Why is the test voltage 250v and min acceptable readings 0.5M ohm for SELV circuits but not none separated ELV?

I thought I'd stick my head above the parapet since nobody else feels inclined......

This is my view,

SELV/PELV circuits follow the general classification (412) regarding double insulated equipment usually supplied from a SELV source to one item of equipment
So test voltage would be 250V between SELV conductors and SELV conductors to earth

SELV/PELV circuits that are separated (418) are likely to be supplied by a separate source where more than one item of equipment is being supplied with a common neutral.
Test voltage here would be 500V between separated circuit live conductors and circuit conducts and earth.
Where the earth is also separated then normal IR tests would apply.

Legh

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Thanks for the reply Leigh but thats not what I was really asking:
SELV cuircuits are 250v, 0.5M
and
ELV circuits (no separation just not more than 50vac) are 500v, 1M.

Can anyone explain why?
Thanks.

Indirect contact, whereas if SELV measures are taken, the risk of indirect contact is much reduced.

In additon, the SELV circuits are not exposed to the same level of surges from switching and lightning, because they are suitably separated and unlike PELV don't share the same earth connection.

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Eur Ing Graham Kenyon CEng MIET

Thanks for the reply Graham.
SELV and PELV are both at 250v and 0.5M Table 61 BS7671. But the table implies that Extra Low Voltage with the exception of these 2 systems be tested at 500v with 1M the min' acceptable reading.

I was hoping someone could explain the reasons for this. The reason I ask is City and Guilds seem to be asking for an understanding of this with recent 2391 exam papers:

"Many candidates failed to appreciate that a 38 V ac circuit would be tested for insulation resistance at 500 V dc. The 250 V test quoted by most is only appropriate for SELV and PELV circuits."

A qutoe from the Chief Examiners Report Feb 2012:
http://www.cityandguilds.com/45768.html?s=4


Does anyone know the reasons for this requirement?
Cheers

Edited: 01 July 2012 at 11:17 AM by deapea

The answer is on the whole reasonably straightforward.

Where an ELV circuit (ie; 38v a.c.) does not fulfill the protection requirements for an SELV or PELV circuit, for example it is not protected from accidental contact with the higher voltages within the system the protection and test requirements for the higher voltage must be applied to the entire circuit.

"Many candidates failed to appreciate that a 38 V ac circuit would be tested for insulation resistance at 500 V dc. The 250 V test quoted by most is only appropriate for SELV and PELV circuits."

So that statement is absolutely correct.

regards

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"Take nothing but a picture,
leave nothing but footprints!"
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"Oh! The drama of it all."
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"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
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Edited: 01 July 2012 at 05:17 PM by rocknroll

SELV and PELV are supplied from a special kind of transformer, that provide separation from live conductors to a known level, and also helps reduce the magnitude of overvoltage impulses.

I believe those are the reasons.

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Eur Ing Graham Kenyon CEng MIET

Ok thanks.
So as I understand it "seperated" means no electrical connection between the primary and secondary sides of the transformer. This means no connection to earth on the secondary side and therefore no chance of an overvoltage resulting from an earth fault on the primary (higher voltage) side.
There is a risk of this if we have ELV with a connection to earth and thats why we take the I.R requiremnts for the higher voltage.
Am I thinking on the right lines ??

It's not so much the ELV connection with earth (PELV circuits may be earthed after all) - but the reliability of the separation from LV (mains) voltage - SELV and PELV must have a 'safety' source (i.e. can't allow mains voltage across to the ELV side even if the device goes faulty) - a feature that other ELV circuits don't necessarily have - e.g. with a conventional transformer a simple insulation breakdown (melted varnish) could allow mains voltage onto the ELV side.
- Andy.