R2 help please

[pencilpusher]

I am doing some R2 testing. I have been removing the cpc from the earth bar and crock clipping it . I am then measuring r2 at the other end of the circuit.

Problem: For example, I removed the cpc for the cooker circuit at the CU. I went to the cooker switch and got continuity. Great. I then realised that the cpc had been placed in the wrong cpc position in the bar. To help me check the other cpc's I decided to do more r2 tests to confirm the cpc ends.

When I tested between removed cpc of cooker (at CU) and removed cpc of immersion heater switch I got continuity. This has happended between cpc's of cooker, immersion and central heating. I suspect parallel paths due to bonding. Am I right? What I don't get is why, if I remove cpc from earth bar and from connection pount, I get bonding coming into it. This is driving me nuts so please help!

Thanks

 

[davie spark]

1) Is the cooker unit feeding the ignition system for a gas oven or gas hob?
2) Have you carried out a separation of circuits test with an insulation tester?

 

[pencilpusher]

Is the cooker unit feeding the ignition system for a gas oven or gas hob?

Yes it is. It feeds the electric cooker and the gas hob.

 

[davie spark]

1) Is the cooker unit feeding the ignition system for a gas oven or gas hob?
2) Have you carried out a separation of circuits test with an insulation tester?​

 

[pencilpusher]

1) Yes
2) No

I am trying to get in my head why the three circuits cpc's give continuity with each other.

 

[davie spark]

Ok, you may have parallel paths via the gas pipe work, the hob will be bonded, the central heating will be bonded as is your cooker circuit in other words there's probably an earth path being provided by a combination of pipework and CPCs. I would try a separation of circuits test on the 3 circuits, i.e. cooker, c/heating and immerser with all CPCs disconnected at both ends of the circuits. The idea being that if you've got all three CPCs diconnected at both ends then there should be no link between them thus allowing you to take a measurement of each individually, does that make sense to you?

 

[davie spark]

I should have added that the water system pipework may be linking your readings also.

 

[pencilpusher]

Thanks. Its making reasonable sense but I need to clarify. Can you explain the insulation test on the cpc.s. I presume I test each cpc seperately to see if I am losing anything to other cpc's? What connections are made with the MFT please?

 

[davie spark]

Make sure you've disconnected 'both' ends of the circuit CPCs before you look for a reading. You could also try disconnecting all the circuit cables, again at both ends, link live and CPC at the consumer unit with a connector strip and test between live and earth at the other end of the same circuit. This reading will also include the resistance of the live/phase cable but if everything is as it should be the reading you get should be acceptable, well under the recommended limits. I'd be looking for under 0.5 ohms.

 

[pencilpusher]

Make sure you've disconnected 'both' ends of the circuit CPCs before you look for a reading. You could also try disconnecting all the circuit cables, again at both ends, link live and CPC at the consumer unit with a connector strip and test between live and earth at the other end of the same circuit. This reading will also include the resistance of the live/phase cable but if everything is as it should be the reading you get should be acceptable, well under the recommended limits. I'd be looking for under 0.5 ohms.

I have done the R1+R2 tests already. I got under 0.5 ohms but I still get different cpc continuity as exlpained. I am not concerned with disconnction times just why I get this continuity problem. Can you kindly help with the IR tests you mentioned. Thanks.

 

[davie spark]

A separation of circuits test is carried out between two circuits, (usually to check for 'borrowed' neutral and 'live' conductors between circuits).The meter is set up as you would for an insulation test, i.e. set on Megohms. You connect the meter leads to the CPC from the two circuits in question at the consumer unit and if you have them already disconnected at the 'user end' of the circuit, there should be no link between them - open circuit. If you do this with the cooker circuit, the immerser circuit and the central heating circuit and you get open circuit between them all then your CPC wiring is ok, you then just have to take the continuity reading that you are looking for before you reconnect them again.

 

[davie spark]

I take it you are disconnecting 'all' the conductors, L,N and E?

 

[pencilpusher]

Thank you very much for your help. I have not disconnected L,N and E but I wil try that. This installation is ten years old, no alteration from what i can see. If I get low IR between cpc's, is this likely to be another wiring issue I have come across with this install?

 

[davie spark]

If i've read your first post correctly the you're saying you've disconnected circuit CPCs at both ends of the 3 circuits concerned, is that right? If so, have you also disconnected the neutrals for these circuits? I don't want to complicate things for you but if the neutrals have not been disconnected I'm thinking there may be a possibility of N-E faults which won't trip an MCB or blow FUSE. Have you done IR tests on any of these circuits?

 

[pencilpusher]

Yes, IR tests on NE as a quick check on board. These circuits show >300 M ohms, so OK as far as I can see.