Conduit R1+R2

sparky_1989 · Jun 18, 2019

Hi guys looking for some advice I'm working in an old property with conduit fed ring mains and lights. I've got a full street of these to test so looking to see the best way to go about it . There is no seperate cpc ran in to any of the switches or lights. The integrity of the conduit is perfect when I do a wander lead test at all the points (0.40) I've also did a visual in loft space and anywhere I can see it . What's the best way to satisfy the inspection and testing requirements. I was thinking R2 test then earth fault loop impedance test on these circuits . A seperate 16mm earth runs from conduit behind board onto the main Earthing terminal. I've not seen this setup since I was an apprentice so brain is a bit fuzzy regarding the parallel paths effecting readings so don't know if R1 + R2 is what others would do. How would you guys go about testing it.

telectrix · Jun 18, 2019

with conduit used as cpc, i'd record R2 readings. the schedule of tests asks for either R1+R2 or R2. obviously with live tests Zs etc. as well.

ipf · Jun 18, 2019

Agree, record R2.
If, as you say, the conduit integrity is perfect, 0.4 ohms R2 seems a bit high. I would think it would be near enough zero, at points near CU at least. Even through out, maybe.

The Ghost · Jun 18, 2019

Paralell paths are a problem or something to be wary of. i.e. between a spur and boiler/water system to name one. A high current continuity tester (25a) a must. I am sure I have seen a spark tester where quite a high current is put through and you can hear joints arcing where they are not properly connecting. Just can't remember the name, sure it's some girls name. So if it is buried in walls you can't ensure continuity between joints as you can't measure across them with high current tester. So if you are picking up a paralell earth from somewhere it may not pick up a bad joint. Of course I have no idea of the structures you are dealing with and whether steel is a part of the structure or what. Bottom line if it was me I would be thinking is it viable to pull in a cpc as it is cheaper than an inquest. But then that is just me. Sorry to be a bit dramatic. I suppose check all the bushes where it enters boxes would be another check. Regs do require such conduit to be exposed for inspection and tests between joints as I undestand it. It is quite a tricky thing to put your name on a cert with the above consideration but if you have covered it all then great.

Des 56 · Jun 18, 2019

Check that the sockets have a fly lead from the conduit box to the front,the front fixing screws are not adequate for earthing purposes

westward10 · Jun 18, 2019

If the board is surface fixed I suspect that poking a long screwdriver through the back will result in hitting a metal surface, either an adaptable conduit box or an original flush fitted fuse board. If the conduit system is sound at all points of connection, I can't remember steel conduit resistances but even for an entire house you should have difficulty registering any resistance.

The Ghost · Jun 18, 2019

Oh yeh I remember now it is a Clare tester 50v/50A. I do remember a case where in a flour mill, an electrician tested conduit and failed to pick up a bad joint which arced and blew the place up. I know this is not a flour mill we are talking about though.

Lucien Nunes · Jun 18, 2019

Bottom line if it was me I would be thinking is it viable to pull in a cpc as it is cheaper than an inquest. But then that is just me. Sorry to be a bit dramatic.

I do think that's a bit dramatic. The OP is doing EICRs on a 'street-full' of properties, where so far the continuity has tested OK. Telling everyone that every run of conduit in the street needs a CPC pulled in, which would presumably warrant at least a C3 for every circuit if not a C2, is not going to be well received if there isn't actually a problem.

End of the day, I work on a lot of installations with conduit / trunking CPC. Only one have we had a problem with hidden joints being high resistance and unusable. I run a CPC in new work as a matter of course, especially where there is a long single run of conduit with a lot of joints, not because conduit is a bad CPC but because I would rather have a minimum number of joints in any CPC. But if it's existing and sound, carry on.

I am sure I have seen a spark tester where quite a high current is put through and you can hear joints arcing where they are not properly connecting. Just can't remember the name, sure it's some girls name.

You might not want to be pumping out enough test current that you can hear conduit joints arcing when buried in the wall. +1 for finding a slightly naff conduit joint, -10 for melting the cables inside it. I test containment joints with a Ductor. Not sure I've met a girl of that name, doesn't sound very feminine. Ductor for a boy and Megger for a girl maybe?

The Ghost · Jun 18, 2019

As a matter of interest @Lucien Nunes how do you rule out any parallel paths? As for the Clare tester that is for when you have just installed the conduit not the cable I think.

Lucien Nunes · Jun 18, 2019

Sometimes you can't, especially in industrial work where containments are all side by side on the same unistrut and / or the structural steel. You can't test every joint independently but on the flipside you can usually see them. But if you do a simple risk assessment, which is more likely to create a shock hazard due to a broken CPC - a radial wired in T+E reliant on one 1.5mm² CPC looped through a dozen points, or one in a 20mm galv that is paralleled with half a dozen others, a massive steam pipe, half the building steel and a bunch of other conductive things?

In a domestic or brick-built commercial location, where there are fewer points in contact with other services, you can often predict where they will be, what influence they will have on the reading, and choose test positions that will best distinguish between them. A 1.0mm² flex from an FCU to the heating controls that are otherwise earthed via the pipework, even if it's only a couple of metres it's only going to have a minor effect on a resistance measurement of a run of tube either side of the FCU.

ipf · Jun 18, 2019

Des 56 said:
Check that the sockets have a fly lead from the conduit box to the front,the front fixing screws are not adequate for earthing purposes

But they are when earthing a back box via a socket. Take the socket off, the back box isn't earthed.......with conduit, the socket's not.
Another good reason never to remove without isolating...….especially with conduit systems.
I would advise linking the socket and box in all situations.

ipf · Jun 18, 2019

Thinking about conduit in domestic, I found this a while back. It used to be a council flat with concrete floors. It was bought privately and this window/outside shelf arrangement installed. I was asked to sort things out so they could internally open the window properly.....politely declined.

sparky_1989 · Jun 18, 2019

Thanks for some great replies. The first flat is tested so I've got an idea of what to expect for the rest . I was on my break when I posted my earlier message so i discovered a bit more information later when I carried out the rest of the tests and opening up all the points . In the kitchen the original conduit backbox had been tiled over and altered with new T&E cable supplying 4 x newer sockets above the worktop all spurred from 1 socket (tut tut) . I think this could be the reason for the higher R2 readings I was getting, The upstairs and lounge was getting around 0.01 R2 and was on a different circuit.

ipf · Jun 19, 2019

By the way.....in those first two photos you can see some orange pyro. They were new, for security lighting......the reason we were on site in the first place. It was a nice little council job, about 20 floods.

Lucien Nunes · Jun 19, 2019

I have visualised screwed conduit mainly because of the mention of RFCs narrowing down the date range a little and the fact the cable was not in question. But it raises the question about slip conduit work that has been rewired in the past in PVC that is sound but without a copper CPC. What then, if the conduit continuity seems OK on test?

Conduit R1+R2

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telectrix

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The Ghost

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westward10

The Ghost

Lucien Nunes

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The Ghost

Lucien Nunes

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ipf

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