Page 384 - Handbook of Electrical Engineering

372 HANDBOOK OF ELECTRICAL ENGINEERING

Cubicles and light duty terminal or junction boxes often use stainless steel or painted mild
steel gland plates for receiving cable glands, occasionally brass plates are used for single-phase and
DC cables. Stainless steel may also be used for special services. These plates are chosen to be
between about 2 and 5 mm thick, to provide adequate rigidity and resistance to the ingress of dust
and liquids, as defined for example in the international standard IEC60529, see also Chapter 10. The
glands pass through plain or clearance holes and are secured by lock-nuts and spring washers on the
inside surface. Since the entry is a plain hole with a painted surface, it is necessary to use earthing
tabs. Each cable should have a tab and all the tabs should be bonded to a common earthing boss
nearby. The tabs may be on either the outer or the inner surface, depending on the type of equipment
and its environment. Care must be taken to ensure that water and other liquids cannot pass along the
entry hole.
Casings and heavy-duty terminal boxes are often made of cast iron or fabricated from thick
steel plates. Occasionally cast bronze may be used, in services offshore where sea water corrosion
may be a problem. In these cases the entry is usually threaded. The gland is screwed into the threaded
hole. A washer may be required between the outer surface of the box and the gland, to satisfy the
requirements for ingress of liquids and particles, and for the hazardous area. When Ex (d) glands are
used with Ex (d) boxes it is necessary to ensure that the prescribed number of threads on the gland
enter the hole.
Most casings, terminal boxes and gland plates are provided with one or two earthing studs for
bonding them to the earthing system. If a terminal box is cast integral with the frame or casing of a
motor, a generator or other machine, then an earthing stud is not necessary at the box, but the frame
will have one or two studs or bosses for the same purpose.

13.4.3 Earthing Only One End of a Cable

Multi-core cables used for control, instrumentation, computers and telecommunications carry very
small currents in their conductors, when compared with power cables, and these currents feed into
very sensitive electronic circuits. The system design of these electronic circuits must take account of
interference that can be induced or circulated in the cable conductors. Screens are provided around
groups of typically two, three or four conductors, which are mainly intended to discharge static charges
that can otherwise accumulate and create noise or damage at the terminal equipment. Screens are
also provided around all the conductors in the cable for a similar reason.

If a screen is earthed at both ends of its cable then a ‘stray’ current may be caused to flow in
the screen. This is because the earth potential at each end of the cable may not be exactly the same or
both zero. A few millivolts difference due to random stray currents, or worst still fault currents, in the
local earth or steelwork is enough to cause difficulties with the electronic signals. The stray current
flowing along the screen will magnetically induce currents into the core conductors. It is therefore
common practice to earth the cable screens only at one end of the cable. The bonding of each screen
is made at a specially designed ‘clean earth’ busbar mounted inside, for example, a control panel or
marshalling box.
High voltage power cables that operate at voltages above about 3000 V are provided with
graphite semiconducting screens at the surface of the conductor and on the outside surface of the
insulation. The purpose of the screen around the conductor is to control the potential gradient, or
electric stress, in the insulation that is close to the conductor. The high surface voltage is accompanied