Handbook of Electrical Installation Practice - BeKnowledge

For all categories of pass the case of the starter must not become live during the
fault, i.e. must not present an electrical shock hazard to an operator, and the enclosure
must be intact, i.e. displacement or opening of doors or covers is not permitted.
For type 2 the contacts of the contactor(s) can be lightly welded, provided
the welds can be broken.
It is therefore essential that after a short-circuit the installation is inspected and
the mechanical movement of all switching devices is checked with the supply isolated
prior to replacing fuses or reclosing the mccb feeding the starter.
Most manufacturers have carried out tests to demonstrate type 2 co-ordination
of starters with their recommended fuses or circuit breakers; in order to maintain
the level of protection the recommended fuse ratings or mccb setting should not be
exceeded without the manufacturer’s approval.
These standards define the test method to a two part short-circuit test sequence
where the performance of, typically, the starter and fuse combination is checked
at two fault levels referred to test currents r and q. In common with the other
tests specified in these standards the short-circuit tests must be performed as a
sequence; all parts of the test sequence must be ‘passed’ for certification to be
possible.
The values of test current are derived from a table; in the case of test current r,
the selection is based on the rated current of the starter being tested.As an example,
devices of rated currents up to and including 16A are tested at a prospective fault
current of 1kA and then the test repeated at test current q which is the full fault
level; the value is typically 50kA, but actually subject to agreement between the
manufacturer and user.
The starter must still satisfy the same operator safety criteria as above. The
standards give detailed guidance on the evaluation of the test samples after the tests,
and the post-test checks to demonstrate the suitability of the devices for further
service also apply.
The pass or fail criteria are generally as in the old BS 4941 and IEC 292 types ‘a’
and ‘c’ co-ordination; as an approximation type 1 is equivalent to type ‘a’ and type
2 is equivalent to type ‘c’. It should be noted that a type ‘b’ pass is no longer recognised;
this is now a form of type 1 pass. The possibility of contacts tack welding
during these tests is still recognised, so the recommendation to examine contactors
after any operation of main fuses or tripping of main circuit breakers must be
observed.
ENCLOSURES
Motor Control Gear 473
Electrical equipment must be constructed to prevent electric shocks to users caused
by accidental contact with live parts; it must also be protected against environmental
influences such as ingress of foreign bodies, dust and moisture. This is normally
achieved by some form of enclosure which may be metallic or non-metallic.
The degree of protection afforded by an enclosure is classified according to the
IP coding defined in BS EN 60529 (which also appears as an appendix to BS EN
60947-1). Extracts from the appendix of BS EN 60947-1 are given in Appendix II
of this book.