17th ed shallow buried non-RCD circuit in metal conduit OK?

[bonebill]

17th edition question

I was thinking to have a fixed wall-mount 30mA RCD at a location other than the CU (call this the Remote RCD). I am not sure if it is actually worth doing now, but it begs a question:

The wiring to the Remote RCD would ordinarily be buried less than 50mm deep, in safe zones.

AIUI under the 17th ed, shallow buried wiring must be after a suitable RCD.

Of course two daisy-chained RCDs with the same trip characteristics is a no-no because you lose discrimination.

So the feed to the Remote RCD would be looking to be non-RCD, but that would break 17th ed rules.

So I wondered if it would be OK if the wiring to the Remote RCD was inside (earthed) metal conduit.

 

[comms]

So I wondered if it would be OK if the wiring to the Remote RCD was inside (earthed) metal conduit.

You will then be compliant with 17th regs.

 

[securespark]

How do you propose to achieve this?

 

[plugwash]

you might also want to consider MICC or one of the other cable types with a built in earthed metal layer.

 

[jason1958]

You could use SWA or BS8436 cable.
- BS8436 looks a bit like FP200 Gold
- Note the limits on bend radius
- Quite hard to find, no-one doing cut lengths of it either

Basically any of the cables/methods named in 526-06-06.

The problem with steel conduit is a) bending + die/tap hire b) size if not on surface since BR AD "A" limits how far you can channel walls.

If your plaster is thick (not the plasterer, the plaster) then you may be fine. Our plaster is 30mm (25mm mostly-sand-cement render & two skim plaster coats). Oops, someone covering me in plaste...

 

[bonebill]

Flexible steel conduit any good?

(The stuff that looks like shower hose coated in black rubber.)

 

[plugwash]

It would be ok but I would bet that SWA would be easier and cheaper. I don't fancy trying to run cable through a long length of that stuff.

 

[Spark123]

Don't think the flexible conduit is OK for use as a CPC so Ill err on the side of caution and say not.

 

[RF Lighting]

I have to agree there.

Non-rigid steel conduit in not suitable for use as a CPC.

 

[GaryMo]

Don't think the flexible conduit is OK for use as a CPC so Ill err on the side of caution and say not.

Agreed, 543-02-01.

 

[bonebill]

I think I need to understand what Consumer Protective Conductor is meaning here.

When we say that flexible steel conduit would not be suitable as a CPC, what are we saying?

Is it that the conduit would not, on its own, be suitable for making the earth connection between the two endpoints?

Or are we saying something like, having earthed the conduit at one end, we can't rely on it to be earthed for its entire length?

If the former, then would that not be solvable by running a separate earth wire through the conduit with the same cross sectional area as the phase & neutral wires?

I will probably look up that bit of the regs tomorrow when I have access to them.

If it matters, the flexible conduit is helical, and therefore continuous metal (as opposed to a series of interlocking segments with the consequent cumulative contact resistance).

 

[securespark]

Flexi conduit is prohibited by the regs for use as a CPC.

Even with a separate CPC run in the conduit, it will not comply. with the 50mm rule.

 

[bonebill]

So ... flexible steel conduit about as much use as a chocolate teapot then.

Is it the same story for an RCD-protected circuit, buried less than 50mm, outside safe zones? i.e. Flexible steel conduit no good, but can use rigid steel conduit, SWA or (if you can find any) BS8436?

 

[bonebill]

So ... flexible steel conduit about as much use as a chocolate teapot then.

Is it the same story for an RCD-protected circuit, buried less than 50mm, outside safe zones? i.e. Flexible steel conduit no good, but can use rigid steel conduit, SWA or (if you can find any) BS8436?

Looking at the 16th ed, I think I can answer my own question here.

Flexible steel conduit is specifically prohibited for use as a CPC by 543-02-01.

But under 522-06-07, we see that, subject to certain other provisos, the protective thing does not need to be a CPC: "... or by mechanical protection sufficient to prevent penetration of the cable by nails, screws and the like ...".

HOWEVER, I can't imagine any practical flexible steel conduit that you couldn't just nail or screw through in fairly short order.

As such, ordinary flexible steel conduit will not be usable for this purpose.

 

[bonebill]

I think I need to understand what Consumer Protective Conductor is meaning here.

When we say that flexible steel conduit would not be suitable as a CPC, what are we saying?

Is it that the conduit would not, on its own, be suitable for making the earth connection between the two endpoints?

Or are we saying something like, having earthed the conduit at one end, we can't rely on it to be earthed for its entire length?

If the former, then would that not be solvable by running a separate earth wire through the conduit with the same cross sectional area as the phase & neutral wires?

I will probably look up that bit of the regs tomorrow when I have access to them.

If it matters, the flexible conduit is helical, and therefore continuous metal (as opposed to a series of interlocking segments with the consequent cumulative contact resistance).

Answering my own question:

The point is that, in essence, the regs say that your protective thing must either (a) be a CPC in and of itself, or (b) if it does not qualify as a CPC, be mechanically strong enough to stop nails and screws going through.

The regs specifically prohibit the use of flexible or pliable (metal) conduit as a CPC (543-02-01).

So your only hope is the mechanical strength which, in general, will not be good enough.

(If you do happen to use flexible metal conduit, then if you need an earth connection at the far end you will tend to need to feed through a separate suitable CPC wire.)