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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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    Yes that's right they will be load sharing type. All outgoing circuits will have RCBOs including any possible column lighting. Not 100% sure yet if the EVC will contain the RCBO as it's going out to tender to suppliers. If it does then I suspect we will feed each circuit via TP Type C MCBs with the RCBO being located within the EVC for selectivity purposes.
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    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? 



    How are you providing protection against electric shock prior to the outgoing RCBOs? Is your switchfuse/isolator, bus-bar chamber and DB (prior to the RCBOs) constructed to provide double or reinforced insulation? (most of the ones I've seen of that kind of size tend to be steel cased, which makes me suspect not). If not you'll presumably want some means of ADS - which presumably means an RCD of some kind at the point you transition from double/reinforced insulation to earthing. If the transition point is within the DBs (i.e. it's double insulation all the way up to and including the DB incomer and ADS within the DB and onward) then an RCCB incomer would make sense, but quite possibly you'd need something upstream of that.

     

    Am I correct assuming the RCBOs must all be 4 pole because the Neutral is classed as a line conductor?



    Depends on the reason for having the RCBOs - if it's to provide additional protection to the socket/EV (as per 722.531.2.1.1) - usually because there's no RCD at the charge point - then yes it needs to disconnect all live conductors, including N. If however it's just there to provide TT-style ADS for the final circuit up to the charge point (where another RCD protects the EV) then I'd say normal rules apply and only the line conductors need be disconnected.

     

    protected by 30mA Type A RCBOs mounted in each of the D/Boards.



    Is there an assumption there that the charge points will have some built-in protection against d.c. residual currents over 6mA - if not you might need type-B RCDs (or similar - e.g. some manufacturers offer EV types).


    BTW - keep an eye on protective conductor currents, RCD ratings and the reliable maximum electrode resistance needed to keep within the 50V requirement.


       - Andy.
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    HDCU to a sw/fused isolator




    that sw/fuse needs also to be double insulated, or look at a MCC breaker with an earth fault relay built in. sit down before you get a price though.

    the spec, (and price) of this are not the exact thing but are close enough to give you the right ballpark to protect the bus chamber.


    there are all-insulated alternatives to bus bars  example I presume these have been considered and ruled out ?


    Where and how   is metering in all this - some  HDCU  example  can take a built on chamber for current transformers.


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    The HDCU is double insulated / reinforced and confirmed by DNO as being made by schneider so presumably this negates the need for both the Sw/Fuse / Bussbar chamber to be double insulated / reinforced. The HDCU also has a Meter Bracket built in with CT`s Inbuilt.
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    Not at all. The HDCU contains the DNOs fuses and has the suppliers metering mounted on the front. The tails from here will go into your fuse-switch before your bus bar unit. The question still stands - how are you providing the shock protection for your fuse-switch and bus bar unit, which I am guessing will be metal? 


    Regards,


    Alan.
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    https://storage.electrika.com/flips/1173-acti9-rti-15-a/index.html#272  Im thinking one of these or an equivalent instead of a metal switched fuse
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    Im thinking one of these or an equivalent instead of a metal switched fuse



    But that is still class 1,  so back to what Alan queried.
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    You need a GRP equivalent to the one in your link. Protek manufactured a metal enclosed fully adjustable 400amp RCBO with a GRP option, but the link I have is no longer valid.

    Found it, there is a GRP option on request
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    Thanks for the advice all. Will take a look at these in either Protek or Schneider but using the GRP type enclosure instead of metal all fed in double insulated meter tails. 300mA Tripping Current Sensitivity although not sure what the adjustable time setting should be set at? New to EV and TT Systems so will try and read up more. I haven't worked out how the Bussbar Chamber can be reinforced / GRP or an alternative option for that.
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    You only need up to the first RCD to be double insulated, after that your method of shock protection becomes ADS, so normal TT rules apply.

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