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Topic Title: The trouble with TT
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Created On: 04 October 2012 08:21 PM
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 04 October 2012 08:21 PM
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jcm256

Posts: 1895
Joined: 01 April 2006

From Autumn Wiring Matters on EV charging.

Quote:
Without data, the committee was
unable to conclude that the risk of using
a PME earth was low enough to consider
a PME earthing arrangement suitable to
supply charging equipment for outdoor
connecting points.
Subsequent to this decision, and
recognising that there are limited
options available to the installer, as well
as other risks associated with using a TT

What do you think the risks associated with using TT would be?
The only things I can think is:
1) RCD may go faulty:
2) May trip during charging, result flat battery in the morning, resulting dent in car wing or charger.
3) From experience having inspected great number of garages statutory Electrical mechanical plant. A number of 13amp double or single socket outlets with integral RCD, were found faulty (Welded RCD) together with bridged plug- top fuse,, because no heed of manufactures instructions was taken , that was the instruction booklet asked for a 16amp socket outlet backed up with a 20amp circuit protective device be provided. Continuous flow of current of many hours does cause overheat damage, as we all know from 13amp FSU for immersion heaters and storage heaters.
4) Anything else you can think off please, of the risks associated with TT.
 04 October 2012 09:29 PM
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davezawadi

Posts: 2713
Joined: 26 June 2002

Oh dear, a lack of competence in a committee? Such is the nature of the beast, just look at BS7671. If PME is OK for a domestic supply, it must be OK for a charging point with no external conductive parts. Another local earth electrode covers local touch potentials, what else is there?

-------------------------
David
CEng etc, don't ask, its a result not a question!
 05 October 2012 09:35 AM
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peteTLM

Posts: 3187
Joined: 31 March 2005

Id say the potential loss of the electrode, or the lead going to it.
In london, the streets are dug up with such regularity its an accident waiting to happen. The civils firms doing the work generaly have complete disregard for other operators plant buried in the ground, mainly due to the congestion in the pavement and elsewhere. There is just no earth(or other material) left, its virtually everything on top of each other, and everyman for himself.
Could this be it? Its a localised problem not found elsewhere.
(ex civils planner + other things)

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----------------------------------------
Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 05 October 2012 10:06 AM
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AJJewsbury

Posts: 11553
Joined: 13 August 2003

If PME is OK for a domestic supply, it must be OK for a charging point with no external conductive parts.

I suspect the connected vehicle might be external, metallic and likely connected to the supply PE when on charge.

4) Anything else you can think off please, of the risks associated with TT.

There's the perennial problem of having two different earthing systems in proximity to each other. You might TT the charge point, and hence the vehicle, but if there's an outside tap, or a lamppost etc not just within reach of the charge point but of any connected vehicle too (which could be 4+ metres away, + lead length), you still risk potential differences.
 05 October 2012 07:58 PM
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davezawadi

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But I dealt with that Andy, local electrode, no significant potential to earth? That is the entire basis of PME, are you disputing its safety? Perhaps you have something other than an earth fault in mind? I am at a loss as to your thoughts here, because the car is insulated from ground, although presumably connected to the local electrode. Just where does this "potential" come from?

-------------------------
David
CEng etc, don't ask, its a result not a question!
 05 October 2012 08:26 PM
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John Peckham

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Joined: 23 April 2005

David

Cars connected to a PME supply are a metal box on rubber tyres just like a caravan. PME supplies to caravans are not permitted I assume for the lost neutral scenario. If you loose the neutral a car connected to a PME supply earth will become live if the neutral is lost depending on if the pipework is bonded or plastic. A person touching the metal work of the car (door handle) in this situation may receive a fatal shock depending on the dialectic strength of their footwear.

I have a nice photo of a seat of a fire started by a lost neutral on a PME supply taken recently by me where the neutral current went down a oil pipeline.

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 05 October 2012 08:59 PM
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UKPN

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---well, it could well be a "caravan"! fed from what is essentially a
"highway" power supply.

we dont give a CNE connection to these items, so it will be interesting
to see what the wiring regs bods come up with on this one.

I have been involved in a couple, where the customer asked for
advice, but our supplies were existing and SNE.

perhaps an all insulated structure, class 2?

but CNE/PME outside? I think we should pass on that.

Regards.
 05 October 2012 10:47 PM
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slittle

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Let's face it, we all know that it will trundle along with every EV owner doing there own thing and a few folk installing charging points for them on pme supplies with no consideration, no certification etc until something happens.

Then one night on the way home from the pub, Joe Smith will touch or possibly worse relieve himself on a vehicle connected to a faulty supply and become another statistic, then there will be public outcry, a need for a scheme to certify installers of electric vehicle charging points and a money making scheme will appear.

I have to admit that although I'm happy to play the game, I'm starting to get a little bored of it when my guys jump through hoops and others don't.

Stu
 08 October 2012 11:12 AM
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AJJewsbury

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But I dealt with that Andy, local electrode, no significant potential to earth? That is the entire basis of PME, are you disputing its safety? Perhaps you have something other than an earth fault in mind? I am at a loss as to your thoughts here, because the car is insulated from ground, although presumably connected to the local electrode. Just where does this "potential" come from?

Local electrodes make little difference to the voltage on CNE - just try Ohm's Law with an electrode of a few tens of Ohms with a load of several kW. The rise in the local ground voltage will decay rapidly with distance from the spike - conditions vary a lot of course, but on the surface 5+ metres away I doubt it would be closer to CNE voltage (perhaps up to 230V in an open CNE condition) than true earth.
AFAIK PME is only considered "safe" where a bonding can establish (what used to be called) an equipotential zone. Outside taps bonded to PME are supposed to have insulating inserts. Electrical gardening equipment is mostly class II. PME outdoors (where earth is needed) is a problem. I guess the assumption is that EVs (with chargers on-board) will be class 1.

- Andy.
 08 October 2012 11:36 AM
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broadgage

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Could not the risks of PME supplies for EV charging be reduced by use of a 3 pole RCD that dissconects all 3 conductors in the event of a fault, including the CPC.

An open neutral on the supply could result in the metallic body of the car becoming live at a dangerous voltage.
A 3 pole RCD would protect against this, since any dangerous leakage of current between the car and the general mass of earth, would trip the device and disconnect all conductors.

Whilst some would argue against the dissconection of the CPC under any conditions, it should be no worse than pulling out the plug.

Presume that under normal conditions the PME terminal is 10 volts above true earth, and that the resistance between the car bodywork and true earth is a few thousand ohms (reasonable for rubber tyres that are wet) That would allow a few ma to pass and not trip a 30ma RCD.

Now consider a fault on the supply that results in 230 volts between the car and true earth.
In wet weather the RCD will trip instantly.
In dry weather, the car might remain at a dangerous voltage, but only until someone touches it, when the RCD would trip instantly.
 08 October 2012 12:44 PM
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timothyboler

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Originally posted by: broadgage

Could not the risks of PME supplies for EV charging be reduced by use of a 3 pole RCD that dissconects all 3 conductors in the event of a fault, including the CPC.

An open neutral on the supply could result in the metallic body of the car becoming live at a dangerous voltage.

A 3 pole RCD would protect against this, since any dangerous leakage of current between the car and the general mass of earth, would trip the device and disconnect all conductors.


Draw out the path of current. What would happen is that the "fault" current would travel down the Line back through the Neutral (RCD in balance) then back down through the earth lead to the car, through the person to true earth.

There's a scheme here that's been proposed before on this forum similar to what you've described. But, as you suggested is not compliant owing to the switching of the PEN conductor. You also need to drive a supplementary rod for it to operate.

Anyway I think the point is that the IET could conclude that the risk of a broken neutral only is in fact a very remote risk anyway for domestic installations (at least for supplies via buried cable).

If it is found to be a significant risk then there's a few options:
1) Supplementary earth electrodes for PME households. This is the standard in America. (Won't help if the earth resistance is high though).
2) Supply the charger through an isolation transformer inside the house.
3) TT the charging point.

Regards, Tim

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Everyone loves a fireman - but hates the fire inspector.
 08 October 2012 01:05 PM
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whjohnson

Posts: 739
Joined: 24 January 2009

The worst problem with TT in my experience is the cutting off and removal of the earthing conductor - "that horrible nasty green wire running around my new flower bed - I cut it off and it didn't seem to do anything"

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