Earth wire needed?

JohnW2 · 31 Oct 2025

Risteard said:
Cables are not "double insulated" though. That is a common misnomer. They are insulated and sheathed. (Obviously precluding non-sheathed cables such as singles.)

I could have added to what I previously wrote ...

... other than in the title of two or three Tables in Appendix 4, BS 7671 does not anywhere mention "insulated and sheathed" cables. It does not permit touchable single-insulated cables ('basic insulation' only) but the only relevant 'solution' to that which it offers is to have "double (or 'reinforced') insulation". That therefore seems to imply that BS7671 regards cables such as T+E (and flex) as being "double insulated".

Risteard · 31 Oct 2025

JohnW2 said:
I could have added to what I previously wrote ...

... other than in the title of two or three Tables in Appendix 4, BS 7671 does not anywhere mention "insulated and sheathed" cables. It does not permit touchable single-insulated cables ('basic insulation' only) but the only relevant 'solution' to that which it offers is to have "double (or 'reinforced') insulation". That therefore seems to imply that BS7671 regards cables such as T+E (and flex) as being "double insulated".

It doesn't imply that it regards them as that at all, but merely that it might afford a like level of protection.

plugwash · 31 Oct 2025

Mythbusters #4 Double insulated cables

Following on from the last Mythbuster article on appliance testing in the workplace in Issue 74 (March 2019), a reader suggested looking at the confusion between protective measures for electric shock and the construction of flexible cables.

electrical.theiet.org

Are meter tails double insulated?

Technically no, because meter tails are not a protective measure in the installation designed to prevent the user from the risk of electric shock, like automatic disconnection, or electrical separation. More correctly such tails should be referred to as ‘insulated and sheathed’ cables, just as flat twin-and-earth or H07-RN-F flexible cables are.

However, it is evident that having two layers of insulation would appear to fulfil the requirements for both basic and fault protection, and this is noted in Regulation 412.2.4:
412.2.4 Wiring systems

412.2.4.1 Wiring systems installed in accordance with Chapter 52 are considered to meet the requirements of Regulation 412.2 if:

i. the rated voltage of the cable(s) is not less than the nominal voltage of the system and at least 300/500 V, and

ii. adequate mechanical protection of the basic insulation is provided by one or more of the following:

a. The non-metallic sheath of the cable

b. Non-metallic trunking or ducting complying with the BS EN 50085 series of standards, or non-metallic conduit complying with the BS EN 61386 series of standards.

So, for meter tails or other insulated and sheathed cables, the basic insulation over the conductor and the non-metallic sheath are deemed to comply with the requirements for both basic and fault protection in Regulation 412.2

Murdochcat · 31 Oct 2025

For the sake of simplicity, any 230v cable carrying a L, N, SL and CPC should have the CPC continuous from the CU

JohnW2 · 31 Oct 2025

Risteard said:
It doesn't imply that it regards them as that at all, but merely that it might afford a like level of protection.

If it doesn't regard 'basic insulation' ('single-insulated') as acceptable, what are you suggesting it does regard as acceptable for an LV cable (and what does it call it), given that 'double insulation' appears to be the only relevant method it mentions as being an acceptable addition to basic insulation and also the fact that it does not mention "insulated and sheathed" at all?

JohnW2 · 31 Oct 2025

Murdochcat said:
For the sake of simplicity, any 230v cable carrying a L, N, SL and CPC should have the CPC continuous from the CU

That sounds like a very reasonable statement of what appears to be 'obvious' - unless your "continuous" means that no joints are allowed in the route back to CU.

Murdochcat · 31 Oct 2025

JohnW2 said:
That sounds like a very reasonable statement of what appears to be 'obvious' - unless your "continuous" means that no joints are allowed in the route back to CU.

nope - prove to be continuous via proper continuity testing .............

JohnW2 · 31 Oct 2025

Murdochcat said:
nope - prove to be continuous via proper continuity testing .............

Fair enough, so, as I implied, I think your statement was not only correct but essentially self-fulfilling - since if the conductor did not have 'continuity back to the CU' (well, to the MET,really), it wouldn't be a 'CPC', would it?

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Earth wire needed?

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