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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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  • 0

    Most UPS units are indeed intended for short term use. Two alternatives exist.

    Firstly, use a standard type but fitted with large external deep cycle batteries. This will give a greatly extended run time. Remember that the inverter in the unit was probably only intended for very short term operation and may be killed by prolonged operation. Restrict the load to less than half of that intended to avoid this.  Secondly be aware that the battery connections may not be isolated from the mains input and must therefore be treated with the same care as any other mains voltage connections.

    Alternatively use not a ready made UPS but a power inverter intended for an off grid home. Select one with a generator input and automatic transfer.

Reply
  • 0

    Most UPS units are indeed intended for short term use. Two alternatives exist.

    Firstly, use a standard type but fitted with large external deep cycle batteries. This will give a greatly extended run time. Remember that the inverter in the unit was probably only intended for very short term operation and may be killed by prolonged operation. Restrict the load to less than half of that intended to avoid this.  Secondly be aware that the battery connections may not be isolated from the mains input and must therefore be treated with the same care as any other mains voltage connections.

    Alternatively use not a ready made UPS but a power inverter intended for an off grid home. Select one with a generator input and automatic transfer.

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