3.4.3 - Contact with
live conductors
In order for someone to get an electric
shock he or she must come into contact with a live conductor.
Two types of contact are classified.
1 - Direct contact
An electric shock results from contact
with a conductor which forms part of a circuit and would
be expected to be live. A typical example would be if someone
removed the plate from a switch and touched the live conductors
inside (see {Fig 3.7}). Overcurrent protective systems will
offer no protection in this case, but it is possible that
an RCD with an operating current of 30 mA or less may do
so.
Fig 3.7 Direct contact
2 - Indirect contact.
An electric shock is received from contact
with something connected with the electrical installation
which would not normally be expected to be live, but has
become so as the result of a fault. This would be termed
an exposed conductive part. Alternatively, a shock may be
received from a conducting part which is totally unconnected
with the electrical installation, but which has become live
as the result of a fault. Such a part would be called an
extraneous conductive part.
An example illustrating both types of indirect
contact is shown in (Fig 3.81.) Danger in this situation
results from the presence of a phase to earth fault on the
kettle. This makes the kettle case live, so that contact
with it, and with a good earth (in this case the tap) makes
the human body part of the shock circuit.
The severity of the shock will depend on
the effectiveness of the kettle protective conductor system.
If the protective system had zero resistance, a 'dead short'
would be caused by the fault and the protecting fuse or
circuit breaker would open the circuit. The equivalent circuit
shown in {Fig 3.8(b)} assumes that the protective conductor
has a resistance of twice that of the phase conductor at
0.6 Ohms, and will result in a potential difference of 160
V across the victim. The higher the protective circuit resistance,
the greater will be the shock voltage, until an open circuit
protective system will result in a 240 V shock.
If the protective conductor had no resistance
during the short time it took for the circuit to open, the
victim would be connected across a zero resistance which
would result in no volt drop regardless of the level reached
by the fault current, so there could be no shock.
The shock level thus depends entirely on
the resistance of the protective system. The lower it can
be made, the less severe will be the shocks which may be
received.
To sum up this subsection: Direct contact
is contact with a live system which should he known to he
dangerous and Indirect contact concerns contact with metalwork
which would he expected to he at earth potential, and thus
safe. The presence of socket outlets close to sinks and
taps is not prohibited by the IEE Wiring Regulations, hut
could cause danger in some circumstances. It is suggested
that special care be taken, including consultation with
the Health and Safety Executive in industrial and commercial
situations.
Fig 3.8 In direct contact:
Left-side) fault condition
Right-side) equivalent circuit, assuming
a fault resistance of zero
