Replacing a 63A Switchfuse

YEDL have just installed a new PME earth in our house. I'm about to install a new CU, but want to check something first.

The current CU is an old Wylex wired fuse job, fed by about 8m of 6mm T&E, which scared the life out of me when we found this out (only bought the house 3 months ago) because there are a number of high power appliances installed in the house. This is protected by a 63A switchfuse (although I haven't checked the fuse rating in it TBH) next to the meter. I assume this is there because of the distance to the CU as much as the under sized feed. The new CU is going next to the meter so I want to ditch the switchfuse because it's only 63A.

The tails from the meter to the switchfuse are too short to feed the new CU directly so I need to extend them, and I thought I could either use a couple of henley blocks to join them (do the DP blocks mean I can just use one?) or mount a 100A isolator module in a single module enclosure (Screwfix 32311).

Which would be the recommended method?

Reply to
Danny Monaghan
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I would recommend installing new tails from the CU long enough to reach the meter, and getting YEDL to connect to the meter, avoiding any joints.

Failing that, henley blocks, yes the DP ones mean you can use just one.

Owain

Reply to
Owain

Are you sure it is 6mm^2 ?

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so, my quick calculation suggests it might be the other way round (i.e. the fuse is protected by the 8m of cable)!

If you take the earth fault loop impedance as 0.35 for the PME earth, plus 10.49 x 8 mOhms for the T&E, you get a fault current of about 530A. That will open a 63A HRC fuse in say 0.4 secs. So the minimum size CPC you need in the cable is sqrt( 530^2 * 0.4 ) / 115 = 2.9mm^2

and 6mm^2 usually has a 2.5mm^2 CPC...

(10mm^2 or 16mm^2 T&E would be ok)

Yup, most suppliers have a limit to the length of feed to the CU - often

2 or 3m. Beyond that you need the switchfuse.

With a DP Henley you just need the one. Personally I would go with the switch to do the join. That way you make life easier should you want to split the tails later (no need to pull the main fuse again) and you can also completely isolate the CU when working on it of for testing.

This was what I did with mine (see pic):

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Reply to
John Rumm

Yup. The feed to the shower uses bigger cable!

It's obviously been in a few years, but we had an electrician check the house out before we bought it and he never even mentioned this.

I've had a large tumble dryer sat waiting to be wired in for 3 months because of this - I daren't put any more load on the current setup.

Heh! I have an 8m fuse!

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Thanks - I was kind of leaning to this option.

Reply to
Danny Monaghan

Underrated, but maybe not as bad as it sounds. 47A is continuous cable rating rather than a short term peak rating.

NT

Reply to
meow2222

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