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
I replaced the fusewire in the cartridge for her today, and everything
is working again.
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
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?
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.
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.
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
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).
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.
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.
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.
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
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
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
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 :-)
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.
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