148 HANDBOOK OF ELECTRICAL ENGINEERING
the standard is 2 for protection against live parts, and for ingress between adjacent units of an assem-
bly, in particular by the fingers of a person. The standard defines four basic ‘forms’ of separation:-
- Form 1. No separation is provided.
- Form 2. Separation is only provided between the busbars and other functional units.
- Form 3a. Separation of the busbars from the functional units. Separation of one functional
unit from another. However, the terminals for the external cables need not be
separated from the functional units, nor from each other. - Form 3b. As for Form 3a except that the terminals as a group are separated from the
functional units. The terminals need not be separated from each other in the group. - Form 4. As for Form 3a except that the terminals are an integral part of a separated
functional unit. The terminals need to be separated from each other in the group.
Also defined in the standard are many terms and expressions that are used to describe individual
parts and components as well as combinations of them, e.g. assembly, functional unit, barrier. The
standard states what is to be achieved but not how and with what materials. An annex has been issued
in the UK that expands the general principles. In June 1996 The Electrical Installation Equipment
Manufacturer’s Association (EIEMA) published Reference 1. The four ‘forms’ were sub-divided as
follows:-
- Form 1. No sub-divisions.
- Form 2. Sub-divisions as:-
Form 2a
Form 2b, Type 1
Form 2b, Type 2 - Form 3. Sub-divisions as:-
Form 3a
Form 3b, Type 1
Form 3b, Type 2 - Form 4. Sub-divisions as:-
Form 4a, Type 1
Form 4a, Type 2
Form 4a, Type 3
These various ‘forms’ and ‘types’ differ in detail regarding:-
- Separation of busbars and terminations.
- Separation provided between the busbars and cable terminals.
- Use of rigid barriers.
- Location of cable glands.
- Whether each functional unit has its own integral glanding arrangement.
Reference 1 has an excellent diagram in the form of a ‘decision tree’ to fully illustrate the
above details. Another good summary which includes a table showing the relationship between
busbars, functional units and terminations is Reference 2.
Oil industry users tend to prefer the various types of Form 4, with an ingress code of IP31,
32, 41 or 42 for use indoors.