Using conduit as a CPC

[pc1966]

A recent discussion on conduit choice wandered over to pros and cons of the 'conlok' style and a link was posted to one of the Efixx videos in this post:

Which conduit holders for metal conduit?

An alternative might be to use brass for an improved aesthetic...pricey but nicey! https://theworkshopbelow.com/collections/conduit-antique-brass-conduit

www.electriciansforums.net

The related video showing them putting in the cables has normal G/Y CPC pulled in along with the live conductors and that got me wondering about the use of the actual conduit as the CPC. I had assumed it was normal and an advantage of threaded conduit that you could use it as the CPC and only have to put in jumpers from back-boxes (suitably fitted with star washers for paint penetration) to the accessories to ensure a good enough connection. Some questions for folks to debate are:

• Is using the metal conduit as CPC considered poor practice now?
• Is it just the 'conlok' style that you would not trust to be a reliable CPC system?
• If using the conduit as a CPC would you consider adding, say, a single 4mm CPC as a supplementary bond from source to furthest point to povide a 2nd CPC path in the (hopefully unlikely) case that it developed a poor connection at a single point?

 

[netblindpaul]

I have in the past installed using only the containment (conduit and trunking) as CPC. However, after a long discussion and analysis with my colleagues, based on real-world findings over multiple installations, we agreed that the number of examples of suspected and measured high-resistance connections in the containment was too high to ignore completely, and since then made it a principle to specify a copper conductor throughout. This was not a matter of achieving compliance, we were happy that the containment was sufficiently reliable to comply and to be safe. It was our own specification that we work to, which exceeds minimum requirements in most aspects. We will accept and re-use existing containment CPC where it is sound.

There is a certain amount of common-sense involved. A stiff backbone of 200x50 trunking along the wall with short 25mm galv drops, is not going to put any one point in jeopardy of losing its CPC. A long run of a special-purpose circuit that snakes around the building in a solitary conduit that's often inaccessible, is an order of magnitude more likely to have one or more of its many dozens of joints loose or subject to corrosion. Here it makes statistical sense to run the copper CPC as ductering and tracing one possible marginally high connection is not going to happen in the future. The actual resistance of the conduit when good will probably beat the copper hands down, but the copper continuity is predictable for the life of the installation in a way that 97 screwed joints, one of which is subject to rampant condensation near an unseen gap in the eaves, might not be.

This is why we used to do a doctor test on the containment when using it as a cpc.
Shame that has gone out of fashion!

 

[pc1966]

This is why we used to do a doctor test on the containment when using it as a cpc.
Shame that has gone out of fashion!

Yes, some PAT testers used to do tens of amps for bond testing but now they are (AFIK) much the same as MFT, etc, in using around 200mA. While it might give a good idea of the DC resistance it is not really saying if it is haning on by a single strand of flex, etc, that the old style of test would reveal.

 

[netblindpaul]

Yes, some PAT testers used to do tens of amps for bond testing but now they are (AFIK) much the same as MFT, etc, in using around 200mA. While it might give a good idea of the DC resistance it is not really saying if it is haning on by a single strand of flex, etc, that the old style of test would reveal.

Quite, luckily, even though I am too ill to use them, I have a high current PAT tester and a 10A Ductor, (Megger DLRO10X).

 

[westward10]

I still have a Megger 101 which has a 25A bond test.

 

[netblindpaul]

I still have a Megger 101 which has a 25A bond test.

OOh, Megger 101, what's that?
Not familiar with it. Any links?

 

[westward10]

There is a fair few on EBay.

Telford Electronics

 

[netblindpaul]

There is a fair few on EBay.

Telford Electronics

Right, sorry, the Megger PAT101, I didn't know.
A useful thing to think about, IMHO.
I use my DLRO for much more than containment testing.
Motors, connections, earthing and bonding wiring.
The high current high accuracy, DLRO10HD, will give 0.1 micro ohm resolution at the 25 milli Ohm range.

 

[westward10]

Right, sorry, the Megger PAT101, I didn't know.
A useful thing to think about, IMHO.
I use my DLRO for much more than containment testing.
Motors, connections, earthing and bonding wiring.
The high current high accuracy, DLRO10HD, will give 0.1 micro ohm resolution at the 25 milli Ohm range.

Sorry I should have put PAT101. A while back we had some suspect IEC leads on some IT equipment, the bond was far higher than expected 0.30 and above. We took one outside and bond tested it until the cable went all floppy and took the earth out, there were about three tiny strands of copper on a cable claimed to be 3×1.0.

 

[DPG]

I reckon far too much stuff is getting PAT tested at 200mA rather than 10 or 25 Amps these days.

 

[Lucien Nunes]

Testing continuity at 25A feels much more effective and serious, but how many have actually discovered a problem by testing at this current, that would not have been found at a lower current. E.g. the IEC leads mentioned by @westward10 revealed themselves to a resistance test, before they gave out on a high current test. I am struggling to recall a genuine case of 'termination hanging on by a few strands' located on test and not by visual, which is a situation we often cite to justify the 25A test.

 

[DPG]

Testing continuity at 25A feels much more effective and serious, but how many have actually discovered a problem by testing at this current, that would not have been found at a lower current. E.g. the IEC leads mentioned by @westward10 revealed themselves to a resistance test, before they gave out on a high current test. I am struggling to recall a genuine case of 'termination hanging on by a few strands' located on test and not by visual, which is a situation we often cite to justify the 25A test.

I agree to an extent, but with the amount of cheap tat available these days I think proper bond testing is just as important as it used to be when it was first specified.

 

[netblindpaul]

Testing continuity at 25A feels much more effective and serious, but how many have actually discovered a problem by testing at this current, that would not have been found at a lower current. E.g. the IEC leads mentioned by @westward10 revealed themselves to a resistance test, before they gave out on a high current test. I am struggling to recall a genuine case of 'termination hanging on by a few strands' located on test and not by visual, which is a situation we often cite

I once found an earth fault loop fault with a high current loop impedance tester rather than one of the modulated soft testers we use these days to avoid tripping RCDs.
That put mains voltage / 10 Ohms down the loop test wiring.
So, 23-25A, or there about.

 

[pc1966]

That put mains voltage / 10 Ohms down the loop test wiring.

Crikey!

I always assumed they had some transformer so it would be tens of volts at most and leakage inductance or whatever as current limit, like old fashioned lead-acid chargers.

 

[netblindpaul]

Crikey!

I always assumed they had some transformer so it would be tens of volts at most and leakage inductance or whatever as current limit, like old fashioned lead-acid chargers.

The old Robin separates are the ones that do this.
Not sure what model mine is.

 

[westward10]

The old Robin separates are the ones that do this.
Not sure what model mine is.

Yes I still use a Robin makes the young lads jump when the leads crack on the terminals plus no batteries to go flat.