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 {Figure 5.14} shows some of the extraneous metalwork in a bathroom which must be bonded.
Table 5.6 - Supplementary bonding conductor sizes
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 concernedto 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.
Found this ditty about supp bonding. As WP said the cpc of a circuit or section of pipework can be used as a supp bonding conductor as long as the joints are permanent. If they are compression fittings you will have to loop a supp bonding conductor across the joint.
