Testing the SELV

[La Poste]

Correct me if I am wrong but if we are to test a SELV transformer do we:

1. Join conductors of low voltage side of transformer together.
2. Join conductors of primary side of transformer together.
3. Test between these at a voltage of 500V look for minimum reading of 1 MOhm.
4. Test between joined conductors of low voltage side and the earth of the primary side at 500V Look for minimum of 1MOhm.
5. Test between conductors of low voltage side at 250V look for minimum reading of 0.5 MOhm.

When testing the conductors on the low voltage side at 250V this is including the transformer winding right?
I mean is this low 250V test a test of insulation resistance of cables but also including the resistance of the winding inside the transformer.
Is that right?
Thanks.

 

[La Poste]

I mean when testing the SELV circuit for insulation resistance do we simply connect our IR tester to the two conductors of the SELV circuit and wack 250V down them and hope for a reading of 0.5MOhm or is there something I am missing here?
It seems to me that if this is the method of testing then all we are really testing is the resistance of the winding of the transformer.

 

[IQ Electrical]

Correct me if I am wrong but if we are to test a SELV transformer do we:

1. Join conductors of low voltage side of transformer together.
2. Join conductors of primary side of transformer together.
3. Test between these at a voltage of 500V look for minimum reading of 1 MOhm.
4. Test between joined conductors of low voltage side and the earth of the primary side at 500V Look for minimum of 1MOhm.
5. Test between conductors of low voltage side at 250V look for minimum reading of 0.5 MOhm.

When testing the conductors on the low voltage side at 250V this is including the transformer winding right?
I mean is this low 250V test a test of insulation resistance of cables but also including the resistance of the winding inside the transformer.
Is that right?
Thanks.

Pretty much spot on but once you've done the primary/secondary tests, there's no need to do the 250 V tests with the secondary included.

 

[Lighthouse Phil]

Hi La Poste, I normally disconnect the tranny, never include it in the tests. (hope I'm right)
:detective:

 

[La Poste]

Thanks for the answers.
The thing that really confuses me is that in GN3 and other publications it says to test SELV at 250v DC but no where does it say to disconnect the transformer.
I have not read this anywhere and I have been reading a lot.
I mean are we supposed to disconnect the transformer and IR test the low voltage side at 250V or are we supposed to IR test the low voltage side at 250V with the transformer in place.
The transformer does not seem to be mentioned in any publication.
Just as a second question, what would the resistance of the transformer winding be if we did a continuity test on it?
The low voltage side I mean.
Thanks.

 

[telectrix]

1. i never IR test the secondary. 2. if you were to measure the winding resistance, the secondary reading would be approx. 1/20 of the primary. the reading though would only be resistive though. no account taken of the inductance, unless you have an LCR meter.

 

[La Poste]

Can you see the confusion though.
GN3 says to test SELV circuits at 250V DC but does not mention if we are to disconnect the transformer or not.
And no other publication mentions it either.
Well none that I have read.
I mean what exactly are we IR testing at 250V?
Is it the resistance between the two live conductors of the supply side?

 

[IQ Electrical]

I wouldn't even consider a transformer test to be neccessary if it could be confirmed by inspection that the transformer complied with BS EN 61558-2-6 or BS EN 60742 or BS 3535-1 for older products.

Other suitable sources for SELV should meet the requirements set out in BS 7671:2008.

 

[Engineer54]

I wouldn't even consider a transformer test to be neccessary if it could be confirmed by inspection that the transformer complied with BS EN 61558-2-6 or BS EN 60742 or BS 3535-1 for older products.

Other suitable sources for SELV should meet the requirements set out in BS 7671:2008.

Just curious, why wouldn't you even consider a transformer test on a SELV TX?? A BS number only gives the TX a build configuration standard. All TX's have a BS number, it hasn't to my knowledge negated them from being subjected to electrical testing as and when required...

 

[La Poste]

Just curious, why wouldn't you even consider a transformer test on a SELV TX??

What test would you do?
Just curious.

 

[La Poste]

:detective:

The test procedure says test between the SELV conductors at 250V.
Minimum insulation resistance reading 0.5 MOhms.

This also brings us onto the subject of isolating transformers.
Testing the isolated circuit insulation resistance.
I guess a SELV circuit is a small version of an earth free equipotential zone.
That must be the answer.
The difference is it is at a voltage of less than 50V.
The only concern about insulation resistance would be a short circuit not a fault to earth.
The important thing being to have an appropriate over current device, I guess that's the heat sensor in the transformer.
The appropriate BS reading would tell us the transformer had been tested for overload much the same as a BS number for an MCB.

I wonder if that is why the reading is only 0.5 MOhms between conductors.

But I still don't know if I am to test insulation resistance at 250V with the transformer in circuit or not.
I'll get there in the end.

 

[Goody]

This is definitely related to this topic but slightly different. I have a shaver socket integrated with a mirror light with pull cord switch; obviously the shaver socket is SELV. How do I test the SELV part of the equipment in this case? Cheers!

 

[malcolmsanford]

Why is it SELV, is the razor plus below 50v or are they rated 115/230v

Don't mix up SELV and separation.

 

[IQ Electrical]

Just curious, why wouldn't you even consider a transformer test on a SELV TX?? A BS number only gives the TX a build configuration standard. All TX's have a BS number, it hasn't to my knowledge negated them from being subjected to electrical testing as and when required...

In general, the requirement is a thorough inspection to make sure that the source of low voltage (usually a safety isolating transformer) complies in all respects with the British Standard concerned, followed by an insulation test between the extra-low voltage and low voltage systems.

Guidance Note 3 (page 40) reads:

The source of the SELV supply should be inspected for conformity with regulation 414.3 and if necessary the voltage should be measured to confirm that it does not exceed 50 V ac or 120 V dc.

 

[La Poste]

This is definitely related to this topic but slightly different. I have a shaver socket integrated with a mirror light with pull cord switch; obviously the shaver socket is SELV. How do I test the SELV part of the equipment in this case? Cheers!

The shaver is not SELV it is electrically isolated from the rest of the installation so the tests you would do would be to make sure the voltage levels are correct with a volt meter connected between live conductors of the electrically separated circuit and also test for electrical separation this would mean isolating the circuit supplying the transformer, joining conductors of the primary side together, joining the conductors of the secondary side together and then testing between both sets of conductors (IR test) at 500V.
Also test between the joined conductors of the secondary (isolated side) side and earth of the primary side at 500V.

Minimum insulation resistance value 1MOhm.