Earthing problem on Domestic lighting cct

[freedomrun]

TN-C-S Domestic, CU change required. Luminaire in a Metal housing used in the Kitchen and owner wants it to stay, seems that some work has been done in the past and i cannot find any Earth continuity on this lighting circuit back to the present CU, although a cpc is connected to each light on the cct. The property has another lighting circuit that is all OK, is it permissible to use the cpc from this circuit to supply the other lighting cct with the broken cpc. If not, what if i supply both lighting circuits from the same MCB? Customer does not want to pay for me to fault find the problem or replace the Kitchen light with a double insulated type. Cheers.

 

[Technician]

I think a safer option would be to combine the circuits

onto one protective device, do the tests from the metal

housed luminaire and prove (R1 + R2) exists. Ensure that

the breaker will handle it of course, and that the conductor

sizes are correct.

On completion do a live Zs test at lights on that circuit.

If you connect this light to the other circuit you will wind

up with one less cpc terminated in CU, which looks like a

"borrowed" cpc. It might not look too serious, but if you

want the board to look right after you have finished that

is possibly your only option.

I have seen two lighting circuits protected by two circuit

breakers and IIRC the net effect is that the allowable value

of Zs is halved for that circuit breaker.

 

[freedomrun]

The lighting circuit in question is supplied from the CU with an old 60's cable with no cpc, just L & N (IR TEST is 170M ohms between them) at some point after the CU, Twin and earth has been used for the lighting circuit including the Metal light fitting in the Kitchen - but the cpc is not connected back to the CU, hence my question about 'borrowing' a cpc from the other lighting cct than does have cpc continuity back to the CU. Also, the owner will not pay for additional cpc to be run back to the CU from the lighting circuit without cpc continuity. Advice please.

 

[Technician]

Yes; I was concerned with appearences.

If the CU is changed and one light fed from

one circuit breaker has its cpc connected to

the earth terminal bar via another lighting

circuit you must prove by test that the c/b

will disconnect after you have finished within

the stated maximum times.

I assume that the CU might have an RCD in

which case put the RCD on a ramp then measure

the disconnection time in the usual manner.

Record the changes you have made in writing

and make the schedule in the new board reflect

them.

It is annoying when people do not take good advice

when it is on offer.

 

[freedomrun]

The new CU will be a 17th ed split load. This 'borrowing' of the cpc is permissible within the regs then? And as long as my test results prove disconnection time within maximum limits and i record as such on the EIC. The install has got to be safer after doing this, than what is there at present.

 

[Technician]

This is permissible but the cpc as installed has to be a larger

size. 2.5 mm sheathed, 4 mm unsheathed.

This makes the value of (R1 +R2) lower.

Supplementary bonding can be connected to an existing

terminal inside an accessory and need not go back to the MET

as a separate conductor.

 

[SLIPSHOD & SLAPDASH]

It is annoying when people do not take good advicewhen it is on offer.

When a lot of your 'advice' is incorrect then i am glad the offer of it was not taken up.

I think a safer option would be to combine the circuits

onto one protective device, do the tests from the metal

housed luminaire and prove (R1 + R2) exists.The OP STATED THERE WAS NO CONTINUATY Ensure that

the breaker will handle it of course, and that the conductor

sizes are correct.

On completion do a live Zs test at lights on that circuit.SEE ABOVE

If you connect this light to the other circuit you will wind

up with one less cpc terminated in CU, which looks like a

"borrowed" cpc. It might not look too serious, but if you

want the board to look right after you have finished that

is possibly your only option.THERE ARE NO CPC AT THE BOARD

I have seen two lighting circuits protected by two circuit

breakers and IIRC the net effect is that the allowable value

of Zs is halved for that circuit breaker. WHERE DID YOU LEARN THAT GEM

Share

You have found a common problem of alterations carried out in Tw&e and the cpc not connected. The best option if possible would be to wire a new cable from the C/Unit with Tw&e to the kitchen light and then disconnect the leg that went to the C/u. 'Borrowing' a cpc is allowed, think about conduit and trunking systems without a cpc cable.

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[freedomrun]

That's true, forgot about trunking and conduit systems - cheers.......... what reg would that be?

 

[batty]

If customer doesn't want to pay what are you messing around at trying to do a bodge job. My advice would be walk away.

 

[Technician]

At the point where the customer did not want a double insulated

light, I would have walked away. As I saw it, "borrowing" a cpc

was permissible under chapter 54.

As for the two circuit breakers on two lighting circuits, that is what

one older man said to me years ago;

"Would one or the other operate in the event of a fault?" he asked.

The answer, in his view was that the fault current was split two ways

and thus in his view the disconnection time was extended.

 

[SLIPSHOD & SLAPDASH]

Do you mean linking 2 circuit breakers together thus joining 2 circuits together ?

 

[Technician]

Slips...yes.

Hope this clarifies.

BTW thanks for the tips.

I have found lighting circuits where

two separate systems have been linked

by the live feeds from two 6A devices.

I have gone to strenuous ends to ensure

they were separated.

 

[ProDave]

What is the customers problem with fixing the lighting circuit properly?

Is it price, or the amount of upheaval and possible damage to decor they are not happy with?

If it's price, then just increase your quoted price for the CU change, and say you will do all the remedial work for free. Sorted.

 

[freedomrun]

It's the posible damage to the decor - and they think i'm trying it on to get more cash out of the job. I'm Just trying to find the simplest solution within the regs - if i can 'borrow' the cpc from the other lighting circuit, then i will place each of the two lighting circuits in 2 dedicated cct protective devices - had a look thru Chapter 54, still not sure if i'm working within the regs by 'borrowing' the cpc from the other lighting cct. I also would'nt class it as a 'bodge' if it still complies with the Regs - what the customer has at present is a complete bodge.

 

[Technician]

Freedom, I agree with you. Have a look at that section and it is

(last time I looked) the one on the adiabatic equ.

On other posts statements you have taken all practicable steps

to ensure that is is safer than you found it.

Good heavens, they all complain about decor. That is why the

average spark has to be a bit of all things to do it.

Knowing where to draw the line is the trick as pro dave has said.

 

[freedomrun]

I think i'm going to 'borrow' the cpc from the other lighting circuit - seems the best way to resolve the issue in this case. I shall also make notice of it on the EIC.

 

[SPECIAL LOCATION]

TN-C-S Domestic, CU change required. Luminaire in a Metal housing used in the Kitchen and owner wants it to stay, seems that some work has been done in the past and i cannot find any Earth continuity on this lighting circuit back to the present CU, although a cpc is connected to each light on the cct. The property has another lighting circuit that is all OK, is it permissible to use the cpc from this circuit to supply the other lighting cct with the broken cpc. If not, what if i supply both lighting circuits from the same MCB? Customer does not want to pay for me to fault find the problem or replace the Kitchen light with a double insulated type. Cheers.

Just been reading this thread

 

[Zeespark]

Can you not run a single G/Y 6491X next to the current cables to provide your earth path at all?

AndyGuinness

 

[Technician]

Like your analysis SL.

 

[freedomrun]

Yes i can run a 6491x G/Y cable to the other lighting circuit with ease - i just wanted some confirmation if it was compliant within the regs as i was having some trouble getting a straight answer, but thanks to the SUPERB post above, i now have confidence in doing this. I will leave all lighting cct's on their own dedicated MCB's and just ensure everything i have done is noted on the EIC. I just know i'm going to find more domestic properties with the same problem in the future.