

Arc flash generates large amounts of heat that can severely burn human skin and set clothing on fire. Temperatures at the arc can reach four times the temperature of the sun's surface. The high arc flash temperature vaporizes the conductors in an explosive change in state from solid to vapour. Copper vapour expands to 67,000 times the volume of solid copper.

 

Arc Flash Protective Clothing

Conductive vapours help sustain the arc and the duration of the arc flash is primarily determined by the time it takes for over current protective devices to open the circuit. For example, fast acting fuses may open the circuit in 8 ms or faster while other arc flash protection devices may take much longer to operate and open. Metal is blasted and splattered from the fault location.

The arcing faults also produce large shock waves that can blow personnel off their feet. The other exposure risks to arcing faults or arc flashes include flying debris, severe sound waves, shock hazard due to touching energized conductors etc.

Electrical Safety