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

[ericmark]

BAS I was basing the construction of bespoke cable on the way portmanteau cables were designed and assembled for various projects.

The inner sheaths are considered as part of the insulation beween LV and ELV ( in one case HV was also involved ) If all the ELV conductors had insulation thick enough to cope with the potential differences between ELV and LV then the overall cable size would be more than doubled.

Not the insulation that is important it is the screening that makes all the difference. Big issue is "Separated" in SELV also means separated from earth. So unless some earthed screening is used the voltage to earth can rise to over the 50 volt limit.

 

[ban-all-sheds]

Yes, but it is a requirement of a cable carrying (e.g.) 230v that the cores are double insulated. That's the core's individual insulation plus the outer sheath.

Blimey.

That's going to b*gger up conduit wiring once word gets out.

 

[ban-all-sheds]

BAS I was basing the construction of bespoke cable on the way portmanteau cables were designed and assembled for various projects.

The inner sheaths are considered as part of the insulation beween LV and ELV ( in one case HV was also involved ) If all the ELV conductors had insulation thick enough to cope with the potential differences between ELV and LV then the overall cable size would be more than doubled.

All the conductors in 4-core flex are insulated enough to cope with the potential differences between ELV and LV...

 

[PBC_1966]

As far as I can see if the ELV supply has one leg earthed then there is little likely hood of either leg voltage to earth raising above the 50 volt limit.

Just what I was thinking reading through this debate. Why would you not earth one side of the 12V supply as a safeguard anyway? And while I have no idea what all the current rules in BS7671 might have to say about it, it's certainly long been considered acceptable to run the two different voltages in the same cable so long as all cores are insulated for the higher voltage. The sheathing requirement for 240V is to the "outside world," not between the 240V and other conductors within the same cable.

 

[JohnW2]

If there is mains voltage in a fixed cable then there must be CPC and that CPC must be earthed even if the equipment supplied is double insulated and doe not require an earth, ....

Surprising though it may be, as often discussed in relation to plumbers, thermostats, etc.,, there is probably no such regulation. There is a requirement for a CPC to be run to 'every point' in a circuit/installation, but that might be achieved by some other cable.

Kind Regards, John

 

[Risteard]

As said there is no requirement for a cable to contain an integral cpc. The only requirement is that a cpc is present at all points in the wiring.

It is also untrue that mains cables need to be "double insulated". There is no such thing as a double insulated cable. A cable is insulated and may or may not be sheathed, which serves as mechanical protection only - even if the sheath is made of insulating material it has not been tested as insulation nor can it be described as such.

 

[ericmark]

As far as I can see if the ELV supply has one leg earthed then there is little likely hood of either leg voltage to earth raising above the 50 volt limit.

Just what I was thinking reading through this debate. Why would you not earth one side of the 12V supply as a safeguard anyway? And while I have no idea what all the current rules in BS7671 might have to say about it, it's certainly long been considered acceptable to run the two different voltages in the same cable so long as all cores are insulated for the higher voltage. The sheathing requirement for 240V is to the "outside world," not between the 240V and other conductors within the same cable.

I think we are all guilty of not thinking about the difference between SELV, FELV and PELV. I read it as SELV must not be earthed, FELV can be earthed and PELV is earthed. I think the internet sites which say SELV stands for Safety extra low voltage which is incorrect it stands for Separated extra low voltage don't help. I see no reason why PELV and LV should not share the same cable as long as insulation is for LV cables. But SELV must not share the same cable unless some earthed shielding is used to stop capacitive and/or inductive links.

As to why we use SELV rather than PELV I really don't know? Other than in an earth free environment. To my mind PELV is safer than SELV so maybe some one can say why we use so much SELV?

 

[stillp]

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.

 

[PBC_1966]

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

 

[JohnW2]

As to why we use SELV rather than PELV I really don't know? Other than in an earth free environment. To my mind PELV is safer than SELV so maybe some one can say why we use so much SELV?

Interesting question. I suppose that the 'attraction' of SELV is that the ELV circuitry is completely isolated from the LV supply/installation - and only if that isolation breaks down would earthing of the ELV circuitry (perhaps) offer some advantage.

As a matter of interest, would you say/feel the same if the proposal was to connect one side of the ELV circuitry to the LV neutral?

Kind Regards, John

 

[bernardgreen]

As a matter of interest, would you say/feel the same if the proposal was to connect one side of the ELV circuitry to the LV neutral?

Which is the same as exporting the "earthed" CPC ( in a PME systems ) if the ELV goes outside the equipotential zone of the property. Which is of course perfectly safe as earth is earth and connot possibly present a hazard of electrical shock even if a person is holding an earthed ( CPC ) item's metal case while standing in bare feet on wet grass. Only d#for some reason it is considered to be unsafe to use anything but double insulated ( none earthed ) equipment in the garden.

Confused ?

 

[winston1]

As a matter of interest, would you say/feel the same if the proposal was to connect one side of the ELV circuitry to the LV neutral?

Which is the same as exporting the "earthed" CPC ( in a PME systems ) if the ELV goes outside the equipotential zone of the property. Which is of course perfectly safe as earth is earth and connot possibly present a hazard of electrical shock even if a person is holding an earthed ( CPC ) item's metal case while standing in bare feet on wet grass. Only d#for some reason it is considered to be unsafe to use anything but double insulated ( none earthed ) equipment in the garden.

Confused ?

Earth may be earth but earth has potential gradients.

With your case of holding an earthed (CPC) items metal case and standing on wet grass in bare feet suppose at the end of the lawn line at 240V got earthed (no RCD) there would be a potential gradient between the end of the lawn and the CPC with your feet somewhere in between say at 120V. Ouch. Worse still would be a HV power line falling at the bottom of the garden or a lightning strike a mile away.

 

[JohnW2]

As a matter of interest, would you say/feel the same if the proposal was to connect one side of the ELV circuitry to the LV neutral?

Which is the same as exporting the "earthed" CPC ( in a PME systems ) if the ELV goes outside the equipotential zone of the property.

That was, of course, partially my point. I had/have a feeling that some of thsoe who would advocate earthing one side of SELV circuitry (making it PELV) might well think differently if it were the LV neutral (a 'live' conductor) - even though, as you say, the two are essentially the same thing in a TN-C-S installation.

Which is of course perfectly safe as earth is earth and cannot possibly present a hazard of electrical shock even if a person is holding an earthed ( CPC ) item's metal case while standing in bare feet on wet grass.

Given the intended membership of this forum, I think that you maybe should add some qualification/explanation to that statement (or, at the least, an appropriate emoticon), since otherwise some readers could well 'believe it'!

Kind Regards, John

 

[RF Lighting]

As a matter of interest, would you say/feel the same if the proposal was to connect one side of the ELV circuitry to the LV neutral?

Which is the same as exporting the "earthed" CPC ( in a PME systems ) if the ELV goes outside the equipotential zone of the property. Which is of course perfectly safe as earth is earth and connot possibly present a hazard of electrical shock even if a person is holding an earthed ( CPC ) item's metal case while standing in bare feet on wet grass. Only d#for some reason it is considered to be unsafe to use anything but double insulated ( none earthed ) equipment in the garden.

Confused ?

Earth may be earth but earth has potential gradients.

With your case of holding an earthed (CPC) items metal case and standing on wet grass in bare feet suppose at the end of the lawn line at 240V got earthed (no RCD) there would be a potential gradient between the end of the lawn and the CPC with your feet somewhere in between say at 120V. Ouch. Worse still would be a HV power line falling at the bottom of the garden or a lightning strike a mile away.

What the heck are you jabbering on about, and how is it even remotely relevant to the subject bring discussed?

 

[JohnW2]

What the heck are you jabbering on about, and how is it even remotely relevant to the subject bring discussed?

People are jabbering about the fact that if the OP left the CPC of the cable connected to the installation's earth/CPC system (rather than using it just for ELV, thereby debriving the cable, and the 'point' to which it ran, of a CPC), as well as using it for one side of his ELV supply (which would then become PELV), there would, at least in electrical terms (and probably regulatory ones, although I'm not certain) probably be no problem in doing what he proposes.

Kind Regards, John