selv pelv felv & testing of them (questions)

[ppelec100]

Hi all hope everyone’s well.

Sorry this a long read but I really want to get my head around this… simplified questions at the bottom of the long read…
I’m struggling to understand it as there’s not many videos explaining it and lack of diagrams in GN3. I’ve been through bs7671 section 414 and read through forums on here and the selv pelv pages on TLC but I’m scratching my head even more lol…

So selv would normally be supplied via a step down transformer which may power downlights for example but do not have a earth on the secondary side so you cannot get a shock if one of the conductors shorts to the metal part… These would be useful in zones 0 providing the transformer is obviously outside of zones and your typical downlights like jcc v50 outside of zones…

Then there is pelv, which does have an earth on the secondary side which can give you a shock,
Q1. What is the purpose of using pelv if you can get a shock?
Q2. Will the Rcd/rcbo operate if there is a fault to earth on the secondary side?

in section 414 it mentions about socket outlets being on a selv pelv system.
Q3. When it mentions socket what would be a a current using equipment on a selv socket?

Q4. Ceiling luminaire couplers are mentioned, where selv should only be 2 pin with no earth contact to separate from earth and from low voltage systems? As pelv is earthed then it would be 3 pin?

Q5. What is an example of felv and where would it be used? From what I read it’s not that common and BT use it?

Testing

GN3 Page 74

Insulation resistance testing between live conductors including other selv and pelv circuits

SELV table 2.12
Q6. so you would disconnect the transformer or before it’s connected, you would put primary lives together and secondary lives together and test between them at 250v DC?

Q7. Then test between the secondary lives together to primary earth to make sure it is actually separated from Earth at 250v DC?

Q8. But it’s saying between other selv/pelv circuits so say if you got selv transformer in a bathroom and selv transformer in a bedroom are you then joining lives of the selv secondary side in the bathroom and lives of selv secondary sides in bedroom then using wander lead method to do IR test between as it will be a distance away?

PELV table 2.13
Q9. Same as Q6? Q7 not followed as it is not separated from earth?

Table 2.14
Q9. So same as Q6 and Q8 again but at 500v DC? Reason it’s now increased is because we did 250v before and it was clear so we increase to 500v and it’s not passing through the equipment but it’s measuring between the cables that should be separate?

Q10. Lastly you IR at 500v between exposed conductive parts on the selv side of the circuit? And between exposed conductive parts of the selv items and any primary exposed conductive parts and extraneous conductive parts to make sure it’s electrically separated from the earth?

Thank you very much in advance to anyone who takes the time to read and respond to these questions!!!

 

[Tom256]

Hi all hope everyone’s well.

Sorry this a long read but I really want to get my head around this… simplified questions at the bottom of the long read…
I’m struggling to understand it as there’s not many videos explaining it and lack of diagrams in GN3. I’ve been through bs7671 section 414 and read through forums on here and the selv pelv pages on TLC but I’m scratching my head even more lol…

So selv would normally be supplied via a step down transformer which may power downlights for example but do not have a earth on the secondary side so you cannot get a shock if one of the conductors shorts to the metal part… These would be useful in zones 0 providing the transformer is obviously outside of zones and your typical downlights like jcc v50 outside of zones…

Then there is pelv, which does have an earth on the secondary side which can give you a shock,
Q1. What is the purpose of using pelv if you can get a shock?
Q2. Will the Rcd/rcbo operate if there is a fault to earth on the secondary side?

in section 414 it mentions about socket outlets being on a selv pelv system.
Q3. When it mentions socket what would be a a current using equipment on a selv socket?

Q4. Ceiling luminaire couplers are mentioned, where selv should only be 2 pin with no earth contact to separate from earth and from low voltage systems? As pelv is earthed then it would be 3 pin?

Q5. What is an example of felv and where would it be used? From what I read it’s not that common and BT use it?

Testing

GN3 Page 74

Insulation resistance testing between live conductors including other selv and pelv circuits

SELV table 2.12
Q6. so you would disconnect the transformer or before it’s connected, you would put primary lives together and secondary lives together and test between them at 250v DC?

Q7. Then test between the secondary lives together to primary earth to make sure it is actually separated from Earth at 250v DC?

Q8. But it’s saying between other selv/pelv circuits so say if you got selv transformer in a bathroom and selv transformer in a bedroom are you then joining lives of the selv secondary side in the bathroom and lives of selv secondary sides in bedroom then using wander lead method to do IR test between as it will be a distance away?

PELV table 2.13
Q9. Same as Q6? Q7 not followed as it is not separated from earth?

Table 2.14
Q9. So same as Q6 and Q8 again but at 500v DC? Reason it’s now increased is because we did 250v before and it was clear so we increase to 500v and it’s not passing through the equipment but it’s measuring between the cables that should be separate?

Q10. Lastly you IR at 500v between exposed conductive parts on the selv side of the circuit? And between exposed conductive parts of the selv items and any primary exposed conductive parts and extraneous conductive parts to make sure it’s electrically separated from the earth?

Thank you very much in advance to anyone who takes the time to read and respond to these questions!!!

I have copied your post and will reply to your questions - don't have time today.

This post is just to make a quick point of clarity. You mention 'step down transformers' and that may be a cause of some of the confusion for there is a little more to them. The intent in all of this is to keep mains voltage on the primary side away from the secondary. Both in voltage value and by separation, in brief with SELV and PELV the transformer serves more than a step down function, it serves more than simple separation.

The transformer on SELV and PELV is known as a 'safety isolating transformer' you may have seen that term but not registered its intent yet it is a phrase not chosen for its fancy sounding words. The safety isolationing transformer requires by standards additional insulation between primary and secondary - which I can go into more - as such it is more than an isolating transformer. More than just separation.

The safety isolating transformer is not required on FELV where an isolation transformer with basic insulation is permitted. In affect you have the '2 faults to danger' principle in operation.
SELV therfore requires high quality components throughout.
Now the other way a voltage can appear on the secondary is due to touch voltages from faults somewhere in the location, this is possible with PELV not with SELV.

I will paste your post into word and work through the questions - which are very good - bit by bit as I get chance over the next day

 

[ppelec100]

I have copied your post and will reply to your questions - don't have time today.

Okay thank you

The transformer on SELV and PELV is known as a 'safety isolating transformer' you may have seen that term but not registered its intent yet it is a phrase not chosen for its fancy sounding words. The safetyisolationing transformer requires by standards additional insulation between primary and secondary - which I can go into more - as such it is more than an isolating transformer. More than just separation.

Yes I saw it mentioned in bs7671 and googled it and saw a bunch of plug top transformers come up… yes please if you could go into it more it’s kind of confused me.

In affect you have the '2 faults to danger' principle in operation.

Bit confused what you mean by this…


Thank you for replying will look forward to your answers

 

[Pretty Mouth]

Q1. What is the purpose of using pelv if you can get a shock?

I'd like to be able to answer this, but I can't. I don't think I've ever knowingly come across PELV, and don't know of any uses for it.

Q2. Will the Rcd/rcbo operate if there is a fault to earth on the secondary side?

No, the RCD protecting the primary side will not see an imbalance. The only way it could happen is if there was also a fault somehow connecting the primary to secondary.

Q5. What is an example of felv and where would it be used? From what I read it’s not that common and BT use it?

I believe hard-wired interlinked smoke alarms use FELV. The interlink circuit is powered by the 9V battery, through the interlink conductor to other alarms, and returns via the neutral conductor, shared with the 230V circuit.
Incidentally I once accidentally wired a smoke alarm with the L and interlink conductors swapped. A loud 'pop' on switch on and the breaker tripped. When I wired it correctly, the alarm still worked fine as a standalone, but would not interlink with other alarms, the 9V electronics presumably fried from putting 230V across it.