Explanation of SELV vs PELV?

[John Lundrigan]

Hi All,
The separated extra low voltage system excludes any connections to the earth on the extra low voltage side on the basis that such a connection would import a potentially high voltage on a protective conductor should a fault an earth fault exist on the low voltage side. PELV then actually introduces the protective conductor. Why?
SELV and PELV can be additionally protected by 30mA RCDs - would these be in the primary side? Also it appears that such additional protection is NOT necessary in FELV.
I seem to have got myself hung up on the various configurations and their rationale. Could anyone point me to a fuller explanations of these please?

Thanks

Jonel

 

[The Ghost]

 

[John Lundrigan]

Thank you very much for that very helpful video clip which has helped me to get my thoughts straight. I would be really grateful if somebody could look at what I think of the relation between these three and perhaps give me an example of FELV?

SELV, PELV and FELV all share the common requirement of having a extra low voltage output and being isolated from the LV systemin order to reduce the current through the body so that for the average person contact with the output lines will not be dangerous.

For SELV this is taken further for situations where these measures are not enough (wet room, swimming pool). The Extra Low Voltage end itself is isolated from Earth so that there is no return path with contact on either of the wires.

For PELV the requirements are relaxed in that a protective earth can now be included at the Extra Low Voltage end. This now introduces a return path and so would not be used in the same situation as SELV (bathroom, swimming pool). I'm not entirely sure why the protective earth is introduced, but I am thinking it might be to do with the characteristics of the equipment being supplied (laptop, radio) where parasitic interference may me a problem with the way they work. Also, because the protective earth has been introduced then the supply side of the transformer will now need ADS in the form of 30mA RCD.

For FELV it seems that the low voltage output was available within the range accepted by BS 7671 as Extra Low Voltage but that all the requirements for SELV and PELV are not complied with. So long as 411.7.2 and 411.7.3 - basic protection by: barriers, enclosures, insulation; fault protection by ADS (in the primary) and the extra low voltage). I'm really not clear on where this would be used though.


Thanks

Jonel

 

[Deleted member 26818]

SELV is general used for single items.
Shaver socket, down lights with transformers, door bells, bathroom fans, etc.
PELV is more for distributed systems or circuits which include multiple items of equipment.
PA systems, telephone systems, data control, emergency stop circuits, etc.
FELV, never had much to do with, so don’t know in what circumstances it would be used.

 

[John Lundrigan]

Thank you. Very helpful

 

[Risteard]

Shaver socket

A shaver socket is NOT SELV. (For a start it's not extra-low voltage.) It utilises electrical separation.

 

[Deleted member 26818]

Yes, I often forget that.
I think it’s because they’re both Seperated.
Also some PA systems are 100V, which again is not ELV.

 

[CrinkleCut]

Hi. All very informative.
From reading all above I don't see a SINGLE advantage of PELV. I must have missed something.

SELV: It is suggested that the lack of protective conductor eliminates the chance of electric shock introduced on the SECONDARY side. Without protective conductor to return to earth that shouldn't eliminate shock.
Without this I cant see why ELV is sufficient protection....try shorting out a 12V car battery, never mind 50V.

So if SELV is so great, why bother with the risks introduced by PELV?

 

[Lucien Nunes]

Welcome to the forum. Feel free to introduce yourself in the welcome section.

SELV: It is suggested that the lack of protective conductor eliminates the chance of electric shock introduced on the SECONDARY side. Without protective conductor to return to earth that shouldn't eliminate shock.

Because there is no connection from the circuit to earth, you cannot get a shock to earth because there is no return path for the shock current from earth to the source of energy.

So if SELV is so great, why bother with the risks introduced by PELV?

SELV only works while the isolation from earth is good, so it tends to be applied to single, stand-alone pieces of equipment that individually have a low risk of developing an earth fault. If there is a significant risk of parts of a system becoming earthed accidentally (e.g. because there are 100 separate pieces of equipment joined together by miles of cable) then SELV is unreliable. If any one piece of interconnected equipment or cable develops an earth fault the SELV is violated at the other 99, but there might be no indication of the problem. In this case PELV is a better choice; deliberately earth the supply and assume it is earthed everywhere, rather than rely on every one of the 100 pieces of equipment always being fault-free to provide protection at the others. This is also why public mains supply is TN and not IT, as you can't rely on the insulation of all customers' installations to maintain electrical separation for each other.

Without this I cant see why ELV is sufficient protection....try shorting out a 12V car battery, never mind 50V.

ELV is protection against shock. You don't get shocks from a car battery, you get arc-flash injuries because the battery itself isn't fused and the short-circuit current is very high. Of course SELV and PELV circuits need fuses / circuit breakers to protect against this. ELV protects from shock, OCPD protects from fire / flash burn, although with OCPD there is always a risk of arc or burn on high current circuits at any voltage.