12Vdc SELV constant voltage powersupply

[olly_k]

Seem to be having trouble sourcing a constant voltage power supply (as in the title) suitable for LED's (no or negligible minimum load) that is SELV and capable of approximately 50W

Am I missing something? I do not want constant current, and it has to be SELV. I could live with AC but then it would have to be approximately 8.5V since converting to DC would then push the voltage up to 12.
There are some Philips units but can only find them on their website atm... RS have a perfect range but I cannot find any reference to isolated outputs or SELV compliance so that's a none starter.


Any pointers appreciated!

 

[RandomGrinch]

https://cpc.farnell.com/tiger-power-supplies/tgr-12v-50w/led-driver-12v-4-2a-50w-constant/dp/PW05035

 

[olly_k]

https://cpc.farnell.com/tiger-power-supplies/tgr-12v-50w/led-driver-12v-4-2a-50w-constant/dp/PW05035

Brilliant thank you! Is there a 'standard' for SELV? I can find one for ELV, but the isolated/safety part (class 2 doesn't seem enough to me) seems a little more elusive!

 

[JohnW2]

Brilliant thank you! Is there a 'standard' for SELV? I can find one for ELV, but the isolated/safety part (class 2 doesn't seem enough to me) seems a little more elusive!

If it's ELV 'Class II) (hence doesn't have an earth connection), then it's got to be SELV, hasn't it?

Kind Regards, John

 

[flameport]

SELV is defined in BSEN61140, and referenced extensively in many other standards.
Unless you are a manufacturer of SELV equipment, those standards are of little to no use.

 

[JohnW2]

SELV is defined in BSEN61140, and referenced extensively in many other standards. Unless you are a manufacturer of SELV equipment, those standards are of little to no use.

Fair enough - but wasn't I right in saying that if its ELV and Class II, then it has to be SELV?

Kind Regards, John

 

[olly_k]

Fair enough - but wasn't I right in saying that if its ELV and Class II, then it has to be SELV?

Kind Regards, John

I'm afraid not. There has to be an assurance that the transformer cannot fail short circuit and present mains to the secondary, so it's more complicated than low voltage/class 2. Output voltage must not be allowed to rise due to fault conditions. There are more technical things like extra creepage on the PCB as well... That is why I am wary of anything that does not specifically carry the SELV logo on it, assuming it is CE and reputable that is!

 

[JohnW2]

I'm afraid not. There has to be an assurance that the transformer cannot fail short circuit and present mains to the secondary, so it's more complicated than low voltage/class 2. Output voltage must not be allowed to rise due to fault conditions. There are more technical things like extra creepage on the PCB as well... That is why I am wary of anything that does not specifically carry the SELV logo on it, assuming it is CE and reputable that is

Thanks, that's interesting.

Does that mean that the BS7671 definition of SELV ("An extra-low voltage system which is electrically separated from Earth and
from other systems in such a way that a single fault cannot give rise to the risk of electric shock.") is an over-simplification - in other words that, in the eyes of BS7671, if an item is Class II but not SELV, then a single fault may "give rise to the risk of electric shock" (which would rather surprise me!)?

Kind Regards, John

 

[bernardgreen]

My way of think is that SELV ( Safe or Separated ELV ) can only be ensured by use of a transformer with either a split bobbin or dual bobbins

If it's ELV 'Class II) (hence doesn't have an earth connection), then it's got to be SELV, hasn't it?

No. lack of Earth to the power supply will allow a faulty supply to put mains onto the output without the RCD ( if there is an RCD ) tripping. Earthing the laminations will improve the safety of the supply

 

[JohnW2]

No. lack of Earth to the power supply will allow a faulty supply to put mains onto the output without the RCD ( if there is an RCD ) tripping. Earthing the laminations will improve the safety of the supply

Whilst I don't disagree with that, it presumably can't be a requirement of SELV, since many (most? all?) SELV PSUs are Class II, and hence have no earth connection.

Kind Regards, John

 

[ericmark]

I seem to remember SELV, FELV, and a third can't remember letters, one must not have an earth, one must have an earth, and one earth is optional.

SELV must not have an earth, but many Class II have a parking place for an earth, and SELV would be Class III.

So only way to know if a power supply is SELV is it to be written on it, likely if marked Class II it is SELV, but like PME and TN-C-S they are not always the same.

 

[JohnW2]

SELV must not have an earth, but many Class II have a parking place for an earth, and SELV would be Class III.

As far as I can make out, Class III probably refers to items which have an external SELV supply ..

Class III equipment. Equipment in which protection against electric shock relies on supply at SELV and in which
voltages higher than those of SELV are not generated (see BS EN 61140).

Is my understanding correct.?

like PME and TN-C-S they are not always the same.

PME and TN-C-S obviously relate to totally different things/concepts but, at least in the UK, they really have to go hand-in-hand - if a supply is PMEd, then it may be regarded (and used) as TN-C-S, whereas if it is not PMEd, it cannot be used as TN-C-S.

Kind Regards, John

 

[ericmark]

Class III equipment. Equipment in which protection against electric shock relies on supply at SELV and in which
voltages higher than those of SELV are not generated (see BS EN 61140).
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.
PELV (protective extra-low voltage). An extra-low voltage system which is not electrically separated from Earth,
but which otherwise satisfies all the requirements for SELV.
SELV (separated extra-low voltage). An extra-low voltage system which is electrically separated from Earth and
from other systems in such a way that a single-fault cannot give rise to the risk of electric shock.

I knew there were three, but could not remember the names. So Class III is SELV. But it is the marking of SELV equipment to show it is SELV, I looked at some items I have and was directed to https://owim.de/ and prompted to enter number, however it came up with error.

All the transformers seemed to have the class II sign

Class II equipment. Equipment in which protection against electric shock does not rely on basic insulation only,
but in which additional safety precautions such as supplementary insulation are provided, there being no provision
for the connection of exposed metalwork of the equipment to a protective conductor, and no reliance upon
precautions to be taken in the fixed wiring of the installation (see BS EN 61140) .

I would have thought the output of a class II power supply would have to be SELV, lamps like these

would need an SELV supply, as the catenary wire can be touched. But at 5 meter long is would need a toroidal transformer anyway or it could transmit. I looked for one and it was IP 68 and said "They can be submersed in water as the winding is completely encased in epoxy resin and water cannot get inside. these transformers come with a 5 year guarantee." but did not say SELV.

So yes I can see the problem, as I can't find a power supply which says SELV.

 

[JohnW2]

So Class III is SELV.

As I said, my understanding of Class III items is that not only must they be powered by a SELV source, but that the SELV has to come from some external source (PSU), hence no voltages above ELV within the Class III item (and with a single fault in the SELV source not able to result in a voltage >ELV appearing within the Class III item) - an example being a laptop. Since the item itself is therefore intrinsically 'safe', there is no requirement for the double/reinforced insulation required of a Class II item.

I suppose that, theoretically speaking, any entirely battery-powered item would probably technically qualify as Class III (and SELV) ?

I would have thought the output of a class II power supply would have to be SELV ....

I don't think so - in fact, thinking that was essentially my initial mistake. As bernard and others have pointed out, and as the definitions you have just posted indicate, SELV goes a a fair bit further than Class II .... Class II only requires that there be double or re-inforced insulation between a user and any parts normally at LV potential but, as I understand it (but would be very happy to be corrected, if wrong), SELV also requires that a single fault within the electronics cannot result in the potential of 'touchable' conductors (such a s the output of an SELV PSU) rising to dangerous levels.

So yes I can see the problem, as I can't find a power supply which says SELV.

You probably have not looked hard enough because I think, for example, that most (maybe even all) of the (constant-voltage) LED PSUs I've seen (often called "drivers" ) have been marked "SELV".

Kind Regards, John

 

[olly_k]

As I said, my understanding of Class III items is that not only must they be powered by a SELV source, but that the SELV has to come from some external source (PSU), hence no voltages above ELV within the Class III item (and with a single fault in the SELV source not able to result in a voltage >ELV appearing within the Class III item) - an example being a laptop. Since the item itself is therefore intrinsically 'safe', there is no requirement for the double/reinforced insulation required of a Class II item.

I suppose that, theoretically speaking, any entirely battery-powered item would probably technically qualify as Class III (and SELV) ?

I don't think so - in fact, thinking that was essentially my initial mistake. As bernard and others have pointed out, and as the definitions you have just posted indicate, SELV goes a a fair bit further than Class II .... Class II only requires that there be double or re-inforced insulation between a user and any parts normally at LV potential but, as I understand it (but would be very happy to be corrected, if wrong), SELV also requires that a single fault within the electronics cannot result in the potential of 'touchable' conductors (such a s the output of an SELV PSU) rising to dangerous levels.

You probably have not looked hard enough because I think, for example, that most (maybe even all) of the (constant-voltage) LED PSUs I've seen (often called "drivers" ) have been marked "SELV".

Kind Regards, John

The problem I had when looking, was that there was no confirmation of SELV apart from some constant current supplies, but I did not want CC!
So really the suppliers should be a little more proactive and clarify if the item is specified as a SELV unit or not. As you say, most probably are, but even the datasheets often don't contain enough information to determine if SELV or not, and so therefore it should be assumed they are not, unless it is printed on the enclosure, and you can be sure the enclosure is of a reputable brand!
Anyway, thanks to all the folks who have responded. Turns out to have been quite an interesting subject!