5.4.4 - Protective conductor cross-section assessment

A fault current will flow when an earth fault occurs. If this current is large enough to operate the protective device quickly, there is little danger of the protective conductor and the exposed conductive parts it connects to earth being at a high potential to earth for long enough for a dangerous shock to occur. The factors determining the fault current are the supply voltage and the earth-fault loop impedance (see{5.3}).

The earth fault results in the protective conductors being connected in series across the supply voltage {Fig 5.16}. The voltage above earth of the earthed metalwork (the voltage of the junction between the protective and phase conductors) at this time may become dangerously high, even in an installation complying with the Regulations. The people using the installation will be protected by the ability of the fuse or circuit breaker in a properly designed installation to cut off the supply before dangerous shock damage results.

Fig 5.16  - The effect of protective conductor resistance on shock voltage

a) effective resistance of a ring circuit protective conductor
b) potential differences across healthy protective conductor in the event of an earth fault

Table 5.7 - Main earthing and main equipotential bonding conductor ----------------- sizes for TN-S and TN-C-S supplies
Phase conductor (or neutral for PME supplies )	Earthing conductor (not buried or protected against mechanical damage)	Main equipotential bonding conductor for PME supplies	Main equipotential bonding conductor
csa mm²	csa mm²	csa mm²	csa mm²
4	4	6	10
6	6	6	10
10	10	6	10
16	16	10	10
25	16	10	10
35	16	10	10
50	25	16	16
70	35	16	25

Remember that lower fault levels result in a longer time before operation of the protective device. Since the cross-sectional area of the protective conductor will usually be less than that of live conductors, its temperature, and hence its resistance, will become higher during the fault, so that the shock voltage will be a higher proportion of the supply potential (see {Fig 5.16}).

{Fig 5.16} shows the circuit arrangements, with some typical phase- and protective-conductor resistances. In this case, a shock voltage of 140 V will be applied to a person in contact with earthed metal and with the general mass of earth. Thus, the supply must he removed very quickly. The actual voltage of the shock depends directly on the relationship between the phase conductor resistance and the protective conductor resistance. If the two are equal, exactly half the supply voltage will appear as the shock voltage.

For socket outlet circuits, where the shock danger is highest, the maximum protective conductor resistance values of {Table 5.3} will ensure that the shock voltage never exceeds the safe value of 50 V. If the circuit concerned is in the form of a ring, one quarter of the resistance of the complete protective conductor round the ring must not be greater than the {Table 5.3} figure. The reason for this is shown in {Fig 5.16(a)}. This assumes that the fault will occur exactly at the mid point of the ring. If it happens at any other point, effective protective conductor resistance is lower, and safer, than one quarter of the total ring resistance.

{Table 5.7} allows selection (rather than calculation) of sizes for earthing and bonding conductors. The rules applying to selection are:

For phase conductors up to 16 mm², the protective conductor has the same size as the phase conductor.

For phase conductors from 16 mm² to 35 mm², the protective conductor must be 16 mm²

For phase conductors over 35 mm², the protective conductor must have at least half the c.s.a. of the phase conductor. The minimum cross-sectional area of a separate CPC is 2.5 mm² if mechanically protected and 4mm² if not.

Note that Regional Electricity Companies may require a minimum size of earthing conductor of 16 mm² at the origin of the installation. Always consult them before designing an installation.