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Very long run of LV Cabling

fiftyhertz

Hi all,

 

im looking at a project where the landlords LV cut out is approx 850 metres from the proposed tenants installation. The earthing arrangement is TT and protected by a time delayed 300ma RCD and a 63A MCCB.

 

The landlords sub main is going to be 2 x 4C 95mm2 in parallel buried in the ground to supply 20KVA worth of power. This achieved a volt drop of 1.6% which leaves 1.9% VD for lighting. From my Amtech calcs I also get about 1ka at the load Dis board (Phase fault)

 

im not worried about Earth Loop because of the RCD, but i am thinking about the adibatic of the phase conductors. If the tenant puts a long run of cabling in, this will comply from a Zs perspective, but could there be issues with the line under a phase to neutral /phase to phase fault as the cable could slowly cook under a lowish load without tripping the breaker. Is this a valid concern? Amtech isn’t throwing up any faults, but just wanted your thoughts.

 

thanks in advance

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    This is an interesting question because it shows the problem with CAD packages. Amtech will happily calculate a ridiculous answer for one, because it does not think, based on an assumed voltage drop that is really too small. You need a supply of about 32A per phase, and the cable design would allow about 400A per phase quite satisfactorily (about 250 kVA), although the volt drop would be rather high, actually extreme. The problem is volt drop, the answer is higher voltage, say 11kV, but Amtech doesn't know this. The Garbage in, Garbage out of computers is well demonstrated!

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  • 0

    This is an interesting question because it shows the problem with CAD packages. Amtech will happily calculate a ridiculous answer for one, because it does not think, based on an assumed voltage drop that is really too small. You need a supply of about 32A per phase, and the cable design would allow about 400A per phase quite satisfactorily (about 250 kVA), although the volt drop would be rather high, actually extreme. The problem is volt drop, the answer is higher voltage, say 11kV, but Amtech doesn't know this. The Garbage in, Garbage out of computers is well demonstrated!

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