pre consumer unit change

[tank]

hello all, if i were to install an rcd protected board into an old premises, should i first do an IR test on the old board to see if i could expect any problems with the new rcds? how low would the L - E reading have to be for me to expect trouble? or am i talking out of my hole? it wouldn't be the first time.

 

[telectrix]

you should IR test first. anything under 2 Meg. is suspect. also test for shared neutrals.

 

[kingeri]

As tel says, definitely IR test first. I normally IR test the whole installation first, then each circuit if necessary. In fact, the last CU change I did involved rewiring the lighting circuits first. The difficulty sometimes can be convincing folk that a CU change can't go ahead without remedial works first. They usually think you are trying to milk them dry.

 

[Guitarist]

Always do a basic check first... Bonding, cpc on lighting, IR @ 250v, shared neutrals etc. That way you can tell the customer in advance what needs doing before a CU change.
Many people on here have been bitten by doing a CU change, THEN finding faults which the customer refuses to have put right.

 

[i=p/u]

for shared neutrals can you not just put lighting on the one side.. how you test for this?

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i can guess and get it right but its good to hear the easy explanation

 

[Richard Burns]

for shared neutrals can you not just put lighting on the one side.. how you test for this?

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i can guess and get it right but its good to hear the easy explanation

Continuity test between the different neutral cables at the CU, they should not be connected (unless they are still in the neutral bar!) if they are then it is likely that they are shared at some point in the circuit, where is another matter!

 

[telectrix]

a quick tip for this is to see if the landing light has it's L kinked from the hall light in the 2 gang switch. or, if the up and down lights are on different circuits, see which breaker/fuse kills the landing light. if it's the "down" one, then you've got the dreaded problem

 

[spark 68]

for shared neutrals can you not just put lighting on the one side.. how you test for this?

There was quite a long thread about this not so long back, and I think the consensus then, was to not only put on the same RCD side, but to put it on the same MCB, so as not to backfeed a different lighting circuit.

 

[ExArmy]

There was quite a long thread about this not so long back, and I think the consensus then, was to not only put on the same RCD side, but to put it on the same MCB, so as not to backfeed a different lighting circuit.

not sure i follow this? aren't all neutrals efectively shared as they are connected in the neutral bar? and all single pole MCB's only disconnect the line but backfeeding isn't a problem there?

 

[telectrix]

think what he means, is that if you put the 2 light circuits on separate MCBs, say, the landing light is fed from the downstairs MCB, and there's a problem with the landing light. you switch off the upstairs lighting MCB, but the fitting still has a live feed.

 

[spark 68]

not sure i follow this? aren't all neutrals efectively shared as they are connected in the neutral bar? and all single pole MCB's only disconnect the line but backfeeding isn't a problem there?

Because the term 'borrowed Neutral' is misleading, it is actually a borrowed live (or more correctly Line)

Also as Tel said, and partly also because if you have a fault on the Neutral side of the circuit, it will still be very much live.

I will see if I can find the thread.

 

[telectrix]

Because the term 'borrowed Neutral' is misleading, it is actually a borrowed live (or more correctly Line)


​that's exacltly the term i prefer.

 

[spark 68]

Yes Tel, borrowed Live and shared Neutral,

Here are the two Threads:

http://www.electriciansforums.net/i...l-unsure-about-testing-borrowed-neutrals.html

http://www.electriciansforums.net/e...37005-lighting-dreaded-borrowed-neutrals.html

 

[tank]

Always do a basic check first... Bonding, cpc on lighting, IR @ 250v, shared neutrals etc. That way you can tell the customer in advance what needs doing before a CU change.
Many people on here have been bitten by doing a CU change, THEN finding faults which the customer refuses to have put right.

guitarist, what is the rational behind the IR @ 250v ? i thought, apart from SELV, it was used before a 500v test as a way to check if you've missed removing one of the loads.

also, when would the L and N to E insulation test be used? is the pass for this test still 1MΩ, check if below 2MΩ? i got told that L and N to E should be done when there are loads or sensitive equipment that cannot be removed. If this acceptable, do people not bother with removing the loads for the full IR test (L-N, L-E, N-E) and just do this instead?

 

[telectrix]

L/N shorted and tested to E is used when loads can't be removed. e.g. high bay lighting. , IT equipment. etc. L-N IR testing is more important on initial verification, on a new job.

 

[Guitarist]

guitarist, what is the rational behind the IR @ 250v ? i thought, apart from SELV, it was used before a 500v test as a way to check if you've missed removing one of the loads.

also, when would the L and N to E insulation test be used? is the pass for this test still 1MΩ, check if below 2MΩ? i got told that L and N to E should be done when there are loads or sensitive equipment that cannot be removed. If this acceptable, do people not bother with removing the loads for the full IR test (L-N, L-E, N-E) and just do this instead?

Exactly what you say. 250v test just a quicky to see if there is anything obvious before agreeing to change the CU, without the risk of damaging anything.