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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

    If 6mm cable is connected to a 32 amp MCB, then unless the cable is hugely de-rated by grouping, thermal insulation, or high ambient temperature, then there is NO  question of thermal damage from low fault currents.

    At say 35 amps, the MCB might not open for days, if ever, but the cable can stand 35 amps forever.

    At say 100 amps the magnetic trip will never operate, but the thermal trip will operate after a few minutes, before the cable reaches a damaging temperature.

    Or put another way, a published cable rating of say 37 amps does not mean “dangerous at 38 amps”  it means “safe if protected by a 37 amp MCB” if you can find one, or in practice by any smaller size such as 32 amps.

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  • 0

    If 6mm cable is connected to a 32 amp MCB, then unless the cable is hugely de-rated by grouping, thermal insulation, or high ambient temperature, then there is NO  question of thermal damage from low fault currents.

    At say 35 amps, the MCB might not open for days, if ever, but the cable can stand 35 amps forever.

    At say 100 amps the magnetic trip will never operate, but the thermal trip will operate after a few minutes, before the cable reaches a damaging temperature.

    Or put another way, a published cable rating of say 37 amps does not mean “dangerous at 38 amps”  it means “safe if protected by a 37 amp MCB” if you can find one, or in practice by any smaller size such as 32 amps.

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