Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Helpful Answer
  • Locked Locked
  • Replies 3 replies
  • Subscribers 31 subscribers
  • Views 1088 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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?

  • +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.

  • 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.

  • 0

    but that exception has been removed now

    Other here will know more of the intention of the committees, so wait to say what they have to say, but my guess (based on almost the same formula being present in the 2018 version) is that the intention was to keep it in, but the juggling of the words didn't quite come out as intended. There have been a few instances of that sort of thing over the years. What's the old saying about being originally written in good English but thereafter modified by committee...

       - Andy.

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines