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TNCS and earth electrode

In order to protect against neutral failures, 18th Edition states that the earth impedance between the vehicle charging earth and the true ground (or earth), for chargers fitted outside the home, must be under 200 ohms to minimise any voltage developed between the two and ensure correct operation of an RCD.
It can be possible to achieve this 200 ohms earth impedance by sinking one or more dedicated earth rods for the chargepoint. Pod Point installation guidance recommends 100 ohms as electrodes and ground conditions are subject to wide variations due to both locality and season.
Since 2013 earth rods have been required on PME systems and the flexibility that some installers used to not fit an electrode was removed from the 18th Edition in January 2019, but the difficulty of successfully fitting an earth rod predates both regulations

I am setting up an assessment for a well known accreditation body and I noted that the assessment required an installation earth electrode with a resistance of 20-200 ohms which the paperwork declared was to simulate the supplementary electrode required for TNCS systems supplying EV chargers. I contacted the body in question and a chap confirmed that was indeed the purpose of the thing. Didn't want to argue as I thought it was rubbish. I then read the above in a technical document for the charger being used and I thought have I been drinking too much wine? Did I miss something? I am beginning to wonder. It cant be for holding down touch voltages to 70v because even at 20 ohms that would only allow 3.5A on loss of PEN. It cant be for measurement purposes as the assessment requires the "earthing conductor" to electrode to be green and yellow 16mm2. Please tell me that my daily glass of red hasnt killed so many grey cells that I have missed this?

0 UKPN over 4 years ago

Oh, not again! I am going to have my supper, I may look in on this later if there is nothing good on the box.

Regards, UKPN

dnoearthingsolutions@btinternet.com
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0 John Peckham over 4 years ago

Lyle

Where is the requirement for an electrode in 722 to have an earth resistance under 200 ohms to hold down touch voltages on PME systems?
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0 gkenyon over 4 years ago
John Peckham:

Lyle

Where is the requirement for an electrode in 722 to have an earth resistance under 200 ohms to hold down touch voltages on PME systems?

Lyle,

It's the note under Table 41.5 ... nothing to do with either PME or EV ... but that would, of course, mean the charging point is being installed in its own 'TT island'.

It's got absolutely squat to do with a reinforcing earth electrode for PME.

BS 7671 does NOT require a supplementary earth electrode for TN-C-S (PME), unless the installer is opting to use exception in 722.411.4.1 (ii), but in that case the earth electrode resistance required must be calculated in accordance with A722.2:

For single phase installations with max demand 100 A, an earth electrode resistance < 1 Ohm is required !

For three phase installations that don't meet 722.411.4.1 (i), an earth electrode resistance in single figures is likely to be required for installations with maximum demand up to 100 A per phase. Over this, you are looking at < 3 Ohms.

Conclusions;

The advice doesn't match BS 7671 (even 17th Ed Amendment 2).

You might as well forget the "reinforcing earth electrode" option!

PM me or get in touch via my company web-site, happy to help you get your head round these things.
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0 Chris Pearson over 4 years ago

lyledunn:

Please tell me that my daily glass of red hasnt killed so many grey cells that I have missed this?

How big is your glass? ?

No, your little grey cells could compete with those of Poirot. Extraordinary, isn't it!
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0 lyledunn over 4 years ago

Thank you chaps, I thought that I was going to have to ditch my little stock of red squirreled away to see me through furlough!

The more serious problem is that this assessment is rolled out in colleges and training centres nationwide. I think I will ask UKPN to call them on Monday to straighten things out.
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0 David Stone over 4 years ago

Lyle, you are in the ROI, aren't you?

Anyway, what is being suggested here is daft, a 16mm² Earth electrode connection would require a substantial lightning bolt to be fully utilised! You need to find out WHO is behind this advice/course requirement, it is they who are having trouble with the red stuff, or perhaps the golden stuff, or more likely 10 pints of the Black stuff...
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0 David Stone over 4 years ago

This one is quite good for new ideas. You think you have a problem! It holds two whole bottles.
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0 Sparkingchip over 4 years ago

The full article
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0 ebee over 4 years ago

davezawadi (David Stone):

This one is quite good for new ideas. You think you have a problem! It holds two whole bottles.

Dave,

proud of you, that`s a decent size glass
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0 mapj1 over 4 years ago

The pod charger in the liked article presumably breaks all cores, including the supply CPC when it detects a fault.

As noted, unless your PME supply is only feeding one lightbulb, then a 200 ohms electrode resistance is not going to hold the exposed metal to a safe voltage. This is very reminiscent of the 18th edition DPC, where there were proposals like this that also seemed to have been written by someone who has never actually installed a real electrode and measured it. Perhaps the same author has found a new job.
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TNCS and earth electrode

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