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.

 

[Tom256]

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

This feels like I am back at school will give this a go now on laptop. I will split this post depending on time. I tend to waffle on.

To understand the testing of these systems and the use of these protective measures it is important to understand some of the nuanced detail.

I mentioned in my post above ‘2 faults to danger’ this is where I will start.
It is as it says: a ‘single’ fault on a SELV system CANNOT produce a shock condition and this applies throughout the entire SELV system.
To achieve this level of protection it is all in the detail and design and as I mentioned in my first post this extends to robust materials with high integrity assured. This needs to be understood to test these systems and I mention this now as I have seen you have question on testing.

Indeed, a point of note that a SELV system is not one fixed set of parameters that define SELV that apply everywhere. The requirements are tweaked [that nuanced detail kicking in already] by location / use to achieve the 2 faults [required] for Danger level of protection for that particular use.
It starts with the source of supply for the SELV system (SELV and PELV share this same requirement so for now I will refer only to SELV). Regulation 414.3 – Sources for SELV and PELV. Note: NOT FELV

Source of Supply: Four sources are listed and for some locations not all of these sources are permitted (nuanced detail).
Source (iv) is not permitted in Bathrooms, Swimming pools, saunas, restricted conducting locations and medical locations. This is because the electronic devices that would be in (iv) do not have the output voltage under fault limited (by the product standard they are produced to) to the lower voltage required [under fault in the source device] for the locations from Section 7 that I have listed.
A ‘safety isolating transformer’ (i) is permitted and, again, as mentioned prior this device is more than an isolating transformer used for simple separation. See 414.3 (i) for BS EN compliance standards.

With the safety isolating transformer the ‘2 faults to danger’ applies in its construction. A single fault within the transformer cannot impose the primary voltage onto the secondary side. A fault in the primary or in the secondary windings will not import the primary voltage onto the secondary side. It would take at least 2 faults – a fault with primary winding AND a fault on secondary winding.

This is not the case with an isolating transformer with simple separation. This is also a key difference that sets apart what is PELV and what is FELV.
Back to that word ‘nuanced’ again - With regards the ELV permitted on the secondary side. ELV is defined in BS 7671 as not exceeding 50 Volts AC or 120 Volts DC (ripple free – important) yet that is not a one size voltage permitted band that fits all with SELV / PELV.

SELV in swimming pools for example: no more than 12 Volt AC / 30 Volt DC ripple free in Zone 0, yet 25 Volt AC / 60 Volt DC ripple free in Zone 1

So it goes on with specific requirements for medical locations, Bathrooms and so on by way of voltage and basic insulation requirements. BS IEC 60479-1:2018 - the effect of current on the human body relates.
That all in mind, onto the questions.

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?

The Earth connection is not intended to protect against shock nor is it there to give you a shock, other measures such as ELV at times below 12 volts ac are in affect don't forget, yet, it is true this PELV system must be considered a risk for import of voltages under fault on LV system something PELV could be affected by but does not affect SELV.
PELV meets all the requirements for SELV - Example: two faults to danger on source, reduced Voltages -
the key difference/exception that it is not separated from Earth (this is not necessarily because of a protective earth connection but could be because of fortuitous contact with Earth or due to functional earth requirements – EMC screening for example).
But let us assume a protective earth connection to devices on the secondary side - a PELV system.

Question: Why does a 230 V system have a protective earth connection?
Answer: Same reason the PELV does in that you may require fault removal on ‘first fault’ – SELV recall is 2 faults to danger so the first fault will not affect operation of a SELV circuit.

Example: This can certainly be the case in machinery / plant production areas where (source safe) multiple systems are used and the designer felt it is safer to remove the fault under first fault conditions. That could be to prevent the danger of second fault elsewhere or it could be first fault could affect operation of the equipment if left unchecked.
In BS EN 60204 – electrical equipment of machines – PELV is a measure that is set out PLUS with the additional requirements on Basic Insulation to the highest Voltage present on secondary cabling. BS EN 60204 section 6.4 refers

There may also be a need for an earth connection - therefore PELV not SELV - for functional reasons as is the case with 230 Volt systems. Examples: EMC screening, static discharge come to mind yet I am sure there are many more that others here can expand on.

Nuanced details all location specific, it is understanding the why I use it not remembering the when I use it

I am sure others will pick up some of the questions yet I will also do more Later starting with q2 although I see that has already been touched on.

EDITS for typos

 

[Avo Mk8]

Just to be pedantic, or perhaps add confusion and clarity at the same time, the "S" of SELV in established standards stands for "safety", but in later standards, and now BS7671, it stands for "separated"!
The Wikipedia article explains it better than I can, and has a little schematic showing the differences.

Extra-low voltage - Wikipedia

en.wikipedia.org

 

[Tom256]

With the information posted so far some of your questions should be answered. I will skip through a couple more now if my head kicks into gear

Q3. Location specific is I guess the consideration – lighting for example, as we know can be LV, ELV, RLV and you get the idea. If the location requires SELV at the stated voltage then someone will make something to comply.

Q4. Sockets on SELV / PELV: This is answered clearly by Regulation 414.4.3. Sockets must not be interchangeable with any other in the premises.
Sockets on PELV may or may not have 3 pins, there is no requirement for a PELV circuit to have a ‘CPC’ as such, they may well have in some situations but the defining factor is they are not separated from Earth - in some measure.
Note the footnotes to regulation 414.4.1; Note 2: the use of the word ‘MAY’ in relation to how a PELV circuit may be earthed.
SELV on the other hand will have no means for any connection to earth Regulation 414.4.4. The use of the word ’SHALL’ is used for means to ensure live parts and earth are kept apart by insulation.

Q5. FELV shares many of the attributes of PELV - already explained, and as already explained the real difference is in the source of the secondary circuit (safety Isolating Transformer) which is all about ensuring primary and secondary supplies by reinforced protection are kept apart.
As such, sat here thinking of other examples of a LV/ELV association where there is no protective separation between systems. FELV is where electronic components in an ELV device fed by LV use the neutral in the LV for reference or comms. Others may be able to offer specific examples but the principle stands.
Coffee time.