steel wire armour as the earth conductor

THE USE OF THE ARMOUR OF STEEL WIRE ARMOURED CABLES AS A PROTECTIVE CONDUCTOR

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The Problem Encountered

1. The use of the steel armouring as a CPC with the cable terminated at each end in brass glands.
2. The use of an internal core as a CPC in addition to the SWA with both ends terminated in brass glands.
3. The use of an external green/yellow single core PVC CPC run in parallel with the SWA cable with the cable terminated in brass glands.
4. The use of an internal core of the SWA cable as a CPC with no cable glands and the cable ends inserted in to enclosures.
5. The use of the steel wire armouring as a CPC with no brass glands and “Tenby” type bonding clamps on to the exposed steel armouring with a green/yellow single core PVC CPC on to the clamp.
6. The brass glands terminated with and without the earthing ring ( Banjo).

Information Requested

I have asked experienced electricians in the field and students at the college where I teach why external CPCs are used and why cable cores are used for a CPC. The answers varied wildly both with and without logical reasoning. I have particularly probed the respondents with the question of the use of 3 core cable on single phase circuits. The cable is manufactured in both old and new harmonised colours for 3 phase. The use of this cable requires the over sleeving of cable cores to single phase colours and green/yellow for the CPC. Often this over sleeving is not provided. I have tested the advocates of the use of 3 core cables on single phase as to why they think 2 core cable is correctly coloured for single phase use.

The STUDENTS responses to questions can be reduced to those listed below or a combination of these.

1. The steel wire armouring cannot be relied on as a CPC.
2. The use of the armouring is as a CPC is not allowed in the “regs” (BS7671).
3. An external CPC is needed as the SWA is not big enough.
4. An internal core is needed as a CPC as the SWA is not big enough.
5. The SWA does not need to be earthed as it is not an exposed conductive part.
6. It is not good workmanship to use the SWA as a CPC.
7. I have always done it that way.
8. Banjos are not needed as the brass gland is the same as terminating conduit or MICC cable.
9. You cannot use the SWA as a bonding conductor.
10. It is dangerous to terminate both ends of the SWA

Defining Protective Conductor

Protective Conductor is defined in BS7671 under Part 2 Definitions as:-

A conductor used for some measure of protection against electric shock and intended for connecting together any of the following parts:

1. Exposed conductive parts.
2. Extraneous-conductive parts.
3. The main earthing terminal.
4. Earth electrodes.
5. The earthed point of the source, or an artificial neutral.

Protective Conductors are divided in to 4 main categories in BS7671

1. Earthing conductor.
2. Main equipotential bonding conductor.
3. Supplementary bonding conductor.
4. Circuit Protective Conductor.

Sizing of Protective Conductors BS7671

543 -01-01 states that the cross sectional area of every protective conductor , other than an equipotential conductor, shall be:

1. Calculated in accordance with Regulation 543 – 01 – 03, or
2. Selected in accordance with Regulation 543 – 01 – 04.