4.3 Hazardous environments

Question 1

Any electrical equipment that may be exposed to the following should be constructed and/or protected as necessary to prevent any danger arising from such exposure: 4

Answer 1

 Mechanical damage.  The effects of the weather, natural hazards, temperature or pressure.  The effects of wet, dirty, dusty or corrosive conditions.  Any flammable or explosive substance, including dusts, vapours or gases.

Question 2

Ingress protection (IP) ratings are developed by the European Committee for Electro Technical Standardisation (CENELEC), and provide an internationally recognised system of classifying: 2

Answer 2

 the degree of protection provided by enclosures against the ingress of solid objects and moisture  the protection afforded against contact with any live parts within the enclosure for all types of electrical equipment.

Question 3

The IP rating normally has two (or three) numbers:

Answer 3

(a) Protection from solid objects or materials (b) Protection from liquids (water) (c) Protection against mechanical impacts (commonly omitted, the third number is not a part of BS EN 60529:1992 +A2: 2013 Degrees of protection provided by enclosures (IP code)).

Question 4

Electrical and non-electrical equipment and installations in potentially explosive atmospheres must be specially designed and constructed so that the risks of ignition are eliminated or reduced. Techniques to do this include: 3

Answer 4

 Sealing electrical equipment so that the explosive atmosphere cannot come into contact with electrical components.  Reducing the power of electrical equipment.  De-energising electrical equipment where a fault or an explosive atmosphere is detected.

Question 5

Equipment and protective systems intended for use in potentially explosive atmospheres are grouped, based on ignition energy, as:

Answer 5

 Group I – Mining: methane (firedamp), for example, and/or combustible dust. Note: Group I is not on the NEBOSH National Diploma syllabus.  Group II – Surface industry which is sub-categorised into: IIA (propane, for example), IIB (ethylene, for example), and IIC (hydrogen, for example). A IIC gas poses a much higher risk than IIA or IIB and is much easier to ignite. Note: Equipment suitable for use with a IIC gas will also be suitable for use with IIA and IIB.

Question 6

Hazardous areas are defined in DSEAR as:

Answer 6

… any place in which an explosive atmosphere may occur in quantities such as to require special precautions to protect the safety of workers.

Question 7

Hazardous areas are classified into zones based on an assessment of the frequency of the occurrence and duration of an explosive gas atmosphere, as follows:

Answer 7

Zone 0 An area in which an explosive gas atmosphere is present continuously or for long periods. Zone 1 An area in which an explosive gas atmosphere is likely to occur in normal operation. Zone 2 An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it occurs, will only exist for a short time.

Question 8

Dust clouds in the explosive region (above the minimum explosive concentration) are categorised into 3 zones, based upon the grade of release:

Answer 8

Zone 20 An area in which combustible dust, as a cloud, is present continuously or frequently, during normal operation, in sufficient quantity to be capable of producing an explosive concentration of combustible dust in a mixture with air. Zone 21 An area, in which combustible dust, as a cloud, is occasionally present during normal operation, in a sufficient quantity to be capable of producing an explosive concentration of combustible dust in a mixture with air. Zone 22 An area, in which combustible dust, as a cloud, may occur infrequently and persist for only a short period, or in which accumulations of layers of combustible dust may give rise to an explosive concentration of combustible dust in a mixture with air.

Question 9

Ex p (purge/ pressurised protection) is intended to

Answer 9

prevent the explosive atmosphere from contacting the ignition source.

Question 10

Purge protection involves

Answer 10

maintenance of a constant flow of air (or an inert gas) to dilute and take away any potentially explosive atmosphere.

Question 11

Pressure protection ensures that

Answer 11

the pressure inside an enclosure is sufficient to prevent the entrance of a flammable gas, vapour, dust, or fibre and prevent a possible ignition.