Replacing a fused CU ?

[robgraham]

I was asked by a friend to investigate a seemingly faulty 20A DP switch for a washing machine today.

In hindsight slightly unwisely, I unscrewed the switch and pulled it forward to be rewarded with a flash, bang and the appropriate cloud of noxious smoke ! Hey isn't it great when you get the pyrotecniks rather than just something going dead.

Anyway my guess is that the line terminal had not been tightened properly originally, had over heated, the plastic broken down and my approach had resulted in the line terminal breaking free and coming in contact with the earth lead.

The fuse in the CU did not rupture !!

The switch was replaced.

The question arises now is should I strongly advise my friend to have his fuses CU replaced with a modern one; the emphasis is on the 'strongly' as my feeling is that despite a large flash/short as this was, the fuse did not protect the system as an RCD or MCB would have done. In fact an RCD would probably have tripped out long before I was asked to look and would have given the protection that the house should have (said having just has my own RCD trip and finding that it was due to the oven fan becoming leaky).

Rob

[John Rumm]

Helps liven things up! ;-)

It might not on a very brief arc over. Much the same happens when a filament lamp blows in some cases.

Well there are pros and cons. Have a look at the lists and see which apply most in this case:

[meow2222]

not unusual.

Rewirable fuses are still fully compliant for new installation. You'd derive very little safety benefit from adding MCBs. The reason the fuses didnt pop is that any trip device takes a cvertain amount of i times t to blow, and the fault, while it may have looked impressive, didnt draw enough i x t, and the fault blew itself open circuit first. You may have been scared by this, but safetywise its pretty much a non- issue.

?title=3DMCB

[John Rumm]

While this is true, there are two caveats worth mentioning: If the cables are the older style PVC with only a 1mm^2 CPC (modern cable has

1.5mm^2) then it is worth moving away from rewireable fuses since the protection will be marginal or insufficient in many cases. If the power (i.e. socket) circuits currently have no RCD protection then again it is worth introducing this, which realistically will also mean a CU swap.

[Dave Plowman (News)]

It's total madness starting to investigate a fault with the power switched on - especially on a strange installation where you've no idea if even the earthing is correctly done.

Most of these sort of faults can be sorted with a simple inspection - loose wires etc - and you don't need the power on for that.

[TheScullster]

"John Rumm" wrote

AIUI, the 17th Edition is now requiring RCD protection for any buried cables that are not either in earthed conduit or >50mm from the wall surface. This applies to new installs/modifications only though.

Phil

[Andy Wade]

To endorse what others have said, I don't think you should look at the consumer unit in isolation. If danger lurks it's likely to be out in the installation - as you discovered - and just modernising the overcurrent protection is a bit like adding airbags to a car with no brakes.

I suggest doing, or paying for someone to do, a full PIR first. Then assess what is found and effect appropriate improvements, prioritising according to budget, if that's a constraint.

You did check that the fuse hadn't been replaced by the proverbial 6 inch nail?

If the CU in question is the Wylex standard type, or one of its clones, a worthwhile low-cost upgrade is to replace the rewireable fuses and fuse carriers with HRC cartridge fuses:

In fact an RCD would probably have tripped out long before I

I guess that if you do replace the whole consumer unit now you'd have to follow the 17th edition and provide suitably divided RCD protection for everything. A sensible minimum though might be to just add a single

30 mA RCD (in separate enclosure, after the fuse) for the downstairs ring circuit, thus covering the sockets likely to be used to feed portable equipment outdoors.

[robgraham]

=A0 London SW

Yes Dave, and hindsight is a great instructor. At 66 and a lifetime's messing about with electrical installations, this is the first time that's happened to me and although your advice is absolutely correct, worth giving and will I assure you be followed in future, I note with interest that none of those whom I hoped would answer my original post felt it necessary to comment on my possible rashness - one could read in that, that they would have done the same.

Otherwise, thanks guys - the topic will be 'discussed' therefore, rather than 'strongly advised'

Rob

[Dave Plowman (News)]

Should hardly be hindsight, then. After my first electrical shock at a tender age I realised it's dangerous stuff and not to be messed with. I'm genuinely surprised you're still here to post if this is your normal way of working.

I'd be very surprised if they did. Only idiots work on live mains circuits

- unless there is no option.

You also have to remember that lots read this group perhaps without contributing. So general advice on best practice is just that and not necessarily aimed at an individual.

[Dave Plowman (News)]

Interesting. Why are non 17th edition CUs so readily available if any replacement must comply?

[John Rumm]

You can use them in compliant ways with RCBOs etc. There are also circumstances where you still want non RCD protected ways. Having said that, many people will carry on working to the older standards for a while, and the retailers will be happy to clear old stock. There will also be an ongoing desire to do like for like swaps on damaged kit etc.

[Peter Crosland]

Perhaps he should have his entire electrical system checked by a qualified professional followed by having any recommendations carried out professionally as well. It seems a classic example of why Part P was introduced.

Dons tin hat and retires to bunker!

Peter Crosland

[Sam Farrell]

Yes Dave, and hindsight is a great instructor. At 66 and a lifetime's messing about with electrical installations, this is the first time that's happened to me and although your advice is absolutely correct, worth giving and will I assure you be followed in future, I note with interest that none of those whom I hoped would answer my original post felt it necessary to comment on my possible rashness - one could read in that, that they would have done the same.

Otherwise, thanks guys - the topic will be 'discussed' therefore, rather than 'strongly advised'

Rob

[meow2222]

1mm2 T&E CPC will not have any difficulty at all blowing rewirable fuses. Its a long way from marginal. What can be marginal is its compliance with disconnect times for new wiring regulations - but thats nowhere near enough of an issue to prompt a CU replacment for 99% of the population.

Again, the benefit exists but is so small that only a miniscule percentage of the population would consider it worthwhile to fit the new CU. Almost no-one rewires their house every time a minor safety advance comes along.

NT

[<me9]

Or even with these?

[ARWadworth]

Exactly. A true pro (ie a good DIYer) as opposed to a sparky would buy a CU with a non RCD main switch and use RCBOs instead of MCBs.

The term "17th edition CU" should read "minimum allowed to meet the 17th edition at the lowest cost possible CU"

Adam

[Frank Erskine]

Haven't you replaced that CU yet? I thought I was the one that procrastinated :-)

I bought a split load Wylex CU over a year ago (from Screwfix) and was going to change it myself, but as I had the kitchen re-hashed a bit later and would have to (well, you know!) go some sort of 'official' route I got the leccy who did the kitchen wiring to change the CU.

It all saved going 17th edition... The supply to the fridge/freezer (using the old cooker supply ) is now a non-RCD radial feed (labelled as such).

[<me9]

It has long since been changed, and the mcbs as above sold on and re-allocated.

MK 12 way split load one here.

Everything bar the cooker (unused bar the socket) and a special circuit for freezer, alarm, doorbell, server and emergency 12 volt supply are now via rcd or rccbs.

[John Rumm]

If it can't achieve the required disconnect time, then it is at best marginal. (and that's just the old 0.4 sec requirement).

As we have both said, the question of whether a BS 3036 semi-enclosed fuse CU should be replaced is a broader question that needs to take account of a number of factors. Compatibility with the installed wiring is one factor.

No, I disagree strongly with this. There is no way you can call a RCD a "minor safety advance". RCD protection makes a significant impact on the survivability of electric shocks, and unlike nonsense like part P, the the use of them has a big impact preventing death and serious injury as a result of (ab)use of hand held appliances (which as we know is a significant number, unlike those attributable to fixed wiring faults).

[Terry Fields]

Are any figures available that show injury and/or deaths rates are less under the new regime (in this case an RCD CU) than the old (a fused CU)?

A casual reading of the group suggests that there are a number potential benefits from the technical advances in going from the former to the other latter - but how does it work out in practice?