Electrical concerns

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 http://www.flameport.com/electric/old_equipment/wylex_standard_brown_wooden.cs4 I 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
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On 09/09/2010 17:35, Martin Pentreath wrote:

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.
--
Cheers,

John.

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Hi John,
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.
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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:-(
--
Adam



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I'll display my ignorance here. Is there a technical reason why this is, or 'simply' a commercial/economic one?
Thanks,
Sid
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On 09/09/2010 21:43, Sidney Endon-Lee wrote:

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).
--
Cheers,

John.

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John Rumm wrote:

Thank-you John - it seems obvious when you point it out so clearly.
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Sidney Endon-Lee wrote:

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.
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On 09/09/2010 18:51, Martin Pentreath wrote:

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.
--
Cheers,

John.

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Martin Pentreath wrote:

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.
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On 09/09/2010 17:35, Martin Pentreath wrote:

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.
--
Cheers,
Roger
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On 09/09/2010 19:12, Roger Mills wrote:

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

John.

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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.
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On 09/09/2010 22:11, Martin Pentreath wrote:

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

John.

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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 :-)
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On 10/09/2010 00:22, js.b1 wrote:

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...
--
Cheers,

John.

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On 09/09/2010 23:41, John Rumm wrote:

Also, she's more likely to be able to reset a trip than to re-wire a fuse!
--
Cheers,
Roger
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On 10/09/2010 10:56, Roger Mills wrote:

Indeed. Personally I would go for type Cs over fuses. Fuses are not immune from popping on a bulb failure (especially with mains halogens), and are harder to replace.
--
Cheers,

John.

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On Thu, 09 Sep 2010 19:12:52 +0100, Roger Mills wrote:

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




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wrote:

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
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