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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.

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)"G" An area in which an explosive mixture is continuously present or present for long periods Class I Division 1 (gases)
Zone 20 (dusts)"D" Class II Division 1 (dusts)
Zone 1 (gases)"G" An area in which an explosive mixture is likely to occur in normal operation Class I Division 1 (gases)
Zone 21 (dusts)"D" Class II Division 1 (dusts)
Zone 2 (gases)"G" 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)"D" 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.


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.

Temperature Classification Maximum Surface Temperature
European/I.E.C. North American
T1 T1 450° C
T2 T2 300° C
T2A 280° C
. T2B 260° C
T2C 230° C
. T2D 215° C
T3 T3 200° C
. T3A 180° C
T3B 165° C
. T3C 160° C
T4 T4 135° C
. T4A 120° C
T5 T5 100° C
T6 T6 85° C

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.

Types of Electrical Equipment Suitable for
use in Potentially Explosive Atmospheres

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 :

. European - Area of use Designation Standard IEC - Area of use Designation Standard USA - Area of use Designation Standard
Flameproof Enclosure – An enclosure used to house electrical equipment, which when subjected to an internal explosion will not ignite a surrounding explosive atmosphere. Zones 1 & 2
Zones 1 & 2
Class 1 Divisions 1 & 2
Intrinsic Safety – A technique whereby electrical energy is limited such that any sparks or heat generated by electrical equipment is sufficiently low as to not ignite an explosive atmosphere. Zones 0, 1 & 2
Zones 1 & 2
Class 1
Divisions 1 & 2
Increased Safety – This equipment is so designed as to eliminate sparks and hot surfaces capable of igniting an explosive atmosphere. Zones 1 & 2
Zones 1 & 2
Purged and Pressurised – Electrical equipment is housed in an enclosure which is initially purged to remove any explosive mixture, then pressurised to prevent ingress of the surrounding atmosphere prior to energisation. Zones 1 & 2
Zones 1 & 2
Class 1
Divisions 1 & 2
Encapsulation – A method of exclusion of the explosive atmosphere by fully encapsulating the electrical components in an approved material. Zones 1 & 2
Zones 1 & 2
Oil Immersion – The electrical components are immersed in oil, thus excluding the explosive atmosphere from any sparks or hot surfaces. Zones 1 & 2
Zones 1 & 2
Class 1
Division 2
Powder Filling – Equipment is surrounded with a fine powder, such as quartz, which does not allow the surrounding atmosphere to come into contact with any sparks or hot surfaces. Zones 1 & 2
Zones 1 & 2
Non-sparking – Sparking contacts are sealed against ingress of the surrounding atmosphere, hot surfaces are eliminated. Zone 2
Zone 2
Special Protection – Equipment is certified for use in a Potentially Explosive Atmosphere but does not conform to a type of protection listed above. Zones 0, 1 & 2
Zones 0, 1 & 2

* This type of protection is only recognised by National Authorities, not as a
European-wide type of protection.

Selection, Installation and Maintenance of Electrical Equipment Intended for use in
Potentially Explosive Atmospheres

International and national standards are published giving details of requirements for the safe use of Electrical Equipment in Potentially Explosive Atmospheres as follows :

General Recommendations

BS5345:Part 1
N.E.C. Chapter 5
Classification of Hazardous Areas

IEC60079-10 . N.E.C. Chapter 5
Inspection and Maintenance of Electrical Equipment IEC60079-1
Requirements for Flameproof Enclosures IEC60079-14 BS5345:Part 3 N.E.C. Chapter 5
Requirements for Intrinsically Safe Equipment IEC60079-14 BS5345:Part 4 N.E.C. Chapter 5
Requirements for Increased Safety Equipment IEC60079-14 BS5345:Part 6 N.E.C. Chapter 5
Requirements for Purged and Pressurised Equipment IEC60079-14 BS5345:Part 5 N.E.C. Chapter 5
Requirements for Non-Sparking Equipment IEC60079-14 BS5345:Part 7 .
Requirements for Equipment with Special Protection IEC60079-14 BS5345:Part 8

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.

The Certification Process

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.


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

World-wide Certification

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.

Ingress Protection

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.

North American practice is to use NEMA standards to describe ingress protection, i.e.:

NEMA 3 is similar to IP 54
NEMA 4 is similar to IP 55
NEMA 4x is similar to IP 56
NEMA 6 is similar to IP 67
© MEDC 2003

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94/9/CE 1° edizione mag 2000
94/9/CE 2°edizione july 2005