5.3.5 - Protective conductor impedance

It has been shown in the previous sub-section how a low-impedance protective conductor will provide safety from shock in the event of a fault to earth. This method can only be used where it is certain that the shock victim can never be in contact with conducting material at a different potential from that of the earthed system in the zone he occupies. Thus, all associated exposed or extraneous parts must be within the equipotential zone (see {5.4}). When overcurrent protective devices are used as protection from electric shock, the protective conductor must be in the same wiring system as, or in close proximity to, the live conductors. This is intended to ensure that the protective conductor is unlikely to he damaged in an accident without the live conductors also being cut.

{Figure 5.9} shows a method of measuring the resistance of the protective conductor, using a line conductor as a return and taking into account the different cross-sectional areas of the phase and the protective conductors.

Fig 5.9 - Measurement of protective conductor resistance

Taking the cross-sectional area of the protective conductor as Ap and that of the line (phase or neutral) conductor as Al , then

Rp = resistance reading x	Al
	Al + Ap

        

For example, consider a reading of 0.72 Ohms obtained when measuring a circuit in the way described and having 2.5 mm² line conductors and a 1.5 mm² protective conductor. The resistance of the protective conductor is calculated from:

Rp =  R x	Al	= 0.72 x 2.5	Ohms
	Al + Ap	2.5 + 1.5
=	0.72 x 2.5	Ohms =	0.45 Ohms
	4.0