The technical team at NAPIT answer a common question that contractors often raise.
This column will be based around a query we recent received. During an EICR, the individual in question came across two instances where bare or green and yellow circuit protective conductors had been used as live conductors:
1. The bare cpc in the PVC/PVC flat twin and earth cable had been used as a two way circuit.
2. The green and yellow conductor used in a multicore boiler control circuit had been used as a control circuit live, and oversleeved.
So, the million dollar question is: can these conductors be used as live conductors?
There are two aspects to this question, so we’ll split them as follows:
PVC/PVC bare circuit protective conductors
Such cpcs in thermoplastic insulated and sheathed flat cable have a reduced cross-sectional area compared to all sizes above 1 mm2. This is permitted under Regulation 543.1.1(i), that a cpc can be of reduced size if calculated under the adiabatic equation.
This would generally prevent the use of a reduced conductor csa for use as a live conductor such as switch-lines, strappers, neutrals and similar conductor applications (see Fig 1).
With regards to a neutral conductor, the requirements of Regulation 524.2.1 do not permit reduced csa and states that it shall not be less than the csa, of the line conductor for single-phase, and in three-phase circuits up to 16 mm2.
The other consideration of a bare cpc is that it is a non-insulated conductor, so its use as a live conductor would result in a single insulated conductor being exposed. As such, single insulated conductors are required to be enclosed in a containment system like conduit and trunking.
Where a flat twin and earth cable are clipped direct/surface mounted, this would leave the cable having only basic protection for a live conductor. The potential for danger is compounded further if the cable is worked on, thinking the bare conductor is a cpc, when in fact it is live conductor.
Where future works are carried out, this non-compliant use of bare cpcs as live conductors can introduce unexpected hazards, where anyone coming into contact with a live protective conductor would receive a potentially lethal electric shock.
Multicore cables
Where multicore cables, including flexible cables, have designated circuit protective conductors meeting the requirements of Regulation 411.3.1.1 and 514.4.2 having a combination of green and yellow bi-colour combination, they shall only be used as a protective conductor.
It is not permissible to over-sleeve a cpc for use as a live conductor to address the omission of an additional live conductor such as a boiler control cable, or to include a permanent feed for an emergency lighting circuit.
Obviously multicore cables without designated cpc can have one of the cores marked with green and yellow sleeving, as normally carried out in three core steel wire armour cables. The correct method for this is the selection of the black conductor as the cpc with green and yellow sleeving.
Mechanical protection
Single insulated cables are considered to have basic or mechanical protection. Where a bare conductor, such as those in flat twin and earth, is over-sleeved, it is unlikely that the sleeving has an adequate mechanical protection rating, similar to that of a manufactured single insulated conductor.
Conclusion
While BS 7671:2018 provides certain requirements for the identification of conductors, it doesn’t permit the use of bare or green and yellow circuit protective conductors as live conductors.
One further consideration is BS 7629-1 cables which also have bare circuit protective conductors; the added danger should these be used as live conductors is the metal screen of the sheath would also become live as the screen would be in contact with the incorrectly used bare cpc.
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