Tools

>> Explosion Protection
>> Comparsion IEC - Nema Protection Class
>> IP Protection Class
>> Unit Converter

Introduction

Potentially Explosive Atmospheres exist where there is a risk of explosion due to mixtures of gas/air, vapour/air, dust/air or other flammable combinations.

In such areas there is a necessity to eliminate sources of ignition such as sparks, hot surfaces or static electricity which may ignite these mixtures.

Where electrical equipment has to be used in these areas it must be so designed and constructed as to not create sources of ignition capable of igniting these mixtures.

Before electrical equipment can be used in a potentially explosive atmosphere a representative sample has to be fully tested and certified by an independent authority such as BASEEFA in the U.K. or UL in the U.S.A.

This information is intended as a guide only and further expert guidance should be sought before placing into service, maintaining or repairing any item of equipment in a Potentially Explosive Atmosphere.

Where comparisons are shown between, for example, European and North American practice this may be an approximation and individual standards/codes of practice should be consulted for precise details.

Area Classification
Process plants are divided into Zones (European and IEC method) or Divisions (North American method) according to the likelihood of a potentially explosive atmosphere being present.

European & IEC Classification	Definition of zone or division	North American Classification
Zone 0 (gases)	An area in which an explosive mixture is continuously present or present for long periods	Class I Division 1 (gases)
Zone 20 (dusts)		Class II Division 1 (dusts)
Zone 1 (gases)	An area in which an explosive mixture is likely to occur in normal operation	Class I Division 1 (gases)
Zone 21 (dusts)		Class II Division 1 (dusts)
Zone 2 (gases)	An area in which an explosive mixture is not likely to occur in normal operation and if it occurs it will exist only for a short time	Class I Division 2 (gases)
Zone 22 (dusts)		Class II Division 2 (dusts)
		Class III Division 1 (fibres)
		Class III Division 2 (fibres)

Gas Groups (plus dusts and fibres)

There are two main gas groups, Group I Mining only and Group II Surface Industries.
 
These categories are used in European and I.E.C. groupings.
Group I is concerned only with underground mining where methane and coal dust are present.
Group II gases occurring in surface industries, are sub-grouped according to their volatility. This enables electrical equipment to be designed to less onerous tolerances if it is to be used with the least volatile gases.

Typical gas/material	European/I.E.C. Gas Group	North American Gas Group
Methane	I	-
Acetylene	IIC	A
Hydrogen	IIC	B
Ethylene	IIB	C
Propane	IIA	D
Metal dust	-	E
Coal dust	-	F
Grain dust	-	G

Note : North American legislation now allows Zones to be used to classify areas, where this practice is used it follows the IEC Zone method.

Temperature

Hot surfaces can ignite explosive atmospheres. To guard against this all Electrical Equipment intended for use in a potentially explosive atmosphere is classified according to the maximum surface temperature it will reach in service. This temperature is normally based on a surrounding ambient temperature of 40 degrees Centigrade (102 degrees Fahrenheit). This temperature can then be compared to the ignition temperature of the gas(es) which may come into contact with the equipment and a judgement reached as to the suitability of the equipment to be used in that area.

Temperature Classification		Maximum Surface Temperature
European/I.E.C.	North American
T1	T1	450°