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For what does PELV require protective conductor? And also what is differences between PELV and FELV?

Former Community Member
Former Community Member
For what does PELV require protective conductor? And also what is differences between PELV and FELV? Both of them require PE conductor


  • 0
    SELV is the double insulated version of ELV and is 2 faults to danger by insulation at the supply end.


    PELV has a CPC  for 2 reasons.

    firstly, for the same reason that the mains does - to provide prompt disconnection of faults from the 'live' side to ground that might otherwise go un-noticed, and

    secondly to prevent a failure of the power supply that generates the extra low voltage from the mains, from impressing the mains voltage on the output and not being noticed. Imagine a transformer with a short at one point part way up the primary and part way the secondary.. This allows a single insulated power supply construction.


    FELV a catch all for any ELV kit that is not really PELV or SELV, including designs where there is no isolation  between the LV and ELV parts of the circuit. Therefore the protection requirements for the higher voltage have to be applied to the entire circuit.

    Examples include LED lights with a resisitive or capacitive dropper from mains to 12V or whatever the LED needs, and the internals of things like PIRs and some linked smoke alarms where the internals are at 9V or 12V, but use the neutral as a common reference and so the ELV parts are not isolated from the mains.

     




  • 0
    PELV may also have earthing for functional reasons, i.e. the protective conductor is a combined protective and functional earthing conductor. This may be, for example, for electromagnetic compatibility, measurement purposes, or discharge of static electricity.


    BUT - that does not make it FELV.
  • 0

    alex.white:
    For what does PELV require protective conductor? And also what is differences between PELV and FELV? Both of them require PE conductor


     




    Have a look at The label on the back of the shower waste pump in the illustration it appears you can have double insulated functional isolation transformers without a PE conductor.


    But they must be 30 mA RCD protected even if the transformer is mounted outside of the shower room, other manufacturers make similar shower waste pumps which are SELV, but the transformer is within the pump unit rather than a separate controller, do they still need RCD protection as the pump unit is generally within the shower room.


     Andy Betteridge

  • 0

    For what does PELV require protective conductor? And also what is differences between PELV and FELV? Both of them require PE conductor



    I'm not sure a PELV circuit requires a protective conductor at all - it may have one if you want (e.g. for functional reasons), but as far as I can see it's not actually required by regs. PELV circuits (unlike SELV ones) may be connected to Earth - but that connection need not involve a protective earth conductor - as it's not required for protection against electric shock as far as the PELV circuit is concerned. Any old means of earthing, or just a potential for fortuitous contact with earth (say lack of insulation on one pole in an earthy environment), would be enough to make a circuit PELV rather than SELV.


    By comparison SELV circuits are definitely 'earth free' so may be preferred in some situations - e.g. where hazardous voltages imposed on your local "earth" connection (e.g. during faults on the LV system) might be a problem - say if your ELV circuit ran outdoors or otherwise beyond the LV installation's equipotential zone.


    Both SELV and PELV systems require a safety source - i.e. something with protective separation between the ELV circuit any LV primary (e.g. the equivalent of a transformer with double/reinforced insulation between the LV and ELV windings) - so neither need worry about being exposed to mains voltage due to a trasformer fault. FELV on the other hand permits only simple separation - e.g. the equivalent of just single insulation between primary and secondary windings - so a single fault could impose mains voltage on the ELV circuit. Hence FELV circuits usually need to have insulation rated at the primary circuit's (e.g. mains) voltage. PELV and SELV circuits need only have insulation for their actual volatge (e.g. 12V).


      - Andy.
  • 0

    FELV on the other hand permits only simple separation - e.g. the equivalent of just single insulation between primary and secondary windings - so a single fault could impose mains voltage on the ELV circuit.




    Not even that , a FELV may be designed to not any isolation at all between the LV and ELV, in normal operation. The example of the ELV signal relative to neutral that between the linked kidde smoke alarms,  is also an example of FELV. So the cable of the ELV link needs the same insulation and respect as the L and N.


  • 0

    I'm not sure a PELV circuit requires a protective conductor at all - it may have one if you want (e.g. for functional reasons), but as far as I can see it's not actually required by regs.


    PELV circuits (unlike SELV ones) may be connected to Earth - but that connection need not involve a protective earth conductor - as it's not required for protection against electric shock as far as the PELV circuit is concerned.



    There are situation where PELV systems require a protective conductor to connect exposed-conductive-parts, see 414.4.5. Arguably, this would necessarily be separate to the Neutral (earthed conductor), see notes to Figures 9H and 9I in Appendix 9, regardless of the voltage concerned.


     Any old means of earthing, or just a potential for fortuitous contact with earth (say lack of insulation on one pole in an earthy environment), would be enough to make a circuit PELV rather than SELV.



    Not where the voltage of the PELV system does not exceeds 25 V AC or 60 V DC, because the lack of insulation on one pole implies that basic protection is not present, and if the voltage exceeds 25 V AC / 60 V DC then basic protection is required or the conductor installed in an earthed metal enclosure or containment system, see 414.4.5. Alternatively, perhaps it becomes a PEN or PEM conductor in those conditions, again where the voltage exceeds 25 V AC / 60 V DC?



    And just to reiterate, where earthing is required for functional purposes, it may well be necessary for this to be a separate conductor (or "screen"), simply because of the job it's doing (e.g. EMC) - basically, not always best to shove the "noise" down the circuit conductor.

  • Former Community Member
    0 Former Community Member

    mapj1:

    SELV is the double insulated version of ELV and is 2 faults to danger by insulation at the supply end.


    PELV has a CPC  for 2 reasons.

    firstly, for the same reason that the mains does - to provide prompt disconnection of faults from the 'live' side to ground that might otherwise go un-noticed, and

    secondly to prevent a failure of the power supply that generates the extra low voltage from the mains, from impressing the mains voltage on the output and not being noticed. Imagine a transformer with a short at one point part way up the primary and part way the secondary.. This allows a single insulated power supply construction.


    FELV a catch all for any ELV kit that is not really PELV or SELV, including designs where there is no isolation  between the LV and ELV parts of the circuit. Therefore the protection requirements for the higher voltage have to be applied to the entire circuit.

    Examples include LED lights with a resisitive or capacitive dropper from mains to 12V or whatever the LED needs, and the internals of things like PIRs and some linked smoke alarms where the internals are at 9V or 12V, but use the neutral as a common reference and so the ELV parts are not isolated from the mains.



     


     

     




    So many thanks for replying. Please help me also to know about these questions:

    1- you said for PELV "to provide prompt disconnection of faults from the 'live' side to ground that might otherwise go un-noticed" and "to prevent a failure of the power supply that generates the extra low voltage from the mains, from impressing the mains voltage on the output and not being noticed". These could happen for SELV, but it have no CPC. In SELV why we have not any concern about un-noticed? Because of double insulated in SELV?

    2- Is double insulated, requirement for SELV?

    3- Please give me some practical application example for PELV. (For SELV I know: ZONE 0 of bathroom and for FELV you gave LED lights example).

  • 0
    There is a general principle with all electrics since about 1970, that equipment and installations must remain safe until two independent things have failed.


    There are basically 2 common approaches that achieve this 'double fault to danger' level of protection.


    For earthed mains equipment and PELV, the two things that might fail are the earth connection, which is needed to blow the fuse or trip if the mains gets out, and the basic insulation, which is the mechanism by which the mains does get somewhere where it may be accessed - remember PELV wire itself does not need insulation to withstand mains voltages, so  in principle if mains is on the bell wire or whatever, it is considered to be already accessible.


    For double insulated mains equipment, and SELV systems, we have basic and reinforced insulation, so two things have to fail - for example in a split bobbin transformer, an insulation fault that makes the transformer core live still does not make the secondary live, unless there is a second fault connecting the secondary circuit to the core as well, as the secondary bobbin is also insulated from the core to the same standard as the primary.


  • 0

    alex.white:




    mapj1:

    SELV is the double insulated version of ELV and is 2 faults to danger by insulation at the supply end.


    PELV has a CPC  for 2 reasons.

    firstly, for the same reason that the mains does - to provide prompt disconnection of faults from the 'live' side to ground that might otherwise go un-noticed, and

    secondly to prevent a failure of the power supply that generates the extra low voltage from the mains, from impressing the mains voltage on the output and not being noticed. Imagine a transformer with a short at one point part way up the primary and part way the secondary.. This allows a single insulated power supply construction.


    FELV a catch all for any ELV kit that is not really PELV or SELV, including designs where there is no isolation  between the LV and ELV parts of the circuit. Therefore the protection requirements for the higher voltage have to be applied to the entire circuit.

    Examples include LED lights with a resisitive or capacitive dropper from mains to 12V or whatever the LED needs, and the internals of things like PIRs and some linked smoke alarms where the internals are at 9V or 12V, but use the neutral as a common reference and so the ELV parts are not isolated from the mains.



     


     

     




    So many thanks for replying. Please help me also to know about these questions:

    1- you said for PELV "to provide prompt disconnection of faults from the 'live' side to ground that might otherwise go un-noticed" and "to prevent a failure of the power supply that generates the extra low voltage from the mains, from impressing the mains voltage on the output and not being noticed". These could happen for SELV, but it have no CPC. In SELV why we have not any concern about un-noticed? Because of double insulated in SELV?

    2- Is double insulated, requirement for SELV?

     



    To understand 1 and 2 fully, I would suggest you read the standards (BS 7671 Chapter 41, which is IEC 60364-4-41 internationally - or alternatively IEC 61140 which is common requirements for installations and equipment) and come back with any questions. There are a number of requirements for SELV, which mean it's "separated" from the mains and from Earth, but, because the voltage is limited, double insulation is not required for protection against electric shock. There are conditions on the source of supply for SELV and PELV.



    3- Please give me some practical application example for PELV. (For SELV I know: ZONE 0 of bathroom and for FELV you gave LED lights example).



    PELV might be used in


    • electronic equipment that requires a functional earth, say for EMC or static control. It's often used in PCs, and some laptop power supplies, for example.

    • data networking equipment operating at high frequencies, which can cause isolation barriers to charge statically

    • machinery, where it is generally the preferred source of supply for DC controls, because faults can be detected (and this could be a safety issue for machinery control systems, for example).

    • other equipment where the detection of faults is key to safety or operation

    • Certain cases where the requirements of SELV can't be met.

  • 0
    Our posts overlap. I am being a bit arm waving, and simplifying a bit, and Graham is more rigorous with the terminology , and yes it is not strictly called double insulation  for SELV - much as a mains lead is not called double insulated, it is insulated and sheathed, but both the sheath and the insulation on the wire inside have to fail, and they are contrasting colours so hopefully damage to the outer is visible before the copper is exposed to touch.

    The supply to SLEV equipment has to be designed to have more things that would have to fail before the primary mains voltage gets to where it should not. PELV permits less insulation, in principle just basic insulation on the primary,  on the clear understanding that the earth connection will cause automatic disconnection if the mains breaks through.

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