Interesting Configuration

[esra_ptrap]

A friend had a problem with one of her ring circuits which I sorted out quite easily (pinched neutral was tripping the RCD) and she wanted one of the spare ways connecting to a radial for a freezer on the non-RCD side ... All okay.

Whilst sorting this out though I thought the configuration of the CU rather odd (and its only a few years old).

It was a 16 way MK split load unit with 100A incomer and 80A 30ma RCD with all the right circuits in all the right places and nicely wired up.

Odd thing was, it had a 100A service fuse and 16mm tails?

1. I thought 16mm tails were only rated to 90A and they should have been upgraded to 25mm?

2. The rating of the RCD is less than the rating of the service fuse. I would have thought that the service fuse should be of equivalent (or less) rating than any of the CU components?

Any thoughts?

 

[megawatt]

Right up BAS's street

 

[Steve]

Even if the RCD and service fuse were both rated 100A, the RCD (without any MCB characteristics) would be damaged anyway by the time the service fuse blew, if an overload occured.

I dont know any exact figures, but i imagine it blows instantly at some insane current like 1KA - service fuses are never in any great hurry. And a fault like this would trip the related MCB downstream of the RCD anyway. Wouldn't worry about it. Same setup in ours, and many others.

When does your friend ever use 90A of appliances at once?

 

[megawatt]

Crafty's right, most TT systems have an 80A 100ma TD incomer in place of the 100A main incomer (i.e. all load goes through it) and most I've seen have 100A service fuses with 16mm tails. New builds would fit 25mm but they are still very much in the minority (as are 100A 100ma TD RCD's).

Even my missus can't use 90A and she's got shares in the REC (we get Xmas cards from them)

I wouldn't worry either.

 

[ban-all-sheds]

Right up BAS's street

 

[Lectrician]

Domestic service fuses in our area are predominantly 80amp. Or even 60. VERY rare to find a 100amp here, even if the fuse carrier says so.

I still always fit 25mm as a rule.