How to get Part P qualified?

[bernardgreen]

50 Volt SELV always has been based on Telephone exchanges,

The Positive pole of the supply is connected to Earth at the exchange so it is ELV and not SELV

 

[JohnW2]

50 Volt SELV always has been based on Telephone exchanges, a nominal 48 Volt system, that when fully charged would have 56.x Volts on it, in recent years more testing and looking at automotive "Load dumps" has led to an increase to 63 Volts, I must have missed it creeping up again to 70! It is based on not being able to have a sustained arc in the dc circuit when a switch is opened.

I have always assumed that the 50V limit of ELV presumably related to a perception of what voltage was 'fairly safe'. However, for someone with, say, wet hands, 50V can be more than enough to kill, so I'm not sure that it is all that 'safe' an approach!

Kind Regards, John

 

[Mike13]

Safety Extra Low Volts has always been about not having to use a "contactor" to break the supply, it is seen as being unable to sustain an arc. In extreme circumstances les than 6 volts can kill! dip each hand in a separate bowl of salty water, connect 6 volt bike battery across both bowls, could easily be fatal.

 

[bernardgreen]

What is Separated Extra Low Voltage (SELV)
Separated Extra Low Voltage (SELV) system is an extra low voltage electrical circuit that is electrically separated from other circuits that carry higher voltages, isolated from the earth and from the protective earth conductors of other circuits.

 

[JohnW2]

Safety Extra Low Volts ....

As bernard has said, NO. SELV stands for Separated Extra Low Voltage.

The "S" of SELV or the "F" of FELV are irrelevant to this discussion. They only relate to earthing (or the absence of it), but both are "ELV" as defined by the voltage alone.

... has always been about not having to use a "contactor" to break the supply, it is seen as being unable to sustain an arc.

I personally find that hard to believe, for any variant of ELV.

In extreme circumstances les than 6 volts can kill! dip each hand in a separate bowl of salty water, connect 6 volt bike battery across both bowls, could easily be fatal.

That's the very point I made. Nevertheless, in a number of contexts BS7671 appears to take the view that 'protection from electric shock' can be achieved by limiting "touch voltages" to 50V (AC).

Kind Regards, John

 

[Mike13]

I stand corrected on the S bit, but I worked on a 50 V 500A battery charger many years ago and the belief then was that the ELV part was about being safe with a simple switch as much as about being non lethal.

 

[JohnW2]

I stand corrected on the S bit, but I worked on a 50 V 500A battery charger many years ago and the belief then was that the ELV part was about being safe with a simple switch as much as about being non lethal.

I think we are probably talking at cross-purposes.

I don't doubt that, in some contexts, a 50V limit is relevant in terms of arcing (particularly for DC) but, as I said, BS7671 (and probably also those who invented the IEC definition of 'ELV') seems to regard a "touch voltage" less than 50V (AC) as being 'fairly safe' in terms of shock risk.

Also remember that in the context of this forum (and associated regulations etc.) we are usually talking about AC, whereas you are primarily talking about DC - and, at least as far as BS7671 is concerned, "ELV" is <50VC AC or <120V ripple-free DC. In other words, 119 volt DC would count as "ELV".

Kind Regards, John

 

[plugwash]

I understood the different types of ELV as.

SELV, separated from the mains and from mains earth by an isolation barrier that meets certain safety standards.
PELV, separated from the "live conductors" of the mains but not separated from mains earth.
FELV, not separated from the mains or seperated by an isolation barrier that is not safety rated..

 

[JohnW2]

I understood the different types of ELV as.
SELV, separated from the mains and from mains earth by an isolation barrier that meets certain safety standards.
PELV, separated from the "live conductors" of the mains but not separated from mains earth.

Those correspond with the spirit of the BS7671 definitions.

...FELV, not separated from the mains or seperated by an isolation barrier that is not safety rated..

In this case, the BS7671 definition is much more vague, so I don't think necessarily restricts FELV to only what you suggest ...

Functional extra-low voltage (FELV). An extra-low voltage system in which not all of the protective measures
required for SELV or PELV have been applied.

Kind Regards, John