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AC & DC Cable segregation underground

TCrabbe

Afternoon, 

I am currently reviewing a design that has 2 X low voltage DC 200KW circuits (185mm Ho7 Singles) run ducted underground at a depth of 600mm along with a couple of ducted 32A single phase circuits (25mm Ho7) (Both Band II circuits) the separation distance between all circuits underground is 0.25m is this ok in terms of AC / DC segregation? the AC circuit is a standard 230V circuit and the DC circuit can have a voltage of up to 1000V (EV Charger) 

using TABLE A444.1 – Summary of minimum separation distances where the specification and/or the intended application of the information technology cable is not available, shows a 200mm separation distance. but I wanted to make sure there was nothing I was missing, 

Thank you in advance 

  • 0

    As an aside, H07 for 1000V sounds a bit odd to me (normally the 07 indicates 700V (or to be precise 750V, between conductors, 450V to Earth).

    Otherwise I don't recall anything specific about AC/DC power separation (any more than between AC circuits) - my only other thought was to check the requirements for whatever means you're using for shock protection for the various circuits - if one depended on electrical separation for instance you might need to look at maintaining reliable electrical separation between the circuits (often not so much a distance separation issue, but might have a bearing). Likewise underground cables usually need a concentric c.p.c. of some kind unless other measures provide equivalent protection.

       - Andy.

  • 0

    1. You don't say what the 32 A single-phase circuits are doing. The DC power cable may well interfere with the low-power AC circuits, particularly at switch-on or switch-off.

    2. For this purpose, the separation distance "clip" from A444.1 you have shown from BS 7671 is for distance between telecomms (which includes control etc.) and power and is based on information in BS EN 50174 series. The actual table you need for separation of the 32 A AC cables from the DC cables to minimize interference would be A444.2 (page 122 of BS 7671) ... although I do accept it isn't 100 % clear it can be used for that purpose, knowing where the data came from initially, I know it could be used for that.

    If you do need to prevent EMC issues on the AC circuits, then going from the 1.5 kV on the DC side, that gives a distance of 1.1 m, and the current gives somewhere between 0.6 m for 100 A and 1.05 m for 400-600 A - so you would choose the 1.1 m (1100 mm) separation.

    Do let us know what the AC circuits are doing and we might be able to help you decide whether separation is needed.

  • 0

    TCrabbe,

    If this installation is on a fuel filling station, there are other documents to be complied with regarding installa nd location of d.c cabling. Im only mentioning this given the number of incorrect EVSE installs on fuel filling stations recently.

    If its not on a fuel filling station then, the others that have replied have given sound advice.

    Cheers GTB

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