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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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    well 95mm2 will carry the fat end of 400A /core all day at pretty much full chat. 2cores in parallel ,more like 700-800A. 

    Adiabatically it will take about 20kA for a second, for an I2t limit.

    What exactly will be overloaded on a 63A cct ? Even with  a massive overload the cable will run cool, just out of spec at voltage drops.

    Not sure what you think will cook

     

    Mike.

    per core 95m of 95mm is about 18 milliohms, so 950m would be 0.18 ohms.  2 in parallel half this, but double it again, as the current flows there and back.

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  • 0

    well 95mm2 will carry the fat end of 400A /core all day at pretty much full chat. 2cores in parallel ,more like 700-800A. 

    Adiabatically it will take about 20kA for a second, for an I2t limit.

    What exactly will be overloaded on a 63A cct ? Even with  a massive overload the cable will run cool, just out of spec at voltage drops.

    Not sure what you think will cook

     

    Mike.

    per core 95m of 95mm is about 18 milliohms, so 950m would be 0.18 ohms.  2 in parallel half this, but double it again, as the current flows there and back.

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