to the 16th Edition IEE Regulations

chapter 5

chapter 6

Installation control and protection
  3.1 - Introduction 3.5 - High temperature protection
  3.2 - Switching 3.6 - Overload currents
  3.3 - Isolation 3.7 - Protection from faults
3.4 - Electric shock protection

3.8 - Short circuit and overload
------- protection

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



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Extracted from The Electricians Guide Fifth Edition
by John Whitfield
Published by EPA Press Click Here to order your Copy

Click here for list of abbreviations