Selv Pelv Felv

[TJC1]

Can you guys give me an example of what selv pelv and felv are?
Cheers

 

[telectrix]

BS7671... part 2... definitions.

 

[overskilled]

Selv - I'm probably going to get this wrong (late n brain mush) Separated Extra low voltage. No connection to earth at all and fed solely from the secondary side of an isolating transformer.
FElV -functional extra low voltage - has a connection to earth, despite being fed by a transformer.
PELV - ...... IIRC Protected Extra Low Voltage, off the top of my head....I don't recall this one. ...It sits between SELV and FELV though.

 

[Richard Burns]

ELV Extra low voltage, this is in general a protective measure because these voltages present a much lower shock risk.

For areas of increased risk of electric shock this may not be enough and so the circuit is electrically separated from the supply by a safety isolating transformer. This is then a SELV system.
The circuit must have no earth connection and no risk of crossover onto LV circuits. Generally used in bathrooms for 12V equipment, but often not well segregated from LV circuits (think shower cables crossing over the 12V cables of the extract fan.

If the circuit is connected to earth then it becomes a PELV system and uses ADS on earth faults and is not as safe as SELV but still needs a safety isolating transformer so that the live conductors do not have an LV element to them. This may be used where the risk of electric shock is not so great and the earthing of the ELV will not cause a significant risk. A control circuit at ELV that must be earthed by the regulations becomes a PELV circuit.

If the circuit just happens to have an extra low voltage but is supplied by a transformer with simple separation and has an earth connection then this is FELV it is not really a protective measure on its own as it is using ADS as normal for LV circuits it may be that in a control panel the control signals are ELV but not otherwise fully separated from the surrounding LV circuits and so this is FELV.