Enclosure protection for electrical equipment is defined in terms of its opposition to the ingress of solid particles and liquids and against mechanical impact. The enclosure protection is defined by the Ingress Protection (IP) Code where a 2 or 3 figure number is used to indicate the degree of protection as shown in opposite. (A 2 figure IP code number indicates enclosure protection against the ingress of solids and liquids only.)
* IP *** index of protection of enclosures of electrical equipment in accordance with standards IEC 529 , BS 5490 , DIN 40050 and NFC 20 010 up to 1000 v ~ and 1500 v = ( standard UTE C 20 010 ) * The contents of the above chart are for guidence only . If you are any doubt as to the interpretation of the information contained there in, please refer either to the standard itself or contact FITCO .

Hazardous areas are classified into 'ZONES' which indicate the probability of an explosive gas-air mixture being present and, therefore, the likelihood of an explosion occurring .

Zone 0      In which an explosive gas-air mixture is continuously present, or present for
 long periods. 
Zone I      In which an explosive gas-air mixture is likely to occur in normal operation. 
Zone 2     In which an explosive gas-air mixture is not likely to occur in normal
 operation and, if it occurs, will exist for only a short time. An area which is 
 not classified Zone 0, I or 2 is assumed to be a non-hazardous  or 'safe' area. 
 Examples of this zoning applied to ships could be : 
ZONE 0    Interior spaces of oil cargo tanks, pipes, pumps, etc.
ZONE I     Enclosed or semi-enclosed spaces on the deck of a tanker, the boiler firing 
area on a gas carrier using methane boil-off as a fuel and battery rooms. 
ZONE 2    Open spaces on the deck of a tanker. The cargo pump rooms of tankers are,
 at present, considered as falling  somewhere between ZONE 0 and ZONE I .
  

This defines the maximum suTrface temperature of the components in the electrical equipment under normal and fault conditions. This maximum surface temperature must not exceed the gas ignition temperature. The temperature class is stated with reference to a maximum ambient temperature of 40°C.
should any other reference temperature be adopted, regulations require that this temperature be shown on the equipment. It is important to note that the apparatus gas grouping and temperature class are not related.
For instance, hydrogen requires very little spark energy to ignite, but the surface temperature necessary for ignition is very high (560°C). The following table relates the temperature class to the maximum surface temperature under fault conditions .

The flammable gases in which explosion protected electrical equipment may have to operate are grouped according to the amount of electrical energy, in the form of an arc. which is needed to ignite the gas.Gases associated with the mining industry are classed as GROUP I. all other industrial gases are classed as GROUP II in four sub-groups according to their ease of ignition.

It should be noted that equipment certified for use in GROUP IIC may also be used for IIA and IIB. Equipment certified for IIB may be used for IIA. Equipment certified for IIA may be used with no other group. The gas grouping can affect the design and construction of some types of explosion protected equipment (Exd and Exi),

preventing electrical equipment causing explosions in hazardous areas. Some techniques, such as flameproof enclosures, have long been established but others, such as intrinsic safety and increased safety, are the result of more recent developments in electronics and materials . It has been internationally agreed that explosion protected equipment be identified by the symbol 'Ex' followed by a letter indicating the type of protection employed. The following table lists the types of protection and their symbols :

Some equipment may use more than one of these types of protection in its construction . In this case, the primary type of protection is quoted first. For example, an increased safety motor with a flameproof terminal box would be marked Exe,d.

A flameproof enclosure is one wich will withstand , without damage , an explosion of the flammable gas within itself , and will prevent transmission of the flame wich could iginte the gas in the surrounding atmosphere.

These are circuits in which no spark nor any thermal effect produced under prescribed test conditions (which include normal operation and specified fault conditions) is capable of causing ignition of a given explosive atmosphere . Generally, this means limiting the circuit conditions to less than 30V and 50mA . Naturally, this restricts the use of Exi protection to low power instrumentation , alarm and communication circuits. The design of the circuit will depend on the type of gas present (gas grouping) .
In the UK two grades of Intrinsic Safety are recognised based on the safety factor of the equipment involved. Exia - the highest category based on a safety factor of 1.5 with two faults on the circuit .
Exib - based on a safety factor of 1.5 with only one fault on the circuit . In addition to apparatus in the hazardous area being rated as intrinsically safe , an electrical safety barrier may also be fitted to the circuit. The purpose of such a barrier is to limit voltages and currents in the hazardous area when faults occur on the circuit. A separate barrier is required for each Exi circuit and they must be fitted outside the hazardous area .

A safety (or zener) barrier comprises :
(a)   A fuse to limit the maximum current through the shunt diodes.
(b)   A set of resistors to limit the maximum current into the hazardous area.
(c)   A set of shunt connected zener diodes to limit the maximum voltage 
       appearing on the circuit within the hazardous area .

This is a specification which was originally devised in Germany . Increased safety equipment is based primarily on the elimination of 'open sparking' as at relay and switch contacts or on the commutators or slip rings of motors and generators, and on the close control of surface temperatures. Also the construction of the equipment is to a very high standard to prevent faults developing. Extra insulation is used, creepage distances between bare terminals are made longer and special enclosures to protect against damage due to entry of moisture and mechanical damage are also specified.

A new British classification derived from the German Exe but less exacting and is therefore more restricted in its application.

Same idea as the oil immersed except that quartz 'sand' is used rather than oil. An HRC fuse-link is sand filled.

This involves placing equipment in oil-filled tanks to prevent gas from reaching any arcs that may be produced.

Air or inert gas is supplied to the equipment slightly above atmospheric pressure to prevent entry of the flammable gas . This method is sometimes used for pumproom lighting.

This includes precautions taken to prevent explosion which are not specifically covered by the previous designations. The table below shows the type of protection which is allowed in the three hazardous zones: