SWA armouring as cpc and Protective Bonding.

[rattlehead85]

Was asked today why we can use the armouring of an SWA as an Earthing conductor but not as a main protective bonding conductor unless we apply the resistivity formula for steel which result is8x that of copper. Can anybody explain this in simple terms because the person i am trying to explain this to cant understand why the same formula isn't used when we employ the armouring as an earthing conductor in its own right. [emoji120]

 

[yellowvanman]

When armour is used as an earthing conductor the adiabatic equation (543.1.3) is used, and this relates to the possible fault current and fault clearing time associated with the protective device of the circuit.

Bonding has no relationship to the protective device of a circuit, when PME is used the size of the bonding conductor is based on the size of the supply neutral conductor (Table 54.8), for all other earthing arrangements the size of the bonding conductor should be not less than the CSA required for the earthing conductor of the installation and not less than 6mm (544.1.1).

eg. For a typical domestic installation with PME, the bonding size is 10mm (for copper), if you supply a garage and need bonding then the CSA of the armour would need to be >90mm (using steel versus copper @ x9)

 

[Richard Burns]

When working out suitability for a circuit protective conductor you are considering how it will perform with a very high current of short duration.
i.e the fault current flowing before the CPD performs ADS. Therefore you consider the thermal properties of the conductor as well as the resistance which gives you the k values for the adiabatic equation.

When designing for bonding you are interested solely in the resistance of that conductor in order to reduce the volt drop between exposed and extraneous conductive parts so as to maintain a low touch voltage (<50V). The current flow in a bonding conductor should be minimal and so there are no effects of heating and EMF forces so no consideration is taken of the thermal properties of the conductor.