EV Charging Calculations

Question

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 I m is the maximum neutral current as calculated in A722.1. 
 And if or the calculated R Aev is >200&Omega; then the above equation should be ignored and the max R Aev is 200 &Omega;. 
 Just from logic and by reading the second requirement, it seems to be that a normal situation would be for I m to be < 70(I L1 + I L2 + I L3 )/U 0 C max . 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?

AJJewsbury · Accepted Answer

I think you might have a point. The original 18th said: 
 The above three-phase formula for R Aev is valid only where I m > 70/U 0 x (I L1 + I L2 +I L3 ) (my emphasis) so it does look to me that logic has been reversed in the "reformatting" (and adding in of C max ). 
 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.

EV Charging Calculations

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