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TNCS and earth electrode

lyledunn
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?
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    The 100 Ohm value comes from somewhere though ...


    Some vehicles - the Renault Zoe is often quoted - look at the N-E loop impedance, and if it's not less than 100 Ohm, the vehicle won't charge. Don't forget, though, this is the sum of the earth electrode resistance at the installation, the Neutral run back through to the transformer, and the transformer earth electrode resistance.


    So if you've a TT installation, or you're providing a TT island for the charging point, you need to get well under 100 Ohm (preferably well under 80 Ohms) or those vehicles won't charge.
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    The Renault Zoe will be happy with many installations in ROI, loss of neutral or not. It is common practice to neutralise the installation and provide an independent earth electrode, the impedance of which is often below that 100ohms when measured with a loop tester. 

  • 0
    The 100 Ohm value comes from somewhere though ...


    Some vehicles - the Renault Zoe is often quoted - look at the N-E loop impedance, and if it's not less than 100 Ohm, the vehicle won't charge.

    Possibly comes from French practice - they usually use TT and their DNOs supply an up-front 500mA RCD - hence consumer electrodes have to be sub-100 Ohms to satisfy the 50V rule.


    I think 100Ω did appear in one of the DPCs for BS 7671 (presumably in the interests of harmonisation) but mutated to 200Ω by the final version.

       - Andy.
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    gkenyon:

    The 100 Ohm value comes from somewhere though ...


    Some vehicles - the Renault Zoe is often quoted - look at the N-E loop impedance, and if it's not less than 100 Ohm, the vehicle won't charge. Don't forget, though, this is the sum of the earth electrode resistance at the installation, the Neutral run back through to the transformer, and the transformer earth electrode resistance.


    So if you've a TT installation, or you're providing a TT island for the charging point, you need to get well under 100 Ohm (preferably well under 80 Ohms) or those vehicles won't charge.


    Graham, do you know how such a test is conducted by the vehicle? Interesting that City and Guilds in their 2919 EV assessment rig require Ra to be 100 ohms.They use the TT island approach which I thought was losing favour fast for all the reasons set out in annex H of the COP. So this is another one! Not much point in providing a TT island if the car wont charge! 


  • 0
    do you know how such a test is conducted by the vehicle?

    Certainly some cars (particularly the Zoe) are reputed to refuse to change if the earth loop exceeds 100Ω - but that doesn't really exclude TT - you just have to have a decent electrode. As I mentioned earlier normal French practice is TT sub 100Ω even in their sometimes thin dry soils further south - although they usually do a far more thorough job of electrodes - such a ring of copper under foundation concrete - which does rather put our 'bang in a 4' rod' approach rather to shame.

      -  Andy.
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    It does a kind of "loop impedance" test ... I believe N to E.
  • 0
    I guess that`d be more than just a simple resistance measurement then? interesting


    (or voltage check)

  • 0
    I suppose two Zeners back to back at a value a little over peak permisible voltage fed by a dropper resistor and a comparitor IC comparing the ends of the resistor to activate a relay or thyristor or some such might negate the need of an earth rod. Mapj1 probably has some ideas

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