Protection Priorities

In deciding on the degree of protection which should be offered by your electrical equipment for the environment in which you intend to operate it, you need to give attention to the following three priorities.

Priority 1

In order to prevent gas explosions and dust explosions you need to consider the probability of the presence of a potentially explosive gas, dust or vapour and air mixture in the area in which the equipment is to operate.

Areas where there is the likelihood of the presence of explosive gas-air/dust-air mixtures are referred to as zones. Zones are classified as shown in the table below. The higher the number in this "zonal classification", the smaller the risk of an explosion.

The atmosphere of the environment in which the hazardous area equipment is to operate will have a bearing on the degree of protection against explosion that should be provided by the equipment.

Zone Classifications

Zone 0 /20

Zone 0:
An area in which an explosive atmosphere is likely to be present continuously for long periods or frequently.

Zone 20:
Continuous presence of dust cloud

Zone 1 /21

Zone 1:
An area in which an explosive atmosphere is likely to occur during normal operation, occasionally.

Zone 21:
Primary grade of a dust cloud release

Zone 2 /22

Zone 2:
An area where the occurrence of an explosive atmosphere is unlikely in normal operation and if will be of a short duration.

Zone 22:
Secondary grade of dust cloud release

Designation of zones depending on presence of combustible gas / vapour

Designation of zones depending on presence of combustible dust

Priority 2

You need to give consideration to the quantity of heat energy needed to produce an explosion of the gas-air/dust-air mixtures.

You also need to give consideration to the intensity of the explosion that would occur if there were to be an ignition of the vapour or gas-air/dust-air mixture in the area in which the equipment is to operate.

Gases and vapours can be grouped according to the energy be needed to ignite the gas and the intensity of the explosion produced by the gas.

A table of some of the gases and vapours listed in gas groups I, IIA, IIB and IIC is shown below.

The chemical properties of the gases or vapour that may be present in the area in which the equipment is to be operated have a bearing on the degree of protection which should be provided by the equipment.

Zone Classifications

Gas Groupings
Gas Group Gas or Vapour
I (mining) methane (firedamp)
IIA
 industrial methane, propane, aviation fuel
IIB
 ethylene, town gas
IIC
 hydrogen, acetylene, carbon disulphide

Priority 3

You need to give consideration to the temperature at which the vapour, gas-air or dust-air mixture would ignite and become a self-sustaining reaction in the area in which the equipment would operate.

The surface temperature of the electrical equipment must be such that it never exceeds the temperature at which the vapour, dust or gas air mixture in the equipment environment will ignite and sustain combustion.

A system of temperature ratings, so called "T ratings", has been devised to cover the range of temperatures in which it is safe to operate the electrical equipment in the explosive environment.

The relationship between the T rating , the ignition temperature of the ambient explosive gas-air and the maximum allowable surface temperature of the electrical equipment is given in the following table.

Temperature Rating

Temperature Rating (often referred to as T rating or temperature class)
T rating
(temperature class)		 Ignition temperature range of the explosive gas-air mixture Maximum permitted surface temperature of electrical equipment
T1 greater than 450ºC 450°C
T2
 between 300°C and 450°C 300°C
T3
 between 200°C and 300°C 200°C
T4
 between 135°C and 200°C 135°C
T5
 between 100°C and 135°C 100°C
T6
 between 85°C and 100°C 85°C

For example, town gas has an ignition temperature of 600°C. From the table you can see that this places it in the T1 temperature class. Therefore the maximum permissible surface temperature for equipment that operates in a town gas and air environment is 450°C. Carbon disulphide vapour on the other hand has an ignition temperature of only 100°C.

This gives the vapour a T rating of T6. Therefore the maximum permissible surface temperature for equipment in this environment is 85°C.The T rating for some of the gases in gas groupings IIA, IIB and IIC is given in the table below.

Temperature Rating

Gas Group T1 T2
 T3
 T4
 T6
IIA
 apropane
toluene
 n-hexane
n-butane
 petrol
aviation fuel
 acetyl
aldehyde
  
IIB
 town gas
 ethylene
  
  
  
IIC
 hydrogen
    
  
 carbon
disulphide

The minimum ignition temperature of a dust cloud is the lowest temperature of a heated impact plate at which the dust blown into the oven ignites or decomposes, producing flames or an explosion.

Powders show a broad spectrum of ignition sensitivity and the vast majority need a very energetic ignition source.The minimum ignition temperature is not a true constant for a given dust cloud, but depends on the geometry of the hot surface and the dynamics of the cloud.

The following table shows some explosive parameters of dust.

Minimum Ignition Temperature of a Dust Cloud

Material
T Cloud
 T Layer
 Ignition Energy (mj)
Flour
 380°C
 300°C
 >30
Wood Flour
 410°C
 200°C
 >100
Aluminium
 560°C
 270°C
 >5
 Sulphur 240°C
 250°C
 >10