Earth rod for EV charger

[ban-all-sheds]

(iii) which is not shown requires a device to disconnect the supply if voltage between earth and the protective conductor exceeds 70V.

That device could be in the charging point apparatus.

Does anybody know if any of them have it?

 

[flameport]

What about "The ... regulation need not be applied if none of (i), (ii) or (iii) is reasonably practicable"?

That could be used - but if doing so, no earth rod would be required, so you have paid for the installation of something you don't need.

However that exception only applies until the end of 2018, and even now it's pretty poor to just ignore certain things even if it is theoretically permitted.
Regulations are the minimum required, not necessarily the only option.

A ~70 ohm earth rod is perfectly acceptable for a TT supply. It will do essentially nothing in the event of a fault on a TNCS supply.

 

[flameport]

That device could be in the charging point apparatus.

The 18th edition adds that exact text, so it's implied that charging points will have it.

Not seen any that have it yet.

 

[Observer]

That could be used - but if doing so, no earth rod would be required, so you have paid for the installation of something you don't need.

However that exception only applies until the end of 2018, and even now it's pretty poor to just ignore certain things even if it is theoretically permitted.
Regulations are the minimum required, not necessarily the only option.

A ~70 ohm earth rod is perfectly acceptable for a TT supply. It will do essentially nothing in the event of a fault on a TNCS supply.

So what will change on 1 Jan? According to the specialist sparkie I spoke to, a rod will have to be installed at or close to the charger, regardless of the earth system. But (he says) it will still be connected to the MET. What resistance standard will be applied? <100 ohms? If so, then for a TN-C-S system, what will have changed?

 

[Observer]

I believe that he is wrong. The "reasonably practicable" get-out clause will have been removed, so a PME earth may not be used, no matter how many local rods you have unless with the local rods you end up in compliance with (ii).

Won’t that mean that it will be very difficult or impossible to make a satisfactory installion of an EV charger in a PME system?

 

[flameport]

So what will change on 1 Jan?

The exception is removed, and the wording for item (iii) is amended to suggest that the required disconnection device could be included in the charging equipment.

What resistance standard will be applied? <100 ohms?

The same as now - so only a few ohms, totally impractical for the majority of installations.

According to the specialist sparkie I spoke to, a rod will have to be installed at or close to the charger, regardless of the earth system. But (he says) it will still be connected to the MET.

They are wrong.

The options are as they are now - for a TNCS system where the TNCS earth is used:
a 3 phase system with balanced loads and other characteristics which are seldom found in any installation and no domestic installs ever
an earth rod or electrode which must have the implausibly low impedance which is near impossible to obtain
or the mythical device which disconnects all conductors including the protective conductor in the event of 70 volts or more between earth and the TNCS earth.

The real answer for actual installations is the same as is done now - an earth rod is installed and the installation is TT. No connection to the TNCS earth at all.
TN-S installation just use the TN-S earth, and a TT one would either use the existing earth rod or a separate one.

 

[JohnW2]

Won’t that mean that it will be very difficult or impossible to make a satisfactory installion of an EV charger in a PME system?

No - neither difficult nor impossible if, as flameport has suggested, you ignore (don't use) the PME earth and, instead, use just a TT rod for the EV charging.

Kind Regards, John

 

[Observer]

The exception is removed, and the wording for item (iii) is amended to suggest that the required disconnection device could be included in the charging equipment.


The same as now - so only a few ohms, totally impractical for the majority of installations.


They are wrong.

The options are as they are now - for a TNCS system where the TNCS earth is used:
a 3 phase system with balanced loads and other characteristics which are seldom found in any installation and no domestic installs ever
an earth rod or electrode which must have the implausibly low impedance which is near impossible to obtain
or the mythical device which disconnects all conductors including the protective conductor in the event of 70 volts or more between earth and the TNCS earth.

The real answer for actual installations is the same as is done now - an earth rod is installed and the installation is TT. No connection to the TNCS earth at all.
TN-S installation just use the TN-S earth, and a TT one would either use the existing earth rod or a separate one.

Okay. So, taking your and JohnW2's comments together, I understand that your (mutually agreed) view is that the EV charging point installation in my case should be TT with an earth rod close to the charging point installation and that will be mandatory from 1 January.

Do you go so far as to suggest I should raise this with my electrician and suggest he reconsider his interpretation? From an electrical safety perspective, is it better to:

(a) leave the installation as it is - i.e. supply cable CPC connected to MET, which has second earth connection (albeit of doubtful value) via earth rod;

(b) disconnect the earth rod and leave the existing supply cable CPC connected to the MET;

(c) disconnect the earth rod from the MET, disconnect the existing supply cable CPC; and install a separate TT earth rod solely for the charging point installation.

 

[JohnW2]

Okay. So, taking your and JohnW2's comments together, I understand that your (mutually agreed) view is that the EV charging point installation in my case should be TT with an earth rod close to the charging point installation and that will be mandatory from 1 January. Do you go so far as to suggest I should raise this with my electrician and suggest he reconsider his interpretation?

That's obviously for you to decide. If I were in your position, I probably would.

From an electrical safety perspective, is it better to:
(a) leave the installation as it is - i.e. supply cable CPC connected to MET, which has second earth connection (albeit of doubtful value) via earth rod;
(b) disconnect the earth rod and leave the existing supply cable CPC connected to the MET;
(c) disconnect the earth rod from the MET, disconnect the existing supply cable CPC; and install a separate TT earth rod solely for the charging point installation.

As has been said, my understanding is that (c) is not only 'safer' than (a) and (b), but it probably the only practical approach that will be regulation-compliant come next year.

Depending upon where the existing earth rod is, you may not have to install a new one - it might be possible to continue using it, disconnect it (and the CPC to the charging point) from the MET and uses just the rod as the earth for the charging point.

Kind Regards, John

 

[Observer]

That device could be in the charging point apparatus.

Does anybody know if any of them have it?

My EV (a Tesla) was supplied with a UMC cable ([Universal?] Mobile Connector) with adapters for 13A domestic power and 32A "commando" socket (which is what I have installed) https://www.tesla.com/sites/default...nstall_guide_industrial_socket_blue_DEC16.pdf. The cable has an in-line diagnostics box which reports a number of fault conditions, incuding "ground fault" and "loss of ground". I don't know whether any of the fault conditions sensed constitutes "a device to disconnect the supply if voltage between earth and the protective conductor exceeds 70V".

 

[JohnW2]

I don't know whether any of the fault conditions sensed constitutes "a device to disconnect the supply if voltage between earth and the protective conductor exceeds 70V".

If, as I understand is your present situation, both the only 'true earth' you have (your TT rod) and the 'protective conductor' (the CPC of the supply cable) are connected to your MET (hence connected together), then there's obviously no way that there could ever be any appreciable potential difference ('voltage') between them, let alone 70V.

Kind Regards, John

 

[Richard1]

As a member of NAPIT, I would say that to comply with the regs 722.411.4.1 as laid down options 1 & 3 were not available to him and only option 2 could possibly work, the location of the earth rod may have been dependent on site conditions, ie local services such as drains, water mains, telephone cables, or the fact that area did not give the lowest possible Ra reading for the rod, where as the other side of the house did, I cant see any one putting the rod further away than necessary costing additional time and materials on the job.

 

[ban-all-sheds]

If, as I understand is your present situation, both the only 'true earth' you have (your TT rod) and the 'protective conductor' (the CPC of the supply cable) are connected to your MET (hence connected together), then there's obviously no way that there could ever be any appreciable potential difference ('voltage') between them, let alone 70V.

But there could be a large PD if it were a TN-C-S supply and the neutral was lost, and that's what this is all about.

It's not about simple earthing of a charging point, it's about what's the best arrangement to have when things go t**s-up.

Spoiler: PM

Yes, I know that you know that - the reply was really aimed at others reading this.

 

[ban-all-sheds]

As a member of NAPIT, I would say that to comply with the regs 722.411.4.1 as laid down options 1 & 3 were not available to him and only option 2 could possibly work, the location of the earth rod may have been dependent on site conditions, ie local services such as drains, water mains, telephone cables, or the fact that area did not give the lowest possible Ra reading for the rod, where as the other side of the house did, I cant see any one putting the rod further away than necessary costing additional time and materials on the job.

I think you're missing the point.

"Lowest possible Ra" is irrelevant - if it isn't low enough then option 2 does not work, and you do not use the supply PME earth for the charging point.

 

[JohnW2]

But there could be a large PD if it were a TN-C-S supply and the neutral was lost, and that's what this is all about.

Quite so. However, the OP said that he didn't know whether the 'diagnostics box' which came with his Tesla constituted "a device to disconnect the supply if voltage between earth and the protective conductor exceeds 70V". I was pointing out that since the nearest to an 'earth' reference he has is his TT rod, the box would not detect any significant pd between that 'earth' and the protective conductor (even if that conductor had a large pd to true earth) if that 'nearest to an earth reference' was a rod connected to his MET, hence also to the 'protective conductor'!