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.

MEDC have spent 25 years designing and manufacturing electrical equipment suitable for use in potentially explosive atmospheres. We deal with all the major testing and certification authorities throughout the world and have a diverse range of internationally approved products.

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.

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.

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

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.

Many MEDC products are certified for use in ambient temperatures up to 55 degrees Centigrade, see individual data sheets for details.

e.g. Butane has an ignition temperature of 365 degrees Centigrade, equipment used in the vicinity of this gas would need a T rating of T2 or higher.

Different techniques are used to prevent electrical equipment from igniting explosive atmospheres. There are restrictions on where these different types of equipment can be used as follows :

MEDC advise that all Explosion-proof electrical equipment is maintained, by suitably trained personnel, in accordance with the Manufacturers' recommendations.

Any spare parts used should be purchased from the original Manufacturer and repairs should be carried out by the Manufacturer or under his supervision, in order that the item remains in conformance with the certification documents.

All Electrical Equipment, intended for use in a Potentially Explosive Atmosphere, should be certified as suitable for such use.

The methods of obtaining certification differ in detail, see below, between each certifying body or group of bodies (e.g. CENELEC). Basically this process consists of supplying a representative sample of the equipment along with a set of drawings to a recognised test/certification body e.g. BASEEFA who in turn test the equipment against a recognised Standard e.g. EN50018 and issue a Certificate. The user of the equipment can then refer to this Certificate to enable him to safely put the item into service in a zone appropriate to the Certification.

European Practice – after 1st July 2003

After the above date the ATEX Directive comes into force throughout the EEC. This becomes a mandatory requirement for all equipment intended for use in a hazardous area. The fundamental difference between current practice and ATEX certification is that ATEX addresses the essential safety requirements for hazardous area equipment and uses Standards as part of the method of conforming to these. Amongst other documentation required by certifying authorities will be Technical Manuals in order that the user is informed of installation methods etc.

ALL EQUIPMENT, BOTH ELECTRICAL AND MECHANICAL, INTENDED TO BE PUT INTO SERVICE WITHIN THE EEC AFTER 1ST July 2003, WILL HAVE TO HAVE BEEN CERTIFIED IN ACCORDANCE WITH THE ATEX DIRECTIVE.

In practice this means re-certification of all currently certified electrical equipment.

MEDC have started this process and all relevant equipment will be covered by the implementation date of 1st July 2003.

It should be noted also that MECHANICAL equipment is covered by the ATEX Directive so for the first time items such as gearboxes will have to carry ATEX certification.

The equipment coding will be as the current practice plus an additional code as follows:

ExII2G i.e.

Ex – Explosion proof in accordance with ATEX.

II – Group II surface industries.

2 – category 2 equipment (suitable .....for use in Zone 1) note:	Category 1 is suitable for Zone 0.
	Category 3 is suitable for Zone 2.

G – suitable for atmospheres containing gas ( D is suitable for atmospheres containing dusts).

Equipment will be CE marked when certified to ATEX.

European Practice – Current – until 30th June 2003

The method is basically as above. In addition all electrical equipment intended for use in the European Economic Community (EEC) must comply with Electromagnetic Compatibility regulations (EMC) and manufacturers must issue, on request, an EC Declaration of Conformity in accordance with the EMC regulations.

When certified, an item of equipment and its' certificate, carry a code e.g. EExdIIBT4. This can be broken down as follows:

E – European certificate in accordance with harmonised standards

Ex – Explosion-proof electrical equipment

d – flameproof enclosure type of protection

II – Group II surface industries

B – gas group B

T4 – temperature class T4 (135 degrees centigrade surface temperature).

North American Practice

Sample equipment and supporting documentation are submitted to the appropriate authority e.g. .U.L., F.M., C.S.A.

The equipment is tested in accordance with relevant standards for explosion protection and also for general electrical requirements e.g. light fittings.

After successful testing a listing is issued allowing the manufacturer to place the product on the market.

The product is marked with the certification details such as the gas groups A,B,C,D the area of use e.g. Class 1 Division 1

Most countries outside Europe or North America use the IEC Standards as a basis for their own national standards.

The Russian Federation certifies equipment to GOST standards, these closely follow CENELEC practice.

There is a scheme in place which will when fully adopted allow for internationally recognised certification to become a reality, this is the IEC EX SCHEME. This uses the IEC standards and IEC recognised test and certification bodies to issue mutually recognised test reports and certificates. The scheme is in its infancy and its level of success cannot yet be measured.

2 digits are used to denote the level of ingress protection that a piece of apparatus enjoys :–
(The first digit denotes the level of protection against solid objects and the second against liquids)

.	Solids	.	Liquids
0	No protection.	0	No protection.
1	Protected against solid objects up to 50mm, e.g. hands.	1	Protected against vertically falling drops of water.
2	Protected against solid objects up to 12mm, e.g. fingers.	2	Protected against water spray up to 15 degrees from vertical.
3	Protected against solid objects up to 2.5mm, e.g. tools.	3	Protected against water spray up to 60 degrees from vertical.
4	Protected against solid objects over 1mm, e.g. wires.	4	Protected against water sprays from all directions.
5	Protected against dusts. (No harmful deposits).	5	Protected against water jets from all directions.
6	Totally protected against dust.	6	Protected against strong water jets from all directions, e.g. Offshore.
.	.	7	Protected against immersion between 15cm and 1m in depth.
		8	Protected against long immersion under pressure.