Published: 8 October 2014
Category: Q&A
Chris asks this question, which concerns twin & earth cable he’s seen in an electrical installation. It is different from other twin & earth cable he’s worked with and he wonders whether it complies with regulations. It has been answered by BASEC (Theme – Cable, Wiring and Wiring Accessories):
This Q & A is one of thousands posted in our Technical Expertise area, and answered on a daily basis by our Voltimum Experts.
Question: I have recently seen two installations where the twin & earth cable is distinctly ribbed, with the centre rib lower than the two outers - not flat on the sides (as I am familiar with).
It looks much more like figure eight ‘zip wire’, but I have checked in an accessory box and there is a circuit protective conductor (CPC) in it.
I’ve not been able to see any printing on the cable to identify it and I’m not in a position to be able to undertake any dismantling or testing.
In view of some of the substandard cables circulating these days, can you tell me whether this is a standard cable profile?
Or does it warrant further investigation?
Answer: This 'figure of eight' type construction of flat twin & earth cable to BS 6004:2012 (PVC) or BS 7211:2012 (LSHF) is permitted by the standards.
Although not widely seen, it is becoming more common as it slightly lowers the amount of sheathing material used and hence keeps manufacturing costs down while providing the same performance.
It is more commonly seen in low smoke halogen free cable (BS 7211) than in PVC cable.
As long as the minimum thickness of sheath material is present over the insulated cores and CPC, the construction should conform to the standard.
The marking requirements for the cable remain the same as with flat-sided cable, so if the cable is to conform to the above standards they will need to be embossed or indented with the required markings.
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