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Topic Title: SELV testing
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Created On: 13 July 2011 12:39 PM
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 13 July 2011 12:39 PM
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postman

Posts: 56
Joined: 03 July 2011

In guidance note three when testing SELV circuits or rather circuits supplied by SELV it states an insulation resistance test should be carried out across the live conductors of the SELV circuit at a voltage of 250V.
The minimum reading should be 0.5MOhms.

Now does this mean we are to take the SELV transformer out of circuit or test with the transformer in situ.
There seems to be no mention of whether the transformer is in the circuit being tested or not.

This is not the test for electrical separation between primary and secondary side of the transformer at 500V this is to IR test the SELV conductors.

There seems to be a little ambiguity in the wording of the regs in this instance.

Any advice would be warmly welcomed.

Thanks.
 13 July 2011 04:42 PM
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peteTLM

Posts: 3085
Joined: 31 March 2005

This test is intended for when you have say a large torrodial transformer feeding dozens of lights with a significant amount of cabling. The transformer and all the lamps would have to be taken out of circuit before the test is done.
It is very rarely used as common practise is to now use pre-leaded transformers which connect directly to the lampholder of a single light fitting.
For a 2391 exam, you need to know how it would be done, but you will never have to do it in this instance. You may however come across some low voltage- selv heating/logic/building controls which are in this category at some time in your carear, so keep it in mind for then.

P

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 13 July 2011 05:58 PM
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postman

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That's great.
Thanks for making that clear.
 16 July 2011 05:41 PM
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postman

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Apologies for raising this old chestnut again but I have been reading this:

"The separation of live parts from those of other circuits and from earth shall be confirmed by measuring the insulation resistance. This can be done by testing between the live conductors of each SELV circuit connected together and those of any adjacent circuits operating at higher voltages which are also connected together.

When the circuit is supplied from a safety source complying with BS3535 an insulation resistance test at 250V DC should be made with the minimum acceptable value being 0.5 MOhms.

In order to confirm compliance with BS7671 an insulation resistance test of 500V DC between live conductors of SELV circuits and protective conductors of circuits operating at higher voltages shall be carried out. The value of insulation resistance must not be less than 1 MOhm"

Taken from my construction skills study notes book.
I read this as a 250V DC test between SELV conductors and conductors of other circuits of a higher voltage and then a separate test of SELV conductors to earth of circuits at higher voltages of 500V DC. A little confusing don't you think.

Now the NIC book snags and solutions part 3 inspection and testing page 131 states:

"Insulation tests are required to be carried out between
1)conductors of each SELV circuit and earth.
2)live conductors of each SELV circuit connected together and the live parts of other circuits.

The voltage should be 250V DC and the minimum insulation resistance should be 0.5MOhm

the IEE book electricians guide to the building regs states:
"test between SELV conductors and live parts of other circuits at 500VDC but does not mention earth

And BS7671 states:
612.4.1 (regarding SELV)
" The separation of the live parts from those of other circuits and from earth according to section 414 shall be confirmed by a measurement of the insulation resistance. The resistance value obtained shall be in accordance with table 61
Table 61 states SELV and PELV test voltage 250VDC insulation resistance 0.5MOhm."

Guidance note three states on page 41.
"To demonstrate compliance with BS7671 an additional insulation resistance test should be applied at 500V DC between the live conductors of the SELV circuit and those of other circuits"
Again no mention of earth.

I am getting fed up with the vague nature of these regulations, they don't seem to be clear at all and it's getting me down.

I mean what about the earth circuit on the primary side of the SELV transformer, the earth circuit of the supply cable that feeds the transformer.
The SELV circuit should be electrically separate from earth so as not to import an earth fault from another circuit into the SELV environment.

I guess the best way to approach this exam is just to read guidance note three and stop thinking about anything else.
Guidance note three is the bible and anything else must be wrong including the NICIEC.

Perhaps the IEE are working on the assumption that a SELV transformer is double insulated and has no earth therefore the test to earth is not needed.

Edited: 16 July 2011 at 06:51 PM by postman
 20 July 2011 01:38 PM
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AJJewsbury

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Guidance note three is the bible and anything else must be wrong including the NICIEC.

Perhaps the IEE are working on the assumption that a SELV transformer is double insulated and has no earth therefore the test to earth is not needed.


I'd say that BS 7671 would have to be the definitive one. 612.4.1 seems to say that the tests should be from the live parts of the SELV circuit to to live part of other circuits AND to earth. Maybe the new GN 3 will be clearer...

GN 3 seem clearer about the voltage to be used though: in 2.7.8 "Where protection is provided by SELV, PELV or electrical separation, the measured insulation resistance should be at least that required by Table 2.2 for the circuit with the highest voltage present in the location. As such, if protection by SELV is being employed in an area which also contains circuits at low voltage, then the minimum acceptable value of insulation resistance for the SELV circuit wiring would be no lower than that obtained for the low voltage circuit and not less than 1 MΩ."

- Andy.
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