BS 7671: 2001 (incorporating Amendments No 1: 2002 and No 2: 2004) has requirements for protection against electric shock, and lists a choice of five basic measures which shall be used to protect against indirect contact.
Introduction
Protection by earthed equipotential bonding and automatic disconnection of supply is the most common measure. Its purpose is that under earth fault conditions, voltages between simultaneously accessible parts are not of such magnitude and duration as to be dangerous.
Main equipotential bonding
Regulation 413-02-02 requires main equipotential bonding to be carried out. Its importance is often underestimated (see Figure 1). An earth fault in the current-using equipment produces a fault current (If) which flows along the circuit protective conductor and back to the source. A small proportion of the current may flow through the main equipotential bonding conductor directly to earth, and then back to the source.
The potential difference between the equipment exposed-conductive-part and the simultaneously accessible extraneous-conductive-part is:
Uf = If R2
Where:
If is the fault current R2 is the resistance of the circuit
protective conductor.
(Ignoring any reactance of the circuit protective conductor, and any small effect of current flowing in the main equipotential bonding conductor) The effect of connecting the main equipotential bonding conductor to the extraneous-conductive-part is to minimise Uf. Without this conductor, the potential difference would approximate to the voltage drop produced by If along the full length of the earth return path, and this could be significantly greater than (If R2). Therefore, failure to install all necessary main equipotential bonding conductors within an installation will certainly increase the shock risk associated with indirect contact.
Installation of main equipotential bonding conductors
IEE Guidance Note 5 recommends that main equipotential bonding conductors should be kept as short as practicable and be routed to minimise the likelihood of damage or disturbance to them. The connections to gas, water and other services entering the premises must be made as near as practicable to the point of entry of each service, on the consumer’s side of any insulating section or insert at that point or any meter. Any substantial extraneousconductive-part which enters the premises at a point remote from the main earthing terminal or bar must also be bonded to this terminal or bar.
First published in Wiring Matters issue 22.
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