Arc Flash Hazards – LV Circuit Breakers

Scope

There are essential requirements for electrical safety. Both domestic and international standards refer to steps and procedures to be taken for protection against hazards from electrical equipment. In particular, high temperatures and electrical arcs or arc flash can cause catastrophic damage.

This document illustrates the approach taken in the United States to safeguard against the hazards derived from electrical arcs and their effects on human beings. It also sums up the recommendations of the US standards for all personnel working on live electrical equipment.

Electrical arcs and their dangerous effects on people

The arc formation in a cubicle can be described in 4 phases:

1. Compression phase – The volume of the air where the arc develops is overheated due to the release of energy. The remaining volume of air inside the cubicle heats up from convection and radiation. Initially there are different temperatures and pressures from one zone to another;

2. Expansion phase – From the first instant of internal pressure increase, a hole is formed through which the superheated air begins to escape. The pressure reaches
its maximum value and starts to decrease from the release of hot air;

3. Emission phase – Due to continued contribution of energy by the arc, nearly all the superheated air is forced out by an almost constant overpressure;

4. Thermal phase – After the expulsion of the air, the temperature inside the switchgear nears that of the electrical arc. This final phase lasts until the arc is quenched, when all the metals and the insulating materials coming into contact undergo erosion with production of gas, fumes and molten material.

Should the electrical arc occur in an open configuration some of the described phases might not be present or have less effect; however, there will always be a pressure wave and a rise in temperature in the zones surrounding the arc.

Being in the proximity of an electrical arc is extremely dangerous:

• Pressure: at a distance of 24” (61cm) from an electrical arc associated with a 22 kA arcing fault a person can be subject to a force of 500lb (225kg); furthermore, the sudden pressure wave may cause rupture of the eardrums or permanent injuries;
• Temperatures of an arc can reach about 34,232 ºF (19,000°C; the surface of the Sun is 6,000°C)
• Sound: electrical arc sound levels can reach 160 db, (a jet engine at 100’ (30m) is 140 db).

The electrical arc lasts until the opening of the overcurrent protective device on the supply side of the electrical arc.