Handbook of Electrical Installation Practice - BeKnowledge

388 Handbook of Electrical Installation Practice
Busbar and circuit chamber assembly
The actual section of the busbars to be used should be quoted on the manufacturer’s
drawings and care must be taken to ensure that the appropriate section is used for
a particular unit.
Some general guidelines on preparation of contact surfaces are given below, but
it cannot be overstressed that at all times the manufacturer’s instructions must be
followed.
Contact surfaces must be cleaned carefully before assembly, using fine emery
cloth where the surface of the contact is of natural finish, and using a proprietary
silver polish if the surfaces are plated. Where the connections have threads these
should be cleaned with a fine scratch brush. After cleaning, the joint faces must be
wiped with a clean lint-free cloth to remove any dust, and the joints assembled as
soon as possible.
Normally, the lower busbar is connected up first, with the joints insulated as
required, ensuring that a good electrical connection is made between all copper
faces and that all securing nuts are tight. The next bar to be assembled will be the
centre one and, finally, the top one.
The same treatment of joint faces must be carried out on all other connection
joints within the circuit chamber, such as connections for VTs.
The manufacturer’s instructions should be checked carefully to see whether any
or all of the joints that have been made on site are required to be insulated. If this
is the case then the components needed should have been supplied with the equipment
and the instructions given by the manufacturer should be followed carefully,
particularly in respect of any recommended safety precautions, with special attention
to the chemical reagents that may be used.
Cabling
The preparation of the cables for connection to the switchgear is a specialist occupation
and an experienced jointer should be employed for this purpose. The cable
has to be laid carefully to avoid tight bends and the length to the cable lug
measured very carefully to avoid any stress on the cable once the joint has finally
been made. Minimum bending radii for cables are specified in BS 6480.
Insulation of the exposed cable and of the joint is frequently provided by means
of heat shrinkable plastics sleeves. These should be applied and terminated in
accordance with the manufacturer’s specific instructions. Where proprietary cable
connectors are supplied for the termination of the cables, apply these with careful
reference to the termination maker’s instructions, or employ a specialist contractor.
Where crimping is employed for the attachment of lugs or thimbles to the cable
core, it is vitally important only to use the type and size specified by the maker for
the core size in use.
Occasionally, paper cables are still terminated in compound-filled boxes and
where this is the case care should be taken to ensure that the compound used is
continually stirred during the melting period, to avoid excessive heating and possible
burning of the compound at the bottom of the boiler. Care must be taken to
ensure cleanliness of the cable box and all utensils associated with this operation.
The cable box itself, and any buckets or ladles which are used in the operation, must
be pre-heated before use and the compound at the appropriate temperature should