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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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    If you are supposed to consider all the options, another to think on is a pair of private transformers - step up to 690 single phase, or even 999V split phase,  (anything not quite HV )  and then step down again at the load end. Allows a smaller, cheaper cable size that may pay for the transformers, and quieter than the genset.

     There is a magnetising current in the transformers but that is mostly inductive and can be tuned out if need be with shunt capacitors.

    Done in large  factories, and at least one poultry farm in Essex.

    Mike. 

    PS  late edit.

    A single 20kVA unit in a box will weigh in at about 150-180kg, be a cube of about 50-60cm per side, and have thermal losses of about 5% at full load.

     

     

     

Reply
  • 0

    If you are supposed to consider all the options, another to think on is a pair of private transformers - step up to 690 single phase, or even 999V split phase,  (anything not quite HV )  and then step down again at the load end. Allows a smaller, cheaper cable size that may pay for the transformers, and quieter than the genset.

     There is a magnetising current in the transformers but that is mostly inductive and can be tuned out if need be with shunt capacitors.

    Done in large  factories, and at least one poultry farm in Essex.

    Mike. 

    PS  late edit.

    A single 20kVA unit in a box will weigh in at about 150-180kg, be a cube of about 50-60cm per side, and have thermal losses of about 5% at full load.

     

     

     

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