Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 33 replies
  • Subscribers 32 subscribers
  • Views 3044 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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

Parents
  • 0

    Provided that the phase and neutral conductors are correctly sized WRT the fuse or circuit breaker, then there is no question of “cooking” the conductors under fault conditions.

    For example, a 95mm sub main protected by a 63 amp MCB can NEVER suffer thermal damage during a fault. A modest overcurrent to say 70 amps might persist for days, but is within the long term current rating of the cable.

    Likewise a 4mm sub circuit on a 32 amp MCB cant be damaged by overcurrent  provided that the full rating of the cable is available. If the fault current is low, then the fault current is reduced and the MCB will take longer to trip. But the cable does not “know” the difference between this and someone plugging in numerous portable heaters. 

     If de-rating for grouping or thermal insulation is required then I would either go up a cable size to 6mm, or go down to 20 amp MCB.

    An earth fault is potentially more concerning since total reliance is placed on the RCD functioning. I would prefer a time delayed RCD at the origin and a quicker acting one at the load end.

Reply
  • 0

    Provided that the phase and neutral conductors are correctly sized WRT the fuse or circuit breaker, then there is no question of “cooking” the conductors under fault conditions.

    For example, a 95mm sub main protected by a 63 amp MCB can NEVER suffer thermal damage during a fault. A modest overcurrent to say 70 amps might persist for days, but is within the long term current rating of the cable.

    Likewise a 4mm sub circuit on a 32 amp MCB cant be damaged by overcurrent  provided that the full rating of the cable is available. If the fault current is low, then the fault current is reduced and the MCB will take longer to trip. But the cable does not “know” the difference between this and someone plugging in numerous portable heaters. 

     If de-rating for grouping or thermal insulation is required then I would either go up a cable size to 6mm, or go down to 20 amp MCB.

    An earth fault is potentially more concerning since total reliance is placed on the RCD functioning. I would prefer a time delayed RCD at the origin and a quicker acting one at the load end.

Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines