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TT Supply for EVCs

Eddie Currents
Assuming a 3 phase 400A DNO underground 300wf Ai/Cu cable to an (HDCU) Heavy Duty cut out located in a cubicle in a Car Park. A DNO earth is not provided.  Double insulated 185mm Meter Tails run from the HDCU to a sw/fused isolator then onto a bussbar chamber feeding 3 x 250Amp rated Type B D/Boards. 13 Electric Vehicle Charger Circuits will be installed from each of the 3no D/Boards so 39 circuits in total. A TT Earth Electrode shall be located next to the cubicle with the consumer earth connected from the electrode directly to the switchgear within the cubicle thus forming it's own TT System. All 39 EVC Circuits shall be 3 phase and protected by 30mA Type A RCBOs mounted in each of the D/Boards.

Am I correct assuming the RCBOs must all be 4 pole because the Neutral is classed as a line conductor? If using Schneider the Acti 9 type requires 6 ways per circuit when using the bolt on RCD element.

Also should each of the D/Boards have a time delay S-Type RCD acting as a Main Switch or will a standard 4 pole dissconnector suffice as a Main Switch?
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    AJJewsbury:




    300mA Tripping Current Sensitivity



    What might the total earth leakage current be for 39 EVs simultaneously?


    300mA device - which might trip at anything over 150mA allows little more than 3.5mA per vehicle...


        - Andy.

     




    Depending on the source of the protective conductor current, in a three-phase system there may well be some cancellation, just as with neutral currents in three phase systems.


    This is certainly true of "static leakage", i.e., linear protective conductor current resulting from capacitors in noise filters or resistive and capacitive leakage through insulation, unless subject to significant power factor differences. It is not true of protective conductor current with significant triple-n harmonics.


    "Static leakage" that is balanced across phases is not always detected by a three-phase RCD, for the same reason it cancels out in the protective conductor.


    Equally, a three-phase RCD may not detect a fault in which all three phases are shorted to earth, with absolutely equal fault impedances at the same instant. Luckilly, such a fault is highly unlikely (unless deliberately arranged) and BS 7671 only requires us to consider single-fault conditions.



    So, anyway ... back to the problem at hand ... we may not need to consider the simultaneous "leakage" of all 39 vehicles - so at least 13 (one phase fully loaded, no points in use on the other two phases)  - but it does require some investigation to discover the triple-n's situation when all phases are fully loaded.

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  • 0

    AJJewsbury:




    300mA Tripping Current Sensitivity



    What might the total earth leakage current be for 39 EVs simultaneously?


    300mA device - which might trip at anything over 150mA allows little more than 3.5mA per vehicle...


        - Andy.

     




    Depending on the source of the protective conductor current, in a three-phase system there may well be some cancellation, just as with neutral currents in three phase systems.


    This is certainly true of "static leakage", i.e., linear protective conductor current resulting from capacitors in noise filters or resistive and capacitive leakage through insulation, unless subject to significant power factor differences. It is not true of protective conductor current with significant triple-n harmonics.


    "Static leakage" that is balanced across phases is not always detected by a three-phase RCD, for the same reason it cancels out in the protective conductor.


    Equally, a three-phase RCD may not detect a fault in which all three phases are shorted to earth, with absolutely equal fault impedances at the same instant. Luckilly, such a fault is highly unlikely (unless deliberately arranged) and BS 7671 only requires us to consider single-fault conditions.



    So, anyway ... back to the problem at hand ... we may not need to consider the simultaneous "leakage" of all 39 vehicles - so at least 13 (one phase fully loaded, no points in use on the other two phases)  - but it does require some investigation to discover the triple-n's situation when all phases are fully loaded.

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