Fire Safety – Public Advice
Petrol Stations - Types of Protection for Electrical Apparatus
Used in Explosive Atmsopheres
Ref
FS- PAN732
Issue/Revision Date
01/07/2011
Review Date
01/08/2014
Version: 1.1
Types Of Protection For Electrical Apparatus For Use In Potentially Explosives Atmospheres.
'Types of protection' is the technique that is applied to a piece of electrical apparatus or part of it, in
order to ensure that it does not provide a source of ignition and that it can be used under specified
conditions, in a potentially explosive atmosphere.
Of the eight 'types of protection' currently covered by national or international standards, it is
flameproof apparatus and intrinsic safety apparatus that are most commonly used in the hazardous
areas associated with petroleum storage.
Flameproof Apparatus (protection symbol 'd')
With this concept, an explosive atmosphere may enter the apparatus but all parts of the apparatus
which might ignite an explosive atmosphere are placed in an enclosure which can not only withstand
the pressure developed during an internal explosion of the explosive mixture but also prevents the
ignition of any flammable atmospheres surrounding the enclosure. Of necessity, the enclosure has to
be very substantial; a typical example is illustrated in Fig 1.
With this protection concept particular attention has to be paid to the means of entry into the
enclosure, which is usually by means of conduit or cable, since this is essential to the integrity of the
protection.
Intrinsic Safety Apparatus (protection symbol 'i')
This protection technique is based upon the restriction of electrical energy within the apparatus and
interconnecting wiring, to a level below that which can cause ignition by either sparking or heating
effects. Because of the method by which intrinsic safety is achieved, it is necessary to ensure that only
the electrical apparatus exposed to the potentially explosive atmosphere, but also any other electrical
apparatus with which it is connected is suitably constructed. It is also necessary even when using
intrinsically safe apparatus to take precautions to prevent a hazard being created by the apparatus
charging a capacitive and inductive circuit to above a safe energy level.
There are two subcategories in this technique: 'i(a)' and l i(b)'. Equipment constructed to the 'i(a)'
standard is less likely under fault conditions to ignite an explosive atmosphere than 'i(b)' apparatus
and thus only 'i(a)' apparatus should be used in the most hazardous areas (for example areas
classified as Zone 0 areas).
Fig 2 shows the 'safety barriers' that are often used with this protection concept. These are
assemblies of fuse and resistor protected shunt diodes which may be used as safety barriers between
intrinsically safe and nontrinsically safe circuits. The purpose of 'safety barriers' is to limit to a safe
level the energy which can flow to the hazardous area wiring.
General
All electrical equipment located in a hazardous area (for example zones 0, 1 or 2) of petrol filling
stations and petrol (can & drum) stores must be examined and tested at periods prescribed by the
Conditions of Licence Part II (24).
NB More detailed information on this subject is to be found in the Health and Safety Executive’s
publication HS(G)22, ‘Electrical Apparatus for use in Potentially Explosive Atmospheres’
(ISBN 0 11 883746 X)
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