Electric vehicle charge point earthing

Tipper · 28 Apr 2016

I'm going to have an EV charge point installed and have already been told there needs to be a separate earth rod if the charge point is outside. The charge point will be fed by the necessary RCD/RCBO/2-pole switch, etc and an underground SWA cable as the charge point will be on an outside wall about 5m from the house.

As a point of interest, does the SWA armouring get earthed via the house earth (which is already via an earth rod due to being a non-PME system) or does it rely on the new earth rod near the charge point? Which earth takes on the cable earthing role? I get the feeling there may be some discussion over this with my electrician who hasn't seen the location of the charge point, etc, yet.

Can anyone advise? Thanks.

EFLImpudence · 28 Apr 2016

Usually for TN earthing systems the armour would be earthed at the origin and kept separate from the charging area if that were TT.

As you already have a TT earthing system (rod) it could be connected at both ends.
You are relying on RCDs for earth fault disconnection.

I get the impression that no one really knows what is best yet.
https://www.google.pt/webhp?sourcei...UTF-8#q=vehicle charging on already tt system

securespark · 28 Apr 2016

I would use the earth from the house rod solely to connect the armour to, then use another rod local to the point to connect the external stuff to.

You want to avoid a voltage gradient between the rods. You could also link the two rods I suppose.

Tipper · 28 Apr 2016

Thanks guys so far.
If the earthing arrangements are separated, ie, cable armour from supply point and charge point via new earth rod, then presumably there is no need for 3-core SWA (ie 'twin&earth') because the earth core would not be connected to anything and a two-core would suffice? (Not sure if this reduction in spec would actually reduce the cost significantly. Most of the cost I suspect will be cable route installation and trenching!)

EFLImpudence · 28 Apr 2016

Yes, two core would do. It may anyway if the armour were big enough for the earthing.

Where is the present rod in relation to the charging point?

As SS said "You could also link the two rods I suppose".
By the armour?

Tipper · 28 Apr 2016

The two earth rods would be either side of a double garage, say, 15-20m apart. The house consumer unit is in the garage about 3m from the house earth rod. The charge point supply cable would be about 15m long. The charge point earth rod would be directly below the charge point say 1m.

Doesn't connecting the earth rods together cause a problem or is that when there are two earthing systems ie PME and a separate earth rod? Sorry my earthing knowledge is very rusty!

I'll be getting my electrician involved shortly but I find the ins and outs of this really interesting.

(The house which is about 40 years old now was found to have no earth at all before it was rewired 2 years ago. It was a shambles electrically hence total rewire carried out including new earthing arrangements. The meter tails to the consumer unit were described as '10mm twin&earth cooker cable'. It is on a single phase O/H line with SWA final incoming cable across the garden from the last pole.)

ban-all-sheds · 28 Apr 2016

EFLImpudence said:
As SS said "You could also link the two rods I suppose".
By the armour?

Or by 5m of this buried in the ground:

The last time I checked the people linked to were considerably cheaper than anybody else.

ericmark · 28 Apr 2016

You say your on TT so what is the problem? Only with TN is there a problem. With TT just link all earth electrodes together.

Tipper · 29 Apr 2016

That's what I had assumed but was looking for confirmation. Thanks.

securespark · 29 Apr 2016

EFLImpudence said:
As SS said "You could also link the two rods I suppose".
By the armour?

If the CSA is sufficient, why not?

Although the extra cost of a copper conductor over that distance would not be significant.

EFLImpudence · 29 Apr 2016

Yes, but you said:

securespark said:
I would use the earth from the house rod solely to connect the armour to, then use another rod local to the point to connect the external stuff to.
rods rather than both connected to the armour.

but:

You could also link the two rods I suppose.

by the armour?

If the CSA is sufficient, why not?

securespark · 29 Apr 2016

Yeah. Senior moment. Dozy wazzock. I am 50 in three months time...
Thanks for pulling that up!

I was thinking TN at first, then thinking about linking the rods together to avoid a voltage gradient. There would not be two rods if it were a TN system...

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Electric vehicle charge point earthing

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