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


A

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 (§522.6.*) 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
 
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J

Jim

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 (§522.6.*) 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?
No, it would require an RCD.
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).
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.
 
A

ARWadsworth

Andy Dingley said:
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 (§522.6.*) 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?
You are 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).
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
 
B

BruceB

Jim said:
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.


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
 
J

Jim

Wrong answers above.
I'd mis-remembered, my apologies.
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.
In practice it's better just to RCD at the CU I should think.
 
J

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).
 
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A

Andy Dingley

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

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