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
  • Replies 17 replies
  • Subscribers 32 subscribers
  • Views 1384 views
  • Users 0 members are here
Options
You may also be interested in

Minimum size of CPC

bakey1959

When using the Adiabatic equation, only to calculate the minimum size CPC, I am thinking that using the fault currents given in Appendix 3 of BS7671 rather than that calculated (UO/ZS) would give a more realistic outcome.

For example: A D6 circuit breaker is protecting a circuit where the calculated fault current is 328A. The table associated with Fig 3A6 in Appendix 3 of BS7671 shows that an overcurrent of 120A will operate the 6A breaker in 0.4s. Using these values and a K value of 115 results in a minimum CPC of 0.66mm2 (1.0mm2), whereas if you use the 328A the calculation results in a minimum CPC of 1.80mm2 (2.5mm2).

I cannot see a problem with using the lower value, but would appreciate confirmation and guidance from others.

Parents
  • 0


    I guess you could consider if the cable is running at a temperature less than 70c, in which case the k value could be more than 115. If the formula is plotted it provides a straight line with k at about 163 at 0C and 115 at 70C. Run at a conservative 50C would give k at about 129.

    The question of minimum csa of cpc to meet thermal constraints is a very common one on the 2396 Design course. There would be no expectation to adjust the k value, but I guess the various methods indicate some calculations in installation work are what Mapj referred to as measuring with a micrometer and cutting with an axe!

    Using the 5900 figure that gives a csa of about 0.59mm2. As a matter of interest, you would be marked correct in the 2396 Design course using the simple method and the 1.38KA and a fixed time to disconnection of 0.1s, which would lead to a minimum csa of  around 3.8mm2. 


  • 0 in reply to lyledunn

    It is rare to see a 1mm2 circuit protected by a 6A breaker running at anything like 70C unless the ambient is unusually hot. Possible in parts of the middle east  - so if 1,1mm seems a squeeze remember the assumption about cable core temp before fault is probably most pessimistic.

    Mike.

Reply
  • 0 in reply to lyledunn

    It is rare to see a 1mm2 circuit protected by a 6A breaker running at anything like 70C unless the ambient is unusually hot. Possible in parts of the middle east  - so if 1,1mm seems a squeeze remember the assumption about cable core temp before fault is probably most pessimistic.

    Mike.

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