Handbook of Electrical Installation Practice - BeKnowledge

292 Handbook of Electrical Installation Practice
actuator. Care is required to ensure there will be no mechanical failure of the
cam plunger mechanism and that failure is limited to a spring breakage or loss
of temper. It must not be possible to defeat the interlock without the use of tools.
Positive make switches are not suitable because the interlock can be defeated by
operating the switch plunger and, moreover, such a switch can fail to danger if the
spring breaks. Two interlock switches, connected in series, are sometimes used as an
additional precaution.
Test pieces, having no exposed conductors, apart from the ends of the leads to be
connected, can be tested on the bench. In this case, the test supply terminals are
enclosed in a box with a hinged lid. When the lid is open, an integral switch opens
the supply circuit, enabling the leads to be connected to the supply terminals which
are usually spring loaded clamps. Closing the lid automatically operates the integral
switch and connects the equipment to the supply. Again, a positive break interlock
switch must be used with a suitable actuating mechanism, to prevent defeat of the
interlock feature and to ensure mechanical reliability.
Interlock switches are often used on doors and the removable panels of control
gear where access is required for maintenance and test purposes by electrically
skilled persons. The necessity for this is dubious as such panels should not be openable
without the use of a key or tools and so access is restricted to authorised and
competent persons who are well aware of the hazards and who will have to defeat
the interlock if they need to carry out ‘live’ testing. However, if such switches are
used, they should preferably be of the ‘fail safe’, positive break type and be provided
with a ‘lock off’ facility which can be used if the switch is in a position where
it could be accidentally operated during maintenance or testing.
Other systems
There are two earth free systems called ‘Protection by non-conducting location’ and
‘Protection by earth free local equipotential bonding’, which depend on their locations
being earth free. As it is virtually impossible to guarantee that an earth free
location is always likely to remain so, there is not much scope for these systems in
the UK except for special applications such as the testing of radio and television
equipment. However, even then additional precautions are still necessary.
For electronic equipment encased in earthed metal, there is a special highresistance
earthing system for testing, designed to eliminate the shock hazard when
a ‘live’ conductor and the earthed metal case are both touched. The system comprises
a safety isolating transformer, feeding a socket-outlet via a circuit-breaker.
The test piece is connected to the socket-outlet. A centre tapped high resistance is
connected across the circuit-breaker output terminals. The centre point is earthed
via a rectifier bridge and relay coil. If either pole of the test circuit is touched when
in contact with the metal case, the fault current is limited by the high resistance to
say 0.50mA which, although insufficient to produce a shock, is enough to operate
the sensitive relay and trip the circuit-breaker (Fig. 11.11).
The principal application of energy limitation is in h.v. circuits used in: (1)
TIG welding to ionise the air gap between the electrode tip and the workpiece,
for arc initiation; (2) static elimination, also to ionise the air gap between the
electrode and workpiece and to provide a conducting path for static dispersal; (3)