Handbook of Electrical Installation Practice - BeKnowledge

580 Handbook of Electrical Installation Practice
under the terminal and, where appropriate, suitable tag washers should be used.
Crimped connections should be made only with lugs or connectors of a size intended
for the conductors concerned and with a tool recommended by the manufacturer
of the connector.
Where cables enter into terminating boxes or equipment, the sheath is taken
through into the box and is stripped off inside. If the box or cover is made of metal
the sheath must be protected from sharp edges by the use of a grommet, and for
surface work or where foreign substances might enter the box, the cable should be
mechanically anchored and the entrance hole properly filled by using a suitably
shaped gland which grips the cable.
If the cable is armoured, the gland must be of a type which provides adequate
mechanical anchorage for the wires or strips. As the armour is often used as a protective
conductor the gland must also provide good electrical contact and in turn
be reliably connected to the enclosure or other earthed metal work. On installations
out of doors or in wet environments the whole gland should be protected
against corrosion by a watertight covering which goes well back over the outer
sheath.
The ends of MI cables must be completely sealed against the entry of moisture.
This is effected by screwing a small brass pot on to the end of the sheath and filling
it with a special non-setting water-resistant compound, as shown in Fig. 20.7 or by
using heat-shrink seals (see BS 6207–2). The conductors, where they emerge from
the seal, are covered with insulating sleeving. The materials used for the seal and
the sleeving must be selected to suit the expected working temperature of the cable.
Where MI cable enters terminal boxes and equipment it should be anchored by
the use of a gland, which also provides electrical continuity for the copper sheath
and, if required, watertightness. Here again it is often appropriate to cover the entire
gland with a heat-shrink sleeve which goes well down over the outer PVC covering
of the cable.
Terminating the sheath of MI cables calls for the same care to ensure adequate
electrical continuity as with the armour of other cables, as both are generally used
as protective conductors and must carry the earth-fault current of the circuit. Singlecore
MI cables involve a further consideration in that a small voltage is induced in
them during normal operation which can be much higher during a short-circuit. It
is normal practice to avoid danger from these voltages by bonding all the sheaths
of each circuit together at each end and earthing them. As a result current circulates
continually around the sheath even during normal operation. Providing care
has been taken to ensure that bonding connections are sound and adequately robust
no harm can arise.
Fig. 20.7 MI cable termination (BICC).