New CU

[caveman2]

The wired fuse CU blew today and necessetated a new one be fitted. It was supplying my utility room and unbeknown to me a 30A fuse had 2 radials supplying washing machine and freezer. It had lasted 20 years but the 30A fuse holder blew out and the back has gone as well.
I intend putting in a new one with 2 X 6A MCB's for lighting and toilet fan etc. but wondered about the radials. Could I use 2 X20A MCB's or would I have to make sure the WM and TD were not on the same radial or would it be best to make the radials into a ring and install a 30A MCB. What would be best?

 

[davy_owen_88]

The wired fuse CU blew today and necessetated a new one be fitted.

What do you mean the CU blew? Do you mean a single fuse or the whole lot just exploded? Before you go fitting a new one you might want to find out the fault that caused the damage in the first place.

It was supplying my utility room and unbeknown to me a 30A fuse had 2 radials supplying washing machine and freezer. It had lasted 20 years but the 30A fuse holder blew out and the back has gone as well.

Are you sure it was 2 radials not a ring circuit? What size cable was used? Have you performed any tests? Continuity and insulation resistance spring to mind...

I intend putting in a new one with 2 X 6A MCB's for lighting and toilet fan etc. but wondered about the radials. Could I use 2 X20A MCB's or would I have to make sure the WM and TD were not on the same radial or would it be best to make the radials into a ring and install a 30A MCB. What would be best?

Fitting a new CU is notifiable work so remember to inform your BCO before you start work.

If they really were 2 radials keep them as 2 radials. Not that a ring circuit is a bad idea, just that you get more capacity with 2 radials than a single ring circuit and you remove some issues with overloading a single leg due to poor design or a broken ring.

 

[luminaire]

There could be a lot more issues at stake here, more than you might at first realise. In any event this work is notifiable and is therefore best left to the professionals.

 

[caveman2]

What blew was where the wire was connected at the top of the 30A fuse. It had been there for 20 years and I assumed may have become slightly loose and if ( I cannot say now as I have temp. rewired) the WM and TD were on one of the radials the current at the time would have been high as they were both on. Unfortunately the back of the fuse holder burnt out and I was unable to replace it.
All the appliances have since been working on extension leads and I assume deteoration at the fuse connection was the problem.
Would I radial for WM anf freezer and the other for TD be OK or go for the 30A ring.

 

[davy_owen_88]

It is likely that the cause of the overheating was due to a loose connection. Remember when you replace the CU to cut back to fresh copper - if you don't expect a similar event in the not too distant future.

You need to confirm that they are radials, and not a ring circuit - a visual inspection behind each socket and a continuity test should confirm one or the other. It is also possible that one is a radial and the other is a spur from the circuit origin. If they are wired as a ring and you feed them from 2 separate 20A MCB's you will make things worse, not better.

 

[caveman2]

They are definitely radials although the fact that they are in a group of five single fused spurs and I can see both of the feeds one to 3 spurs and the other to 2 it would be easy to just complete the ring.
The only problem I see with the ring is if on holiday and the MCB goes off then I lose my freezer.
I assume a radial of 20A is not suitable for a WM and TD and if I uprated the MCB to 30A I would need larger than 2.5 cable is that correct?

 

[ricicle]

You sound like you know a little, but not enough IMO so I suggest you seek the services of a competant electrician