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You would prefer to have a defective cable / joint remain live and in service without knowing the nature or extent of the defect?
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You would prefer to have a defective cable / joint remain live and in service without knowing the nature or extent of the defect?
I was hoping that somebody could explain to me the electrical theory behind the assertion that one cannot take a cpc for one circuit from another circuit, all the good tested cpc's return to exactly the same point, i would think the functionality of the cpc protection is retained irrespective of the route it takes.
No, not necessarily. The CPC of a cable does not suddenly go open-circuit by itself. Something is significantly wrong with that cable run, but you don't know what. You know only the symptom that the EICR has thrown up, i.e. open-circuit CPC, and you propose to alleviate that symptom by providing an alternative CPC. Anything else that is wrong will remain live and in service.
As I said back in post #6, the cable might have been partially severed by a screw or nail which is now live. Or it might contain a thoroughly defective joint made by an unskilled person using sellotape, of which one of the three cores has so far given way. Or it might have been ripped to shreds by catching on a Skil saw when somebody was cutting floorboards, and now the bare length of line conductor is resting against a non-extraneous gas pipe. You don't know whether any of these apply to you, only that the broken CPC has alerted you to a problem with the cable. Do you intend to ignore any other ramifications once your CPC is reconnected to the MET?
The gas and water/CH bonding has passed
The CPC is completely severed. I'm not so worried what that might or might not be in contact with, as one side is already earthed and the other side is going to get earthed. It's the other bits that might be involved. What was the insulation reading from line to shelf bracket?How about line to shower-curtain rail? (we had one of those some years back, the rail was solidly live from where a fixing screw had penetrated a cable.)
I won't bother trying to convince you any further.
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You would prefer to have a defective cable / joint remain live and in service without knowing the nature or extent of the defect?
I have asked the electrician the following questions
Which light pendant is the first in the run (in which room) ?
have you disconnected the cpc from this pendant and at the CU and measured for continuity, what was the result ?
which wiring REG will be contravened if an earth is taken from a different circuit ?
to the first 2 there is no answer, to the 3rd there is no ref to a wiring reg, but the explanation is that if he takes a connection from the socket then somebody could in the future remove those sockets and in doing so you lose the cpc. Which of course is true, but that argument could equally apply to any other cpc if somebody decided to "remove" it.
He should be able to determine which rose is the first in the circuit, but to do this i imagine you must disconnect each rose in turn until you can measure continuity back to the mcb. If he does this and measures the associated cpc on this first run rose and it is open cct then obviously either the conductor is broken somewhere under floorboards or maybe loose at an underfloor junction box. If that is the case then either one lifts the floorboards at several places in the house to check where the cpc is lost or perhaps one can borrow another cpc. I would agree that taking the cpc from elsewhere is not the most elegant solution but lifting carpets and floorboards is a very expensive hammer to crack a nut.
in order to counter his argument about somebody removing those sockets I suggested that the incoming supply to the loft radial sockets could be cut (in between the loft joists),a junction box introduced, preferably under a loft board, the radial cct is reconnected and from the junction box run a separate cpc to either the light pendants in the loft or the light pendants for any of the 1st floor which are accessible from the loft.
Which light pendant is the first in the run (in which room) ?
have you disconnected the cpc from this pendant and at the CU and measured for continuity, what was the result ?
which wiring REG will be contravened if an earth is taken from a different circuit ?
to the first 2 there is no answer, to the 3rd there is no ref to a wiring reg, but the explanation is that if he takes a connection from the socket then somebody could in the future remove those sockets and in doing so you lose the cpc. Which of course is true, but that argument could equally apply to any other cpc if somebody decided to "remove" it.
He should be able to determine which rose is the first in the circuit, but to do this i imagine you must disconnect each rose in turn until you can measure continuity back to the mcb. If he does this and measures the associated cpc on this first run rose and it is open cct then obviously either the conductor is broken somewhere under floorboards or maybe loose at an underfloor junction box. If that is the case then either one lifts the floorboards at several places in the house to check where the cpc is lost or perhaps one can borrow another cpc. I would agree that taking the cpc from elsewhere is not the most elegant solution but lifting carpets and floorboards is a very expensive hammer to crack a nut.
in order to counter his argument about somebody removing those sockets I suggested that the incoming supply to the loft radial sockets could be cut (in between the loft joists),a junction box introduced, preferably under a loft board, the radial cct is reconnected and from the junction box run a separate cpc to either the light pendants in the loft or the light pendants for any of the 1st floor which are accessible from the loft.
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I would be using using the earth from another circuit as a last solution. Someone needs to establish where the problem is as nine times out of ten it can be rectified. I suspect you are wasting your time with this electrician.
no that is not what i was suggesting, as a C1 failure it has to be resolved, my preference is for a more elegant solution than his suggestion, tacking in an extra wire via external trunking. If a cpc is derived from another cable then the induced voltage collapses and the circuit is no longer live and is safe, is it not ?
I was hoping that somebody could explain to me the electrical theory behind the assertion that one cannot take a cpc for one circuit from another circuit, all the good tested cpc's return to exactly the same point, i would think the functionality of the cpc protection is retained irrespective of the route it takes.
the problem for me is how many qualified people do I need to employ until I find one who is capable of faultfinding. If the conductor can be shown through measurement to be o/c in the leg from the consumer unit, and this cable is largely inaccessible then there are only two options open 1a) lift all the carpets, floor boards, buried trunking and run a new cable to the first ceiling rose in the loft 1b) run a new cable in external trunking to the first ceiling rose in the loft 2) take a cpc from somewhere adjacent to the first ceiling rose in the loft. What would you suggest to your customer if this was the 1 time out of 10 where it could not be readily rectified ?
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No, not necessarily. The CPC of a cable does not suddenly go open-circuit by itself. Something is significantly wrong with that cable run, but you don't know what. You know only the symptom that the EICR has thrown up, i.e. open-circuit CPC, and you propose to alleviate that symptom by providing an alternative CPC. Anything else that is wrong will remain live and in service.
As I said back in post #6, the cable might have been partially severed by a screw or nail which is now live. Or it might contain a thoroughly defective joint made by an unskilled person using sellotape, of which one of the three cores has so far given way. Or it might have been ripped to shreds by catching on a Skil saw when somebody was cutting floorboards, and now the bare length of line conductor is resting against a non-extraneous gas pipe. You don't know whether any of these apply to you, only that the broken CPC has alerted you to a problem with the cable. Do you intend to ignore any other ramifications once your CPC is reconnected to the MET?
the proposal to use another cpc is not mine it is the suggestion of the author of the fail certificate, I am simply suggesting a less obtrusive version of his suggestion. If the conductor has been partially severed and is now live would an insulation test L/E indicate >500Meg ? The electrician has confirmed that the voltage present on the fascia screws is induced, if it really was line voltage then would not the connection of an cpc from elsewhere to this potentially partially severed cpc cause the MCB to trip ?
The gas and water/CH bonding has passed
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On the question of which regulation states that you cannot use a CPC from one circuit to add a CPC to a different circuit, you will not find any such regulation. On the contrary 543.1.2 and onward shows the requirements for where two or more circuits share the same CPC. So in some circumstances it might be perfectly acceptable.
However, as Lucien and others have pointed out, this would not be an appropriate solution to the "missing" CPC in your situation. You would only be masking the symptoms, not correcting the fault.
A bit like taking painkillers to treat a bleeding artery. You need to find the problem and repair it.
However, as Lucien and others have pointed out, this would not be an appropriate solution to the "missing" CPC in your situation. You would only be masking the symptoms, not correcting the fault.
A bit like taking painkillers to treat a bleeding artery. You need to find the problem and repair it.
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The CPC is completely severed. I'm not so worried what that might or might not be in contact with, as one side is already earthed and the other side is going to get earthed. It's the other bits that might be involved. What was the insulation reading from line to shelf bracket?How about line to shower-curtain rail? (we had one of those some years back, the rail was solidly live from where a fixing screw had penetrated a cable.)
I won't bother trying to convince you any further.
Thanks for that reference
I appreciate your engagement. Thanks.
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Having pressed the point about not leaving a damaged joint / cable in service, perhaps I should give my approach to locating one.
1. Walk the course. Is there any evidence of work carried out since the installation in areas where the cable is likely to be? Sometimes it's a no-brainer.
2. Inject tone into cable and search with tracer along likely routes. If it can be found at least at a few waypoints, estimate total length along that route.
3. Ping cable from both ends (CU and first lighting point) with a TDR or capacitance meter to discover distance to the break. Compare sum of lengths with measured route. Take average, measure along assumed route, look carefully around epicentre for likely causes of damage.
4. If possible, make access hole behind some other fitting or fixture that will conceal it later. Insert borescope. If nothing interesting visible, return to step 2.
etc.
1. Walk the course. Is there any evidence of work carried out since the installation in areas where the cable is likely to be? Sometimes it's a no-brainer.
2. Inject tone into cable and search with tracer along likely routes. If it can be found at least at a few waypoints, estimate total length along that route.
3. Ping cable from both ends (CU and first lighting point) with a TDR or capacitance meter to discover distance to the break. Compare sum of lengths with measured route. Take average, measure along assumed route, look carefully around epicentre for likely causes of damage.
4. If possible, make access hole behind some other fitting or fixture that will conceal it later. Insert borescope. If nothing interesting visible, return to step 2.
etc.
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Lucian what do you use to Inject a tone into the cable?
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Three choices:
The normal AF warbler, ideal for close-up work, finding a core or pair in a cable, following it around a panel etc, but range limited to about 12".
Then there's one of these around the place somewhere . Ironically, when it's lost, you can't locate it by waving a piece of cable around.
Many moons ago I made one myself out of the guts of a couple of transistor radios. Depending on how one were to do that, one might end up with a device that radiates on bands where one is supposed to keep quiet. One would need to use discretion as it might have significant range.
The normal AF warbler, ideal for close-up work, finding a core or pair in a cable, following it around a panel etc, but range limited to about 12".
Then there's one of these around the place somewhere . Ironically, when it's lost, you can't locate it by waving a piece of cable around.
Many moons ago I made one myself out of the guts of a couple of transistor radios. Depending on how one were to do that, one might end up with a device that radiates on bands where one is supposed to keep quiet. One would need to use discretion as it might have significant range.
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Been looking for one of these for a while: Socket and See FFCB100UKA Fuse Finder | ISSWWW.CO.UK - https://isswww.co.uk/socket-and-see-ffcb100uka-fuse-finder?gclid=Cj0KCQjwutaCBhDfARIsAJHWnHtzziofqm8WHz1L4ITtnx4ylS8TlwtITySmtjAXp1RnOctjGdMYPWgaAnNTEALw_wcB ideal for me as it has a removable cable so can be configured for UK and EU via what looks like a standard appliance plug.
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All depends on the accuracy required ! I once used a bit of wire wrapped around a lawnmowers HT lead and and attached to a cable conductor , and an AM radio to track a cable run underground. (Not that I’m suggesting that for a domestic lighting circuit of course!)
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Personally I think the OP was only looking for someone to say it was OK to take a CPC from somewhere else , not wanting to entertain any other sound advice and now we won't hear from him now someone has said it is OK.
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