Whilst the primary function of the metallic armouring of an armoured cable is to provide overall strength, it may be used as a circuit protective conductor (cpc), main protective bonding conductor or, where practicable, an earthing conductor.
Whichever of these three functions is to be performed by the armouring of a cable, all the relevant requirements of BS 7671 need to be satisfied. The armouring of a cable may be used for more than one of the functions if all the appropriate requirements for each function concerned are met.
Use as a circuit protective conductor (cpc)
As with any conductor used as a cpc, the armouring will experience some thermal stress (heating effects), if it is subjected to earth fault current. To withstand this stress safely, the cross-sectional area (csa) of any armouring b used as a cpc must be not less than that determined by one of the two methods referred to in Regulation 543.1.1 of BS 7671, namely calculation or selection.
- Calculation is the more precise method of determining the minimum csa required for a cpc, and uses the adiabatic equation, given below: (Regulation 543.1.3 refers)
- Selection is the simpler, but less precise method, which almost invariably produces a larger minimum csa requirement than by calculation. Selection uses the csa of the line conductor, and the ‘k’ values of both the line conductor and the cpc, to give the minimum csa required for the cpc. Selection must not be used where the line conductor(s) are sized only by considerations of short-circuit current or where the earth fault current is expected to be less than the short-circuit current.
Where the armouring of a cable is to be used as a cpc, consideration also needs to be given to its end-to-end impedance as this will contribute to the earth fault loop impedance (Zs) at the exposed-conductive-parts of all Class I electrical equipment supplied by the circuit. The value of (Zs) at all such parts (including the cable armouring, its glands and terminations) needs to be sufficiently low to satisfy the requirements of BS 7671 for fault protection.
Use as a main protective bonding conductor
The armouring of a cable may be used as a main protective bonding conductor. An example of such use is given in Fig 2, which shows extraneous-conductive-parts bonded to the main earthing terminal (MET) of a distribution board. The distribution board is supplied from the main switchgear at the origin of the installation by an armoured cable.
The armouring of which, as well as being used as the circuit protective conductor of the distribution circuit, is also used as the main protective bonding conductor between the distribution board and the main switchgear.
In an installation where Protective Multiple Earthing (PME) conditions apply (that is, where the means of earthing for the installation is a PME earthing facility), main protective bonding conductors have to carry sustained currents under certain conditions, which may result in heating of the bonding conductors.
The armouring (or a core) of a cable should not be used as a main protective bonding conductor where PME conditions apply, therefore, unless it has been determined by the electrical installation designer that heat produced in the armouring or core due to such use will not cause overheating of the live conductors of the cable.
Where armouring is to be used as a main protective bonding conductor, it is required to have a copper equivalent csa of not less than the minimum required by Table 54.8 of Regulation 544.1.1 of BS 7671.
Use as an earthing conductor
An earthing conductor is defined in Part 2 of BS 7671 as:
A protective conductor connecting the main earthing terminal of an installation to an earth electrode or to other means of earthing.
For practical reasons, it is unusual for armouring to be used as an earthing conductor; nevertheless, BS 7671 does not preclude such use.
Since an earthing conductor performs the function of the protective conductor for the main circuit supplying the installation concerned, armouring used as an earthing conductor is required to have a csa not less than that given by one of the two methods referred to in Regulation 543.1.1 of BS 7671and where relevant the particular requirements of the network operator.
Furthermore, where PME conditions apply, it is necessary for the csa of the armouring which is used as an earthing conductor to meet the requirements for a main protective bonding conductor (Regulation542.3.1 refers).
Preservation of continuity
BS 7671 requires that all protective conductors are suitably protected against:
- mechanical damage and vibration,
- chemical deterioration and corrosion,
- heating effects, and
- electrodynamics effects (mechanical forces generally associated with fault currents)
Armouring is therefore required to be protected against the above detrimental influences and any other stresses which could impair its continuity. Where used as a protective conductor, cable armouring must always be made off into a suitable gland. In addition, electrical joints such as those between armouring, cable glands and earthing terminals, must be soundly made (mechanically and electrically), and, where necessary, suitably protected.
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