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

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?

 

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).

• 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.

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?

 

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).

• 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.