5.4.3 - Bonding conductors

The purpose of the protective conductors is to provide a path for earth fault current so that the protective device will operate to remove dangerous potential differences, which are unavoidable under fault conditions, before a dangerous shock can be delivered. Equipotential bonding serves the purpose of ensuring that the earthed metalwork (exposed conductive parts) of the installation is connected to other metalwork (extraneous conductive parts) to ensure that no dangerous potential differences can occur. The resistance of such a bonding conductor must be low enough to ensure that its volt drop when carrying the operating current of the protective device never exceeds 50 V.

Fig 5.13 Main bonding connections

Thus	R <	50
		Ia
where	R is the resistance of the bonding conductor
	Ia is the operating current of the protective device.

Two types of equipotential bonding conductor are specified.

1. - Main equipotentiol bonding conductors
These conductors connect together the installation earthing system and the metalwork of other services such as gas and water. This bonding of service pipes must be effected as close as possible to their point of entry to the building, as shown in {Fig 5.13}. Metallic sheaths of telecommunication cables must be bonded, but the consent of the owner of the cable must he obtained before doing so. The minimum size of bonding conductors is related to the size of the main supply conductors (the tails) and is given in {Table 5.6}.

2. - Supplementary bonding conductors
These conductors connect together extraneous conductive parts - that is, metalwork which is not associated with the electrical installation but which may provide a conducting path giving rise to shock. The object is to ensure that potential differences in excess of 50 V between accessible metalwork cannot occur; this means that the resistance of the bonding conductors must be low (see {Table 5.7}). {Figure 5.14} shows some of the extraneous metalwork in a bathroom which must be bonded.

Table 5.6 - Supplementary bonding conductor sizes
Circuit protective conductor size	Supplementary bonding conductor size
-	Not protected	Mechanically protected
1.0 mm²	4.0 mm²	2.5 mm²
1.5 mm²	4.0 mm²	2.5 mm²
2.5 mm²	4.0 mm²	2.5 mm²
4.0 mm²	4.0 mm²	2.5 mm²
6.0 mm²	4.0 mm²	4.0 mm²
10.0 mm²	6.0 mm²	6.0 mm²

 

Fig 5.14 Supplementary bonding in a bathroom

The cross-sectional areas required for supplementary bonding conductors are shown in {Table 5.6}. Where connections are between extraneous parts only, the conductors may be 2.5 mm² if mechanically protected or 4 mm²if not protected. If the circuit protective conductor is larger than 10 mm², the supplementary bonding conductor must have have at least half this cross-sectional area. Supplementary bonding conductors of less than 16 mm² cross sectional area must not be aluminium. {Fig 5.15} shows the application of a supplementary bonding conductor to prevent the severe shock which could otherwise occur between the live case of a faulty electric kettle and an adjacent water tap.

There will sometimes be doubt if a particular piece of metalwork should be bonded. The answer must always be that bonding will be necessary if there is a danger of severe shock when contact is made between a live system and the metal work in question. Thus if the resistance between the metalwork and the general mass of earth is low enough to permit the passage of a dangerous shock current, then the metalwork must be bonded.

The question can be resolved by measuring the resistance (Rx) from the metalwork concerned to the main earthing terminal. Using this value in the formula:

Ib =	Uo
	Rp + Rx

will allow calculation of the maximum current likely to pass through the human body where :

Ib - is the shock current through the body (A)

Uo - Is the voltage of the supply (V)

RP -is the resistance of the human body (Ohms) and

Rx - is the measured resistance from the metalwork concerned

to the main earthing terminal (Ohms)

The resistance of the human body, RP can in most cases be taken as 1000 Ohms although 200 Ohms would be a safer value if the metalwork in question can be touched by a person in a bath. Although no hard and fast rules are possible for the value of a safe shock current, Ib, it is probable that 10 mA is seldom likely to prove fatal. Using this value with 240 V for the supply voltage, uo, and 1000 Ohms as the human body resistance, RP, the minimum safe value of RP calculates to 23 kOhms. If the safer values of 5 mA for Ib and 200 Ohms for RP are used, the value of Rx would be 47.8 kOhms for a 240 V supply.

Fig 5.15 Supplementary bonding conductor in a kitchen

To sum up when in doubt about the need to bond metalwork, measure its resistance to the main earthing terminal. If this value is 50 kOhms or greater, no bonding is necessary. In a situation where a person is not wet, bonding could be ignored where the resistance to the main earthing terminal is as low as 25 kOhms. To reduce the possibility of bonding conductors being disconnected by those who do not appreciate their importance, every bonding connection should be provided with a label like that shown in Fig. 5.17.