Electrical concerns

[Martin Pentreath]

Hi all,

A friend of mine had the fuse in her lighting circuit blow last night. When I say fuse I do mean fuse, she has an old Wylex fusebox dating I guess from the early eighies when the flat was converted, with proper fusewire. I'm talking about this sort of thing

replaced the fusewire in the cartridge for her today, and everything is working again.

Question 1: I was a bit concerned when she told me what triggered the trip. She was on her way out of the flat, she had her keys in her hand, she went to turn off the light at the switch, and as the keys brushed against the switch there was a blue flash and everything went dark (she didn't get a shock). That sounds rather scary to me, any ideas what could be behind this?

Question 2: Subject to the whole place not being condemned as a deathtrap by your answer to question 1, I think as a minimal nod to bringing the arrangements up to a reasonably modern level it would be a good idea to replace the fusewire cartridges with RCBOs. Is this actually doable, and if so can anyone point to the type of RCBO that would fit?

Cheers!

Martin

[John Rumm]

What sort of switch was it? To get the symptom described it would probably need to be a live to earth fault caused by the keys. This would suggest a metal clad switch of some sort that is earthed, and something else on the switch (such as the toggle) being live. Short them with the keys and bang!

If the place is carpeted and she was wearing shoes its entirely possible that she was completely isolated from earth and hence felt no shock.

80s wiring in itself is unlikely to be seriously dangerous through age alone, but then again one does not know what older vintage wiring was left in place when the flat was converted. It sounds like inspection and testing of at least that circuit is required urgently.

I am aware of plug in MCBs for the old wylex boards, but not RCBOs. To an extent however this is not addressing the root cause of the problem.

[Martin Pentreath]

Thanks very much for the reply. The light switch in question is an unremarkable plastic one - the only exposed metal being the two screws that hold it in place.

Of course alarm bells immediately rang in my head when she said what had happened. She understands so little about electricty that she was totally unfazed by it. I think she regards all electricity as some sort of black art, and just expects these sorts of fireworks as part of the deal with the devil. I didn't want to put the fear of god into her until I had taken some advice on here about the best way forward.

[ARWadsworth]

It is worth checking behind the lightswitch. It could be a short that cleared itself when the fuse blew.

Also, did the bulb blow? A lamp blowing can take out a fuse and the flash would be visible at the switch due to the high current that was passed.

It would need a new CU to provide proper (17th edition) RCD protection to the installation. There is no such thing as a retro fit RCBO for the old Wylex fuseboxes:-(

[Roger Mills]

Are you sure that it was caused by contact with the keys - or could that have been a coincidence? Did she actually operate the switch?

It's not uncommon for a bulb to fail and take out a fuse or breaker when you switch a light *on* - and I'm wondering whether something similar can happen when you switch *off* - particularly if the contacts bounce. If that happened, the blue flash could possibly have come from inside the switch.

[John Rumm]

In which case, the situation Adam describes seems most likely - could be as simple as she bounced the switch a bit and the bulb decided that was enough to blow. Big current surge, which took out the main fuse, and also could cause enough arcing in the switch to be visible.

ordinary bulbs or halogens?

Do you know if the bulb blew?

I would agree, a peek behind the switch to check for anything untoward and to check the screws are tight would be worthwhile.

[John Rumm]

especially if the keys are causing you to fumble with the switch a bit...

[Sidney Endon-Lee]

I'll display my ignorance here. Is there a technical reason why this is, or 'simply' a commercial/economic one?

Thanks,

Sid

[Martin Pentreath]

Thanks to all, yes the bulb blew. I'll have a look behind the switch, but for now it sounds like a relatively harmless occurence, and probably just as well I didn't frighten the life out of her by telling her to move into a hotel immediately. She's lived there for ten years without ever needing to replace any fuse wire, so on the whole the electrics seem fairly stable and fault-free, and nothing has happened recently to change that.

I still think it might be an idea to swap the old fuse box for a modern RCD protected version, but I don't think that's likely to happen imminently.

[Dave Liquorice]

I'd also expect there to be a lump out of one of the keys... no mention of a bang or pop either.

[Tabby]

When you look at the daeths saved per pound spent on a range of things, retrofitting RCBOs, or a new CU is a real waste of money & time. With the same funds there are way more useful things one can do.

NT

[John Rumm]

Technical really...

An RCD (and hence by extension a RCBO) measures the imbalance in current flow in the live an neutral conductors. If it senses more imbalance than its trip threshold, it trips.

This means there is a prerequisite that it be connected inline with both live and neutral. The fuse "sockets" on the wylex boards only give access to the live terminals, and hence you can't support a RCD.

A DIN rail mounting RCBO for a modern CU, has terminals to connect to both the live and neutral of the protected circuit, and then another flying lead that connects it to the neutral busbar (in addition to its live busbar terminal at its base).

[John Rumm]

She may not thank you - nuisance trips of MCBs with lightbulbs blowing are more common. (mitigated a bit if you spec type Cs) ;-)

[Dave Osborne]

The physical size of an RCBO precludes its implementation in an old-style Wylex consumer unit. Also, whereas a fuse and MCB have only two connections, an RCBO has four or five, and the Wylex box does not have provision for the extra wires.

[js.b1]

Actually despite a modern CU with RCD protection, you could use a DIN rail fuse-carrier and suitable fuse.

It would mean a departure re "Type Approval Fundamentalism". I would also require a fuse-carrier which fits properly.

Hager used to do one a fuse carrier for their stuff. MEMshield-3 probably do one, and in metalclad look to be the best CU out there. MK will take Merlin Gerin stuff ok, and they do fuse carriers.

Had I known the MK consumer unit was polystyrene (PS) I would have run to MEMshield-2 at the time - just on principle :-)

[John Rumm]

Indeed, but bulbs can take out fuses as well (as the OP found), so on balance I think I would go for the type C MCB.

Well not really - if you can find a carrier for the particular brand of CU. They do seem to be getting rarer though.

Yup, I have used those in in the past, handy for the head of a submain that's going to have a MCB at the far end close to current rating of the fuse - lets you keep discrimination usually.

Nice but pricey usually...

[The Natural Philosopher]

My vote goes for a monetary connection and disconnection, an arc and a bulb going down. I often see sparks from light switches due to arcing if its dark when I put em on.

A well shorted bulb can take out a fuse.

[John Rumm]

thing

> I replaced the fusewire in the cartridge for her today, and everything

When considering any wiring changes where the justification is "safety", one has to look at the overall picture. A blanket statement that "a new CU is a real waste of money" is potentially reckless IMO. Indeed there are many times where this will be true, but there are also some times where there is real value in a change of CU (or other electrical upgrade).

While deaths from electrocution are indeed very rare, death is not the only bad thing that can happen; shocks, burns, and house fires etc are also undesirable while also being more common outcomes of electrical accidents. Trips and falls (especially in the dark) even more so.

Things to consider when looking at risks are factors like:

Are there kids or old or infirm people living in the house?

Is there a garden or garage where electric power tools might get used?

Is there an excess of extension leads in use posing a trip hazard?

Is there poor discrimination - especially the 15th edition style "whole house" RCD or even older VO ELCB?

Is a lack of sockets resulting in dangerous overloading using old unfused adaptors?

Are leads being draped over hot surfaces due to poor positioning?

Are people likely to abuse rewireable fuses?

Is the lighting adequate and in the right places?

Are there mains powered interlinked smoke alarms?

There are also less obvious modern trends to look at, like upgrading thermal insulation in houses. It may mean that circuits need their current rating downgraded (especially if rewireable fuses are in use), and or need dividing into additional circuits. Replacements to service pipes in plastic may have knock on effects on earthing and bonding and also the correct functioning of protective devices.

[Tabby]

Around 20 deaths by electrocution & 400 or so by fire a year. RCDs or RCBOs could at best only stop a percentage of those. Even =A310 on smoke alarms would stop more.

Compare that to a quarter of the entire population dying from easily preventable causes, 6 figures per year. Half the population dies from heart disease & cancer, and the general concensus is that half of those are easily preventable by improving diet & similar measures.

Its all about lives per pound and per hour spent. If you look at the top 10 killers and narrow it down to the easily preventable ones, it becomes clear that updating a fusebox in good working order is bordering on an exercise in denial.

NT

[Sidney Endon-Lee]

Thank-you John - it seems obvious when you point it out so clearly.