Meaning of "mechanical protection" for 17th Ed and RCD requirement

[Andy Dingley]

As is widely known hereabouts, 17th Ed introduced a requirement that cables shallow-buried in walls needed to be protected by either earthed metal conduit / sheath, or RCD.

How does this apply to cables surface-mounted on walls and _not_ buried? I don't have a full 17th ed (=A7522.6.*) and my various on- sites are unclear on this.

I'm assuming that plastic conduit on the surface is acceptable and doesn't require an RCD (for that reason anyway, there could of course be other reasons). Is that correct?

What about surface-clipped T&E? Although I'd generally avoid doing this on principle, this house has a great deal of it as past-owner legacy and it's the only cabling that's still usable (i.e. relatively recent, inspectable and passes insulation tests).

Thanks

[Jim]

No, it would require an RCD.

No, it would require an RCD.

AIUI MICC would be acceptable without an RCD.

We wanted to have a RCD-less spur in our recent installation, we used

1.5mm^2 armoured, it was buried in the wall.

[ARWadsworth]

You are correct.

No RCD is needed for cable protection, you just need to use your common sense with regards to possible impact damage and trunk/conduit the cables if needed.

Cheers

[BruceB]

Wrong answers above.

Cables in plastic conduit or surface mounted T&E are not shallow buried, therefore do NOT require a RCD for that reason.

They may require a rcd for another reason, for example they power sockets for general use.

Regards Bruce

[Jim]

I'd mis-remembered, my apologies.

In practice it's better just to RCD at the CU I should think.

[js.b1]

To add, if you do want it buried without an RCD you need to use a cable such as SWA or BS8436 or steel conduit for the entire route (or run on surface otherwise). If you use SWA the glands need to be accessible for inspection and testing, not buried. BS8436 does not require glands, but it has a large bend radius compared to SWA

Steel conduit is a PITA in domestic and reserved for "unusual situations" :-) Pyro is another alternative, but TBH BS8436 is a lot easier (subject to mandatory type-b circuit breaker re let-through current and suitable size, eg, 20A for 2.5mm radial or 32A for 2.5mm ring).

[Andy Dingley]

In general yes (except the trade-off of nuisance trips on lighting circuits leaving you in the dark). However for this circuit, my real question is "Given that I can't RCD it, how much mechanical protection do I need to supply and is my surface-mounted plastic conduit (20mm tube) adequate?".

As it happens, I think I'm by chance running it entirely within the prescribed zones anyway.

[John Rumm]

Surface mount (either clipped or in trunking etc) is fine without RCD protection.

Yup.

also fine since in both cases there is little chance of drilling through it accidentally!