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Before one considers the selection of the different types of protected equipment, we should first look at ways in which the dangers of a hazardous location can be reduced.
Wherever possible, electrical equipment should be situated in a ‘safe’ area, but only where this will lead to a reduction of the hazard. A good example is an electric motor situated in a Zone 1 or 2 location remotely controlled by a starter in a 'safe' location.
Explosion proof is used as a general term and relates to the various methods of protecting electrical apparatus for use in hazardous locations.
Flameproof or (Ex) d equipment is manufactured in such a way that it can withstand an internal ignition. Any gaps or 'flame path' in the enclosure are designed in such a way as to dissipate sufficient energy so that by then time the energy from the ignition reaches the outside of the enclosure, there is not sufficient energy to ignite the surrounding flammable atmosphere. This means that the equipment needs to be heavy and hence expensive, comprising wide flanges on the mating machined surfaces and deeply threaded entries providing a labyrinth like flame-trap.
Type Ex s specially protected is designed and manufactured for special requirements and shall be accompanied by a test certificate issued by the SABS to prove it is safe.
The principle of intrinsically safe equipment is that both the voltage and the current are limited by means of an electronic circuit so that even under fault conditions, neither the voltage nor the current could ever exceed parameters that would allow an arc or spark large enough to ignite the inflammable atmosphere.
This type of apparatus separates the inflammable atmosphere from arcs, sparks or hot surfaces by pressurizing or purging the enclosures with air or an inert gas. The pressure inside the apparatus is kept above the ambient pressure outside.
In this type of protected electrical equipment, special precautions have been taken to prevent sparking, arcing and the occurrence of temperatures high enough to ignite a prescribed gas- or vapour-to-air mixture. A typical type ‘e’ piece of apparatus for use in a coal mine would have non-sparking cable terminal connections, be classified IP65 (dust tight) and be rated temperature class T1 (methane gas).
This apparatus is normally used in Zone 2 locations where, in normal operation, it is not capable of igniting a surrounding explosive atmosphere and a fault capable of causing ignition is not likely to occur. As for increased safety type equipment, non-sparking apparatus is classified with IP and temperature class ratings.
This type of apparatus should be rated at least to a degree of protection IP65. This type of apparatus may be certified for use in atmospheres containing metallic and non-metallic dusts, or non-metallic dusts only, i.e. equipment approved for non-metallic dusts may not be used in atmospheres contaminated with metallic dusts.