IET logo
 
IET
Decrease font size
Increase font size
Topic Title: "Submain" to house
Topic Summary: Switch fuse requirements
Created On: 28 July 2016 03:52 PM
Status: Read Only
Linear : Threading : Single : Branch
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 28 July 2016 03:52 PM
User is offline View Users Profile Print this message



Delta2

Posts: 23
Joined: 14 December 2015

So, have a property with external meter (approx 10m from house).
I have to get the cable into consumer unit, which is 30m from said meter. Supply fuse is 1361 80A. DNO is saying that they will not provide comment on protecting the submain (25mm armoured), which is fair enough.
I've read that a hundred amp fuse is good for 3m in the eyes of the DNO, but they stated the same figure for 80A. I'm just curious as to the reason- I'll most likely fit an 80A KMF after a couple of calcs, so it is protected from my end, but obviously can't guarantee "mine" will go first in the event of a short. Is the no comment just for liability reasons, or am I missing something? Although a long run, can anyone see a problem.
Any information welcome,

Kind regards.
 28 July 2016 04:32 PM
User is offline View Users Profile Print this message



christait

Posts: 86
Joined: 08 February 2013

For very short runs, it is possible to derate the cable, but for any great distance you will start to get issues with voltage drop. Using another 80Amp fuse would seem to me to be fairly pointless, as the chances are the DNO fuse will be the one to blow (according to Murphy's law). It might be worth looking to see if you can get a device that will discriminate with the DNO fuse, an 80Amp BS60898 Type B will be more likely to trip before the DNO fuse does (but will not discriminate fully), and if you could live with a 63A BS 60898 Type B I would expect to achieve full discrimination throughout most of the trip curve, likewise for a 60Amp BS1361 fuse. Alternatively if you can protect the sub main cable mechanically to the consumer unit, and outgoing ways are protected at less than 60Amps, then CU devices will discriminate with 80A BS1361 fuses

Edited: 28 July 2016 at 04:51 PM by christait
 28 July 2016 04:34 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16113
Joined: 13 August 2003

No problem that I can see - usual way of doing it in fact. Since there are no branches between the meter and your KMF whichever fuse goes the result is the exactly the same so no discrimination issue. It's certainly no worse than an ordinay install with the DNO's fuse protecting everying (up to 3m) to the CU. The DNO will just be keeping their options open - i.e. so they can upgrade to 100A in future if required without having to tell the customer to get their electrician back in and check the calculations of the submains. (The 3m limit also ties in with the BS 7671 limit for postponing short circuit protection.)
- Andy.
 28 July 2016 04:37 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16113
Joined: 13 August 2003

an 80Amp BS60898 Type B will be more likely to trip before the DNO fuse does

Yes, but will offer almost no discrimination against the downstream MCBs in the CU. Since faults are far more common on final circuits (or appliances) than on submains, that arrangement would almost certainly result in a whole-house blackout far more often.
- Andy.
 28 July 2016 06:02 PM
User is offline View Users Profile Print this message



weirdbeard

Posts: 3116
Joined: 26 September 2011

Fit a 100 amp fuse to your 25mm SWA, this will prevent discrimination in the event of an overload, as it will always be the service fuse that goes Not so sure about what would go first if the SWA is pick axed, could depend on if it's raining or how shiny the pick axe is etc.....

-------------------------
:beer)
Statistics

New here?


See Also:



FuseTalk Standard Edition v3.2 - © 1999-2017 FuseTalk Inc. All rights reserved.

..