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Wilko

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Hi All, I'd appreciate your feedback and advice please : Customer has asked to relocate CU (and upgrade). I've done some basic tests and installation seems ok. It's TNS, today Ze 0.54, Ipf 0.55kA. FF has helped me and so I know it's got a BS1361 60A fuse. Existing 16mm tails about 3m long. New location may need 15m or more. Thinking 25mm 3core SWA. So, I will design check installation voltage drop and confirm increased 'Ze' doesn't knock out any final ccts. Should be fine. But protecting the new cable? Already today it would take about 0.5 sec for cutout fuse to discon I reckon. Many Thanks, David.
 
Phone the DNO and get them out for a PME availability check. If its not available 100mA S type RCD at the origin and switch fuse. Dual RCD board or all RCBO board inside that's either single pole switched neutral or double pole.
 
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Why do you want to rcd the swa submain.? Not necessary aslong as armour is correctly terminated and earthed, but at 15M you will need to fuse the submain.
Because the Ze is above the maximum permitted value already for the switch fuse without adding the length of the distribution circuit. Using the 100mA RCD will give a max Zs/Zdb of 500Ohms. The RCD will provide earth fault protection and the fuse will provide short circuit/over load protection.
 
Surely the tails are a distribution circuit and the 60A BS1361 would have a max Zs of 0.67 ohms, with a Ze of 0.56 ohms and 0.04 ohm increase from 15m of 25mm² SWA this would give 0.6 ohms and be OK.
So installing a 60A switch fuse to protect the tails should be a solution.
What have I missed?
 
Thankyou Lee and Richard for your thoughts, that's perfect ! Cheers, David
 
Interesting subject. The final circuits will need some consideration then; 40m of 2.5/1.5 will have R1+R2 of 0.78, so and RFC (32a) may have a Zs of 1.32, exceeding the max of 1.08. So reduce ocpd to 20a or 16a, or RCBO. Or have I got that wrong?
 
Thanks MW, I will check the actual as next step. I'm thinking it won't be too bad as there is the /4 to go from r to R if that makes any sense. But it's def a limit here somewhere and I'm sure they won't want their kitchen ring running at 20A. Cheers, David
 
Thanks MW, I will check the actual as next step. I'm thinking it won't be too bad as there is the /4 to go from r to R if that makes any sense. But it's def a limit here somewhere and I'm sure they won't want their kitchen ring running at 20A. Cheers, David
Zs = Ze + (R1+R2), so in your scenario 0.54 & 0.78 = 1.32ohms. Came up with 40m R1+R2 for 2.5/1.5 as example length of average RFC. Just saying.
 
Zs = Ze + (R1+R2), so in your scenario 0.54 & 0.78 = 1.32ohms. Came up with 40m R1+R2 for 2.5/1.5 as example length of average RFC. Just saying.
A ring final with an end to end length of 40m would provide an overall R1+R2 of 0.195 ohms, because of the ring path. ((r1 +r2)/4 = (0.48+0.30)/4.
If the ring was 160m end to end then you would get 0.78 ohms R1+R2.

However a 40m radial circuit would give a similar result but would be just in on 25A breaker with Zs as 1.40 ohms.
 
A ring final with an end to end length of 40m would provide an overall R1+R2 of 0.195 ohms, because of the ring path. ((r1 +r2)/4 = (0.48+0.30)/4.
If the ring was 160m end to end then you would get 0.78 ohms R1+R2.

However a 40m radial circuit would give a similar result but would be just in on 25A breaker with Zs as 1.40 ohms.
True enough, forgot about the /4. However, spurs potentially can raise the R1+R2 up. Just thinking out aloud.
 

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