PELV Earthing

[Billq]

AS/NZS 3000 Clause 1.4.76 describes protected extra-low voltage (PELV) as an extra-low voltage system that is NOT electrically separated from earth, but otherwise satisfies the requirement for SELV

AS/NZS 3000 Clause 4.4.1.2 Plugs and Socket outlets for SELV and PELV systems shall NOT be provided with an earthing contact or pin and shall comply with clause 7.5.10

AS/NZS 3000 Clause 4.4.4.3 (iii) Exposed conductive parts need not be earthed if supplied by an SELV or a PELV system in accordance with clause 7.5

AS/NZS 3000 Clause 7.5.6 PELV Notes (3) The earthing of circuits may be achieved by an appropriate connection to earth within the source itself

AS/NZS 3000 Clause 7.5.10 (C) PELV Sockets shall not have a contact for a protective earthing conductor

My question is:
What reason is there for NOT wanting a Earth Contact on the Plug or Socket Outlet for a PELV circuit bearing in mind one leg of the secondary of the transformer is earthed at the point of supply and by not having the ability to connect an protective earth conductor all the way from the source of supply thorough the Socket Outlet and Plug to the appliance then I cannot see the point of having the secondary of the transformer earthed at the point of supply.

[AlecK]

Your 3rd reference appears to be in error; there is no 4.4.4.3 (iii).
There is a 4.4.3 that's relevant, but it doesn't have any sub-paragraphs.
The words quoted appear to be from 5.4.1.1.
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Perhaps a better question is what would be the advantage / benefit of providing an earth connection at sockets, or earthing ELV appliances?
After all; it's not needed for fault protection by automatic disconnection of supply (which is the main purpose of the PEC in an LV circuit).
Automatic disconnection is about limiting the duration of the shock, and also limiting the touch voltage that arises from the high fault currents involved. But the downside is that things don't keep working once the supply is removed.
With ELV we're still protecting against shock; just doing it in different ways.
We can leave things operating and instead limit the maximum current that can pass through a person

SELV provides a greater level of shock protection than PELV.

Video clip here may help: www.electriciansforums.net/threads/expl ... lv.124246/

[Billq]

AlecK,

4.4.4.3 (iii) is a valid clause on AS/NZS 3000:2007

You give a valid reason for not providing an earth connection at the sockets but it makes me wonder then what is the purpose of of having the secondary of the transformer earthed at the point of supply on PELV ??

I did have a look at the thread and video and it seems to insinuate that the earth could be there because of the characteristics on the appliance been supplied. That seems strange as the appliance will not benefit from being earthed as there is no way to earth it as there is no Earth pin/socket to accomplish that.

[JamesM]

Billq - I think your reference to clause 4.4.4.3 (iii) is actually 5.4.1.1(iii)

[AlecK]

We're used to mostly thinking in terms of LV supply from MEN; and for that earthing - in conjunction with protective devices - is the most common means of providing 'fault protection". What used to be called protection against indirect contact.

That system involves very high fault current to operate over-current devices within time limits.

On ELV we're not doing that; our fault protection is just by it being ELV.
So that's Step 1: why we don't need to earth all those parts.

Step 2 is why earthing of the appliances / sockets is prohibited
My understanding is that it's because anything that is connected to earth can suffer from earth potential rise (EPR).
which is something most sparkies seem not to think about; even though differences in earth potential it's the reason we do equipotetial bonding.

In our LV circuits, one cause of EPR is those high currents in an earth fault situation.
For that; basically the fast operation required limits the time that there's difference in potential between the various parts along the circuit.

But there are many other other things that can cause earth potential rise; and if the potential of our ELV earthing conductor rises, then anything connected to it also rises.
The risk is that EPR can result in hazardous touch voltages between simultaneously accessible conductive parts.
And EPR can be caused by conditions that have absolutely nothing to do with the circuit in question.
Eg event on the distribution system that raise the potential of distribution N,
which then gets carried across via MEN link to the installation's earthing system.

That's one of the down sides of the MEN system we use;
and for the LV stuff we generally achieve an adequate level of safety by earthing (almost) everything and putting up with any EPR.
Dealing with the immediate hazard of things being live at LV takes priority;
and if everything is bonded together then even if it's not at zero V at least it's mostly at same potential.

For ELV, that's a compromise we don't have to make.
By not earthing the appliances; we avoid adverse effects of EPR.
The bodies of the appliances are actually safer not earthed.
In which case the prohibition makes sense.

That may not be all of the reason; but I'm sure it's at least part of it.

-------
The MEN system, that Oz & NZ have used for many years, is in some situations no longer considered the best option.
There are plans to have some installations configured as TT instead of MEN.
A TT system doesn't have a N-E link; so EPR can't be imported into installations from network faults.
However NZ is still some way from making this move - and Oz doesn't want a bar of it.

This is probably not the right place for getting into that more deeply.
Just trying to explain what I think is the reason (or at least a reason) for prohibiting, rather than merely not requiring, earthing of appliances on PELV circuits

[pluto]

The fundamental reason for the use of Extra Low Voltage (ELV) needs to be considered first and AS/NZS 3000:2007 clause 1.5.7 (a) and (b) gives details of the basic requirements.

The next consideration is the electric shock protection of the ELV circuit, there are two options, Safety Extra Low Voltage (SELV) and Protective Extra Low Voltage (PELV), and AS/NZS 3000:2007 section 7.4 gives the detailed requirements of each option.

The reasons for no earth pin on any ELV socket outlet or plug are:-
(i) for SELV the ELV supply is earth-free (or isolated) and earthing is not required or permitted;
(ii) for PELV the ELV supply is earthed on one side of the supply is earthed to limit the maximum voltage to earth, thus a separate earth pin is not required.

[Billq]

The clause I mentioned earlier, 4.4.4.3 (iii) should indeed be 5.4.1.1(iii) (I apologise for the confusion)

Thanks to AlecK and Pluto for the in-depth analysis on this subject it certainly has cleared the methodology behind it.

[AlecK]

The definitions from IEC are useful:

PELV
electric system in which the voltage cannot exceed the value of extra-low voltage:
– under normal conditions and
– under single fault conditions, except earth faults in other electric circuits

SELV
electric system in which the voltage cannot exceed the value of extra-low voltage:
– under normal conditions and
– under single fault conditions, including earth faults in other electric circuits