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EV Charging Calculations

Rach J

This is in relation to Annex A722.

The calculation for minimum resistance to earth for a three-phase charging installation in A722.3 Earth electrode resistance is:

Note that Im is the maximum neutral current as calculated in A722.1.

And if  or the calculated RAev is >200Ω then the above equation should be ignored and the max RAev is 200Ω.

Just from logic and by reading the second requirement, it seems to be that a normal situation would be for Im to be < 70(IL1 + IL2 + IL3)/U0Cmax. However, if that is the case, then in normal situations the required maximum resistance to earth comes out as a negative number.

Am I missing something here?

Parents
  • +1

    I think you might have a point. The original 18th said:

    The above three-phase formula for RAev is valid only where Im > 70/U0 x (IL1 + IL2 +IL3)  (my emphasis) so it does look to me that logic has been reversed in the "reformatting" (and adding in of Cmax).

    I'm not really surprised it's not been spotted - the formula (when correct) usually comes out with such maddingly low numbers,  that this approach (while useful for very low load installations like a street light) is all but useless in a decent sized installation (such as one that has EVSE). Most people I suspect either go down the o-pen detection method of just TT it.

        - Andy.

Reply
  • +1

    I think you might have a point. The original 18th said:

    The above three-phase formula for RAev is valid only where Im > 70/U0 x (IL1 + IL2 +IL3)  (my emphasis) so it does look to me that logic has been reversed in the "reformatting" (and adding in of Cmax).

    I'm not really surprised it's not been spotted - the formula (when correct) usually comes out with such maddingly low numbers,  that this approach (while useful for very low load installations like a street light) is all but useless in a decent sized installation (such as one that has EVSE). Most people I suspect either go down the o-pen detection method of just TT it.

        - Andy.

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  • 0 in reply to AJJewsbury

    Ah I see - previously it meant the system was balanced enough that a dedicated earth connection wasn't required (simplifying hugely here I think!), but that exception has been removed now so the equation doesn't work in some circumstances.

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