1.0 PRESENCE OF GASES IN COAL MINES

 Firedamp is a mixture of naturally occurring flammable gases that forms the most common potentially explosive gas in mines. It is commonly predominantly methane and burns or explodes at concentrations between 5% and 15% in air. However, other flammable gases that can occur and may need taking into account include:

• Mixtures of higher hydrocarbon gases in the vicinity of oil bearing strata;

• Hydrogen released during battery charging and by spontaneous combustion (water gas) – explosive limits are between 4% and 74% by volume in air;

• Carbon Monoxide – explosive limits between 12.5% and 74% by volume in air.

 2.0 AREA CLASSIFICATION:

 Group II area is divided into zones as per severity of hazard. There is no guideline for such divisions within Group I. However, Indian Standard classifies coal mines based on rate of emission of gas & percentage of flammable gas in hazardous area formed by gas emission. This guideline may be used for zone classification of coal mines as under: 

 This will facilitate use of Ex equipments other than Flameproof (Ex d) & Intrinsically Safe (Ex i), which are not being presently used as is being done in Group II area. This is logical to use equipment having different level of protection in area based on severity of hazard.

 It should be appreciated that % of Methane (firedamp) in case of Mine of Degree I & II is normally much below 1.25%, which is target level for sounding alarm. Even 1.25% is much below LEL of 5% thus these categories of mines are quite safe. Degree III mines do call for greater safety measures. However, here also necessary action pursuant to sounding of alarm at 1.25% Methane (firedamp) can supplement safety levels.

 A consideration also needs to be given to fact that within a particular degree of mine large areas may not pose even the degree of hazard anticipated at working face, which may be classified as Zone of lower hazard even within same mine. This is akin to extent of Zone as in Group II, where all the Zones are present with geometry defined based on source of release. In case of Coal Mines working face becomes source of release. This needs very careful consideration due to fact that coal mines are not like fixed installation but working face keeps shifting as per development. At same time this point needs consideration as deployment cost of equipments will be considerably reduced due to most judicious use of Ex equipments of almost all type of Ex protection. 

3.0 CATEGORY OF EQUIPMENT:

ATEX divides Ex mining equipments in M1 & M2, Category M2 also covers non electrical equipments.  However, exact co-relation of Category M1 & M2 with IEC / EN 60079 Series of code is not clear in HSE Guidelines.   

IEC 60079-0 divides Group II equipments as IIA, IIB & IIC for Ex d & Ex i based on degree of hazard. Methane (firedamp) is having Ignition Temperature of 595ËšC thus even T1 Class equipment are suitable for Group I. This code also considers hazard from coal dust in addition to methane (firedamp) and accordingly temperature classification T1 is no more applicable to Group I but maximum surface temperature has been specified as under: 

1. 150ËšC on any surface where coal dust can form a layer.
2. 450ËšC where coal dust is not expected to form a layer. 

This can become a logical ground for subdivision of Group I Ex equipments based on difference in hazard/ safety level. We can thus subdivide Ex equipments for Group I as IA & IB to signify IA as equipments on which coal dust is not expected to form a layer and IB as such equipments on which dust can form a layer. 

4.0 MONITORING OF HAZARDOUS AREA: 

Indian Standard prescribes monitoring of methane (Limit - 1.25%) & Carbon Monoxide (Max 5ppm) to sound an audio visual alarm. Further course of action may be specified. 

Under ATEX directive Equipment of Category M2 is required to be de-energized at 1.25 % methane concentration. However, Category M1 equipments are allowed to work. 

5.0 SELECTION OF Ex EQUIPMENTS: 

Based on proposed Zones: