Electrical equipment in hazardous areas

In electrical engineering, hazardous locations (sometimes abbreviated to HazLoc, pronounced Haz·Lōk) are defined as places where fire or explosion hazards may exist due to flammable gases, flammable liquid–produced vapors, combustible liquid–produced vapors, combustible dusts, or ignitable fibers/flyings present in the air in quantities sufficient to produce explosive or ignitable mixtures. Electrical equipment that must be installed in such classified locations should be specially designed and tested to ensure it does not initiate an explosion, due to arcing contacts or high surface temperature of equipment.

The introduction of electrical apparatus for signaling or lighting in coal mines was accompanied by electrically-initiated explosions of flammable gas and dust. Technical standards were developed to identify the features of electrical apparatus that would prevent electrical initiation of explosions due to energy or thermal effects. Several physical methods of protection are used. The apparatus may be designed to prevent entry of flammable gas or dust into the interior. The apparatus may be strong enough to contain and cool any combustion gases produced internally. Or, electrical devices may be designed so that they cannot produce a spark strong enough or temperatures high enough to ignite a specified hazardous gas.

A household light switch may emit a small, harmless visible spark when switching. In an ordinary atmosphere this arc is of no concern, but if a flammable vapor is present, the arc might start an explosion. Electrical equipment intended for use in a chemical factory or refinery is designed either to contain any explosion within the device, or is designed not to produce sparks with sufficient energy to trigger an explosion.

Many strategies exist for safety in electrical installations. The simplest strategy is to minimize the amount of electrical equipment installed in a hazardous area, either by keeping the equipment out of the area altogether or by making the area less hazardous by process improvements or ventilation with clean air. Intrinsic safety, or non-incendive equipment and wiring methods, is a set of practices for apparatus designed with low power levels and low stored energy. Insufficient energy is available to produce an arc that can ignite the surrounding explosive mixture. Equipment enclosures can be pressurized with clean air or inert gas and designed with various controls to remove power or provide notification in case of supply or pressure loss of such gases. Arc-producing elements of the equipment can also be isolated from the surrounding atmosphere by encapsulation, immersion in oil, sand, etc. Heat producing elements such as motor winding, electrical heaters, including heat tracing and lighting fixtures are often designed to limit their maximum temperature below the autoignition temperature of the material involved. Both external and internal temperatures are taken into consideration.

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