Page 47 - Electrical Installation in Hazardous Area

26 Electrical installations in hazardous areas

2.4 Classification of sources of release

Sources of release are now classified using the following basis:

Continuous grade of release

This a point or location from which a flammable gas, flammable vapour,
flammable mist or combustible dust may be released continuously or for
long periods into the atmosphere, so that an explosive atmosphere could
be formed during the period of release;

Primary grade of release

This is a point or location from which a flammable gas, flammable vapour,
flammable mist or combustible dust may be released periodically or occa-
sionally in normal operation into the atmosphere, so that an explosive
atmosphere could be formed during the period of release;

Secondary grade of release

This is a point or location from which a flammable gas, flammable vapour,
flammable mist or combustible dust is not expected to be released in normal
operation, but at which release may be expected infrequently and for short
periods or where dust layers form to an extent such that if disturbed an
explosive atmosphere would be formed.
The three release levels listed above are the basic elements contributing
to the identification of the grade of any particular source of release. One
company code5 did introduce numerical guidance in 1973. This guidance
never achieved sufficient support to be included in any national or inter-
national Standards or Codes. It has nonetheless been widely used since its
inception and, given its intent to identify a division between types of release
to assist in their identification rather than be a strict numerical division, has
become generally accepted if used with care.
What is identified is the following:

Continuous grade release is a release which is present for more than
1000 hours per year (more than 10 per cent of the year approx);
Primary grade release is a release which is present for between 10 and 1000
hours per year (between 0.1 per cent and 10 per cent of the year approx);
Secondary grade release is a release which is present for less than 10 hours
per year (less than 0.1 per cent of the year).
The reason why these figures need to be treated with care is that they are
very general. In a given circumstance, for example, a rotating seal may only