Metal conduit as earthing - is that OK

[Noddy67]

Hi,

Just been to a block of flats - 1920's - testing the landlords lighting (that's all that there is communal area run off a timer, no switches for users).

There's no earthing to the lights - although they're all out of reach - but I got excellent Zs reading as I guess all the earthing is provided by the metal containment that goes around the property.

Firstly, if I'm doing a test cert should I say there are 'no earth' despite the fact I've got a reading to suggest otherwise (Zs) and...

Secondly, doing the insulation testing came up well as well, but then it should do i guess. Can I test between the earthed conduit and L & N to report IR or is that giving it more credence than it really has.

Thirdly, I don't think a landlords communal area is a 'special area' so presumably like household lighting earthing isn't essential if the fittings are double insulated (there are no light switches as it's all on a timer).

I've not come across this sort of problem before - is it common in properties of this age?

thanks in advance.

RN

 

[Knobhead]

I've not come across this sort of problem before - is it common in properties of this age?

What problem?

Every thing is fine

 

[Adam W]

It was very common in properties of that age, in fact in properties built up until the 1980s or so.
I prefer to draw in a separate CPC, as is the modern style.

 

[johnboy6083]

it is still permitted in BS7671 to use metal conduit, trunking, ducting, private water pipes, and even exposed conductive parts as CPC. Its not a poplularmethod nowadays, probably because designers feel that people copme along and alter steelwork, trunking conduit ect, with no thought for its use as CPC, and can leave installations unsafe.

 

[Noddy67]

thanks all - very much appreciated.

 

[ExArmy]

i have recently been encountering some conduit situations, might aswell stick them in this thread instead of making another.

a radial in a kitchen serving 2 points has no earth at the board, it is piggy backed from somewhere else (zs is super low) and i can't find where. theres a bit or 6491x poking out at both sockets, but god knows where it goes to. the whole house is wired in steel conduit which is earthed at the board and there are flyleads at every backbox and switch i have removed so far. so, I have encountered this on a board change,what do i record on the cert?

another house, doing minor works on it and i noticed the steel conduit is not all earthed at the board, it's just been cut off , a plastic consumer unit fitted below, and they are all hanging there looking like a mess, but they are not all earthed together, am i right in saying they have to be earthed as they are exposed conductive parts? most of them are earthed as the backboxes are connected the the CPC's of the t&e at each point served, but i don't think it's adequate as if someone removed them, they they could become live.

and i found some really old cable recently enclosed in conduit, a shiny silver colour stranded cable with cloth insulation. looked very dodgy, and the steel conduit(CPC) was almost rusted to peices. wasn't working directly on this so didn't do anything but what sort of cable is this and how old is it?

 

[Knobhead]

http://www.electriciansforums.net/e...ectrical-regulations/23233-history-cable.html

 

[Engineer54]

Hi,

Just been to a block of flats - 1920's - testing the landlords lighting (that's all that there is communal area run off a timer, no switches for users).

There's no earthing to the lights - although they're all out of reach - but I got excellent Zs reading as I guess all the earthing is provided by the metal containment that goes around the property.

Firstly, if I'm doing a test cert should I say there are 'no earth' despite the fact I've got a reading to suggest otherwise (Zs) and...

Secondly, doing the insulation testing came up well as well, but then it should do i guess. Can I test between the earthed conduit and L & N to report IR or is that giving it more credence than it really has.

Thirdly, I don't think a landlords communal area is a 'special area' so presumably like household lighting earthing isn't essential if the fittings are double insulated (there are no light switches as it's all on a timer).

I've not come across this sort of problem before - is it common in properties of this age?

thanks in advance.

RN

Why are you saying that these light fittings have no earth, when you clearly say that the conduit supplying these circuits has an excellent Zs value?? You don't need to run a separate CPC in metal conduit containment systems, never have done and never will.

For the life of me i can't understand why any electrician would even think of running in a separate CPC. All your doing is effectively wasting time and money and increasing your conduit fill factor...

I can understand connecting a CPC flying lead between the conduit box and the fitting or whatever, but that is ALL you need!!!

 

[grantr37]

on Conduit in old buildings this is very common, especially 1920's and 1930's buildings, sometimes you will find an extra earth conductor pulled in there which is a bare wire, I have seen galvanised steel wire in there.....the type of set up you are speaking about is probably all surface mounted with light fittings on conduit through boxes all fitted down the walls of the corridor and stairs? the fittings have most likely been changed a few times over the years? I have seen old conduit and wiring with new Dtype CFL lights fitted, and I have also seen a lot of old glass bulkhead type fittings as well which take bayonet type bulbs/lamps ...

 

[Engineer54]

on Conduit in old buildings this is very common, especially 1920's and 1930's buildings, sometimes you will find an extra earth conductor pulled in there which is a bare wire, I have seen galvanised steel wire in there.....the type of set up you are speaking about is probably all surface mounted with light fittings on conduit through boxes all fitted down the walls of the corridor and stairs? the fittings have most likely been changed a few times over the years? I have seen old conduit and wiring with new Dtype CFL lights fitted, and I have also seen a lot of old glass bulkhead type fittings as well which take bayonet type bulbs/lamps ...

Maybe you are talking about the old slip conduit, the old electricians sometimes used to run a separate bare CPC into them...
If standard black or galvanised screwed conduit, never seen it!! ...Simply isn't any need on a continuous screwed metal conduit system. The conduit is also suitable as the CPC, even for several circuits contained within the conduit run. What some seem to be saying here, is if 3 separate circuits are installed in a single conduit, they would run 3 separate CPC's, ...Absolute nonsense when you think about it!!! lol!!

 

[somersetsparks]

Maybe you are talking about the old slip conduit, the old electricians sometimes used to run a separate bare CPC into them...
If standard black or galvanised screwed conduit, never seen it!! ...Simply isn't any need on a continuous screwed metal conduit system. The conduit is also suitable as the CPC, even for several circuits contained within the conduit run. What some seem to be saying here, is if 3 separate circuits are installed in a single conduit, they would run 3 separate CPC's, ...Absolute nonsense when you think about it!!! lol!!

Have to agree with that, once saw an electrician 'rodding' a perfectly good flush conduit system in a 1980's flat because 'he didn't like it' shame all he actually did was strip a line conductor and cause half a days work in the process.
Another argument I sometimes here about conduit is that you are relying on a mechanical joint, I always say 'what are terminals then?'

 

[johnboy6083]

part of my job at the moment is to install steel conduit inside steel kiosks. The spec is that we run seperate CPC's per circuit, but i agree Eng, there isnt any need. If they did want a seperate CPC running through, theres no reason why one couldnt be used that is sized to the largest circuit

 

[needasparks]

Was it a practice some years back to do an induced current test on conduit for that reason, seem to remember no one would do it for safety reasons and installed a cpc.

 

[Engineer54]

Obviously the guy that wrote that Spec, didn't trust you to install the conduit system very well. ...lol!! That or he's too young to know any better!! The latter is probably nearer the truth!!

I can understand to some degree, if the conduit has been installed in a corrosive environment without protection. I think you work in the water industry don't you, some of the chemicals used in that industry can be highly corrosive to bare base metals.

 

[johnboy6083]

i do work in the water industry, but the kiosks i talk about are covering boreholes, so its raw water that has had no chemicals added at that point.
I agree that if in a chlorine rich environment for example, then i would install PVC conduit/trunking.

 

[Engineer54]

Was it a practice some years back to do an induced current test on conduit for that reason, seem to remember no one would do it for safety reasons and installed a cpc.

High current Ductor testing (25A). It's been the standard conduit ohm resistance test for donkey's years and still used today!! Please explain to me, for what safety reason, no-one would use this test. The only time i wouldn't perform this test is if i were in a working explosive area.... For obvious reasons!! ...lol!!!

 

[Engineer54]

i do work in the water industry, but the kiosks i talk about are covering boreholes, so its raw water that has had no chemicals added at that point.
I agree that if in a chlorine rich environment for example, then i would install PVC conduit/trunking.

Chlorine will eat through standard PVC conduit in quite short periods of time. Seen a PVC compound covered steel conduit used in a chemical plant before now, all joints etc have heat shrink tube seals over them. Looked a pretty decent installation too...

 

[nickblake]

If you think about it , if the coduit is fully intact and sound what would the point of installing a seperate earth as the cross sectional area of the conduit is far geater than a piece of 1.5 or 2.5 singles , even if you have to change a consumer unit use a metal clad ensure you have good connections and as long as the tests are satisfactory then no point sweating putting another earth in

 

[Engineer54]

If you think about it , if the coduit is fully intact and sound what would the point of installing a seperate earth as the cross sectional area of the conduit is far geater than a piece of 1.5 or 2.5 singles , even if you have to change a consumer unit use a metal clad ensure you have good connections and as long as the tests are satisfactory then no point sweating putting another earth in

Exactly!! Jesus, it's been the fundamental means of transporting electrical circuits and providing means of CPC very successfully for well over a 100 years now. What is it with you new electricians, do you really think that New/Modern thinking is always better than a well proven working system??

I have no idea what is being taught in trade collage these day's, but it certainly doesn't/can't include any theory on conduit installations that's for sure, not by some of the posts on this thread anyway....

 

[johnboy6083]

In chlorine rooms, the PVC conduit is a bluish colour, like PVC pipe. It's not standard stuff.