Electrical Safety - Secondary Ex-Protection

If electrical equipment has been kept under protection against explosion, then it does not necessarily mean that it would be a totally sealed or encapsulated unit.

Types of Protection Techniques

There are seven known types of protection techniques. Let us take a look at the different techniques −

Type M or Type h (Hermetically sealed)

This is a design where the equipment is kept under a complete sealed environment. The parts that can possibly ignite gas or vapour by being exposed to atmosphere are hermetically sealed with resin. It is suitable for Zone 0, 1 and 2 areas. Usually, small compressors, miniature motors and small lamps get this kind of protection scheme.

Type q (Sand Filled or Powder filled)

Here, an enclosure is filled with quartz sand of 1.6mm size having weight of 0.1% of water. The electrical equipment is placed inside of it. When any arc generates inside because of any ignition, it gets absorbed by the sand itself. It is mainly used for Fuse banks and Capacitors. It is suitable for Zone 1 and 2.

Type O (Oil Immersion)

It is similar to type q with very little difference; here sand is replaced by mineral oil. The enclosure shows high and low oil level on its body. It is used for circuit breakers, transformers and switching units. Zone 2 areas go through similar safety procedure.

Type P (Pressurized Apparatus)

In this case, the enclosure gets pressurized with a gas that is more in value than the atmosphere pressure. By so, the equipment present inside remains safe from outside gas and vapour. The process is called purged gas technique. It is used for Zone 1 and 2 areas.

Type I (Intrinsically Safe)

Unlike the above ones, it is not an enclosure; rather, it is a circuitry design. The concept is to limit the current and voltage input within the ignition energy required to ignite the inflammable gases/vapours or air mixtures under normal or anticipated fault condition. It is used for Zone 0, 1 and 2 areas.

Type e (Increased Safety Design)

This safety scheme is solely for Zone 2 areas. Here, the enclosure designed is usually of cast metal or Mould Polypropylene or fabricated sheet metal. The size of the enclosure is decided in such a way that the surface temperature can be limited within the planned temperature class.

Type d (Flameproof or Explosion proof design)

It is a very popular technique. Zone 1 areas usually go through such safety scheme. Here, the equipment to be protected is kept inside a cast metal of CI or LM-6 type. Sometimes, a moulded reinforced polypropylene GRP enclosure is also used. Precautions are taken to avoid the occurrence of explosions. Even if it occurs, it will get extinguished by moving inside.