Discuss Disconnection time for distribution circuit in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

C

cadspark

I'm looking at remedials from a PIR and one thing is a switch fuse with an 80 amp BS88 is serving a 16mm run of 2 core and armour SWA which serves 3 sub boards in 3 seperate outbuildings and they have been wired so they loop in and out at henley blocks until the final sub board. The total run is over 100 metres and it's exported TNC-S (Please don't start this one again) The Zs at the final board is 1.17 ohms which is way over the 0.46 to meet the 5 second disconnection time. Any thoughts on a relatively cheap solution as the client is on a tight budget.
 
The only cheap solutions would be to reduce the size of the fuse but then that's probably not practical. Other than that i can only think of more expensive options.
 
You will either have to increase the size of the conductors feeding the boards, or make the last board TT.

It would have to be the sub main to the last board that was made a TT.
 
It's probably an unlikely solution, unless any exposed/extraneous-conductive-parts relating to circuits of the TT board can be adequately isolated from the main installation earth.
 
In order to negate the high Zdb at final board, possible solutions:

1) Rigorous supplementary equipotential bonding (415.2, 415.2.1, 415.2.2) to ensure disconnection in <5s.

or

2) Protect the SWA sub-main with an 80A, 300 or 500mA RCD. Time-delayed S type if you have upstream RCDs on the boards.
 
Last edited by a moderator:
It's a feed to a small holiday cottage from main house then it goes to stable and finally a barn. Running in another cpc is not really an option as it would mean a lot of digging. The time delayed 300/500 ma RCD sounds like the winner here. Not ideal obviously but better than the situation at present.

Thanks for your replies.
 
I'm looking at remedials from a PIR and one thing is a switch fuse with an 80 amp BS88 is serving a 16mm run of 2 core and armour SWA which serves 3 sub boards in 3 seperate outbuildings and they have been wired so they loop in and out at henley blocks until the final sub board. The total run is over 100 metres and it's exported TNC-S (Please don't start this one again) The Zs at the final board is 1.17 ohms which is way over the 0.46 to meet the 5 second disconnection time. Any thoughts on a relatively cheap solution as the client is on a tight budget.

Could you not just put a switch fuse into the last SWA after the last Henly block so your protecting the tails, and then your Zdb at that last board should be low enough to to trip that fuse.?
 

Reply to Disconnection time for distribution circuit in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock