Acceptable to use earth in 3C+e to carry 12v DC? (read before replying!)

[ericmark]

OK from what is said there have been changes with amendment 3? With BS7671:2008 we have:-

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.

and

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.

What are the new definitions in amendment 3 then?

 

[Echo the husky]

There are no changes to those 2 definitions in the amendment 3:2015 book.

Don't you have your own copy?!

 

[ban-all-sheds]

I have a copy but right now CBA to look in it - would implementing Bernard's oft repeated suggestion to tie a SELV output to earth conflict with what the regulations require v-a-v special location zones?

 

[PBC_1966]

What do special locations have to do with it, unless the thermostat wiring is in the bathroom?

 

[ban-all-sheds]

Again, subject to the CBATLRN, the requirements for SELV are only relevant to special locations.

 

[Risteard]

I wish you were sorrect Eric, but it seems the official meaning of SELV is now Safety Extra Low Voltage. I always knew it as Separated ELV too.

It's both actually.

In installation standards it is Separated Extra-Low Voltage, but I believe appliance standards may refer to Safety Extra-Low Voltage.

 

[Risteard]

I'm losing track of all these changes - What's PELV?

Protective Extra-Low Voltage.

 

[Echo the husky]

Here's some:
414.4.2 iii
Circuit conductors at voltages higher than band I may be contained in a multi conductor cable or other grouping of conductors if the SELV/PELV conductors are insulated for the highest voltage present.

So a standard 4/5 core flex would be OK?

414.1.1 v
For SELV systems only, basic insulation [required] between the SELV system and earth.

414.4.1
SELV circuits shall have basic insulation between live parts and earth.

And in inspection and testing
612.4.1 Protection by SELV
The separation of the live parts from those of other circuits and from earth, according to section 414, shall be confirmed by a measurement of the insulation resistance.

For special locations:
Where SELV or PELV is used, whatever the nominal voltage, basic protection shall be provided by:
i Basic insulation complying with 416.1 or
ii Barriers or enclosures complying with Regulation 416.2

would implementing Bernard's oft repeated suggestion to tie a SELV output to earth conflict with what the regulations require v-a-v special location zones?

Apparently not, though the circuit is no longer SELV it is now PELV.
I can't find anything that mentions that PELV is prohibited in special locations, though I was always taught it was prohibited. Would SELV not be safer than PELV when it comes to reducing the risk of electric shock?

 

[JohnW2]

Would SELV not be safer than PELV when it comes to reducing the risk of electric shock?

As I implied in waht I wrote earlier, I think it's probably 'swings and roundabouts'. So long as the SELV remains 'separated', it's probably safer than PELV, not the least because it reduces the number of earthed things around which one could touch at the same time as, say, an LV line conductor. However, if the SELV 'goes wrong', and LV gets into it, then to have it connected to earth is probably safer.

Kind Regards, John

 

[bernardgreen]

So long as the SELV remains 'separated'

When source of the ELV is a wound transformer with separate bobbins for the primary (mains) winding and the secondary (ELV) winding then loss of separation is very unlikely and would only occur if the transformer became hot enough to melt both the bobbins and the insulation on the wires.

A switched mode power supply unit ( SMPSU ) is a totally different matter when considering the risk of loss of separation. Mains and ELV tracks on a PCB are separated only by a few millimetres and in the high frequency transformer the mains and ELV windings may be separated by a few microns of insulation on the wires. One component failure can create deposites that bridge the separation on the PCB or cause the transformer's insulation to break down. Mains side and ELV side are now connected. If the fuse supplying the SMPSU blows then the ELV side is "safe" but it is not always the case that the fuse does blow. The burnt out SMPSU may have no connections left on its PCB between Live and Neutral so takes no power to blow the fuse but can and often does have "new" unwanted connections between the Live and the ELV via bridges of soot across the separation barrier(s) and / or via failed insulation in the transformer. The ELV is Live relative to Ground but appears to be dead as the lamps or bells or what ever uses the ELV are no longer working..

Grounding one side of the ELV would when separation is lost provide a path for current from Live to ground which would probably trip an RCD if there was one protecting the supply to the PSU

Wound transformers where the windings are in the same bobbin do carry a risk of loss of separation if they over heat but a well designed one will have a metallic screen between the windings and this screen is normally earthed to provide a path to ground should insulation fail and thus RCD or fuse shopuld operate.

 

[JohnW2]

So long as the SELV remains 'separated'

When source of the ELV is a wound transformer with separate bobbins for the primary (mains) winding and the secondary (ELV) winding then loss of separation is very unlikely ...

Indeed, but, in terms of the big picture (domestically), that is a relatively rare situation.

A switched mode power supply unit ( SMPSU ) is a totally different matter when considering the risk of loss of separation....

Quite so - and since SMPSUs represent the great majority of domestic ELV sources, that was my point.

Grounding one side of the ELV would when separation is lost provide a path for current from Live to ground which would probably trip an RCD if there was one protecting the supply to the PSU

Again that is the 'swings' part of the point I made. The 'roundabouts' part arises if the ELV equipment has any exposed conductive parts which are in electrical continuity with 'earth'/'ground'/CPCs. In that situation, the grounding of one side of the ELV increases the number of possible 'second points of contact' for someone was was simultaneously in contact with the LV supply.

Kind Regards, John