I Nearly Died

Cleaned and repaired, but still looking somewhat Daliesque

Sorry, I suppose I shouldn’t have intimated that I was nearly killed and then not signed in for two days. You might have thought I’d succumbed to my injuries. However exactly one goes about succumbing.

In fact it’s Christmas shopping that’s kept me too busy to write. I wasn’t injured at all. It was just my mother’s house that nearly burned down.

Fortunately I was staying at my mother’s. Even more fortunately, I was still awake at half past two in the morning. OK I’m usually awake at half past two in the morning, but it’s lucky that this is usual.

The power went. A bit unexpected, but it is the middle of winter. I thought I heard one of the trips go in the circuit breaker box though (or fusebox, as we still call it).

Then another click, and the power was back on. Weird. Another, and it was off again. OK this was not good. Especially not when you begin to smell smoke.

I went to look at the fuse box, and saw something that put the fear through me. Picture the biggest wire in the house, the one that carries power in from the mains supply and is several times thicker than the ordinary domestic wiring.

Now picture that glowing like the filament of a light bulb. That’s pretty scary, isn’t it? And the only reason I could see it was that the casing of the fusebox was already starting to melt. This cable was coming off what looked like a combination of really heavy-duty circuit breaker and a huge fuse. Yet neither had stopped the current flowing – the breaker seemed to be stuck in the on position.

Fortunately it wasn’t too hot to touch, and I could trip it manually. The glowing and the smoking stopped, and the wooden board the box was mounted on survived with only a scorching. But clearly there had nearly been a fire. If my mother had been in the house alone, I dread to think.

Next day, after we replaced the failed breaker and the one that’s supposed to back it up, I also bought an extra smoke alarm. And replaced the battery of our existing one even though it still passed the test. And we are going to change the bedroom windows to ones you can escape through. It still doesn’t feel like enough though. Apparently this is not a freak occurrence. They age and fail – even though this installation was only about twenty-five years old.

I’m astonished and disturbed, frankly. With well over a century of design behind them, I would have thought – hell, I’d taken for granted – that household power systems would not have a single point of potentially lethal failure.

12 replies on “I Nearly Died”

Scary stuff. We always tend to worry about the slim chances of being confronted with terrorism or homicidal axe-murderers, and then it’s the small domestic things that end up being the real threat.

Right, let me not derelict my duties as full time blog commenter and occasional building services engineer.

Sounds like a loose contact. It’s surprising that sort of failure isn’t more common, actually. You install that cable and never think about tightening those lugs again. In fact, they loosen up due to temperature-led dilation and vibrations from the transformer. Poor contact pressure leads to overheating of the cable. Because the temperature increase is very localised the breaker or fuse may not see this heat. Instead they see normal intermittent current. The protection device thus is not getting enough heat to trip whilst the cable continues to overheat. This is particularly the case with fuses which are slow to burn in order to avoid nuisance loss of power.

The second device is not something I’m sure of from the info at present. It may be a breaker, but if it is after the fuse then it may already be too far from the heat source to trip. But I’m thinking it’s not a breaker in the traditional sense. That is, it doesn’t have a bimetal for thermal tripping. It looks like an RCD (residual current device), also known as ground fault breaker in the USA. If so then it won’t react to heat; it’s monitoring the difference in current that is being fed via the live cable and the returned current on the neutral. If that current is around 5-30 milliamps then the RCD trips. It’s supposed to protect against a person being electrocuted by a small current that would cause cardiac arrest. So to recap, if that device is an RCD (is that blue thing a test button?) then it wouldn’t protect against loose contacts.

So what protects against nascent fire on live cables? An arc fault circuit interrupter. These are mandatory in many applications in the USA but not so in Europe where they remain virtually unknown. Basically anything like a loose or severed cable causes arcing which over time can ignite a slow burn of combustible material. The arc currents can be very low, as low as microamps. But the temperatures in the arc are very high (it’s plasma). The arcing, thankfully has a distinct waveform with high frequency transients. So an AFCI can detect a harmful arc and trip.

BUT! An AFCI would not have protected you in this case. They look for arcs downstream, and your loose contact is upstream. Since the utility company will not install an AFCI on their end of the cable you have a blind spot which will always be vulnerable to something like this happening. The only way to prevent this failure would be either to carry preventive maintenance tightening screws, or predictive maintenance by getting your hands on one of those fancy infra-red cameras. I imagine many safety inspectors forget to tighten screws when they inspect a home. So maybe you need to go home and check your fuseboard now.

Thanks droog, I’d hoped you’d come bearing technical enlightenment. Your explanation makes a lot of sense, and I’m glad that this sort of thing can possibly be prevented simply by regular tightenings. Rather than, say, having to regularly replace the entire board just in case.

The secondary device is indeed an RCD. I’d forgotten the name. It probably wasn’t at fault but we had it replaced because it was of the same vintage as the master breaker (or whatever you call that) – and the electrician happened to have one with him. We were still pretty shocked by the event so a precautionary €25 seemed well worth it.

I’m a little unclear about what counts as “upstream”. No, let’s be honest, I’m perplexed about what counts as upstream. But my description might have been misleading. It appeared that the arc occurred on the connection from the main breaker to the rest of the board.

P.S> Transformer vibration? There’s no transformer on this board is there?

Upstream would be before in terms of where the energy flows. The transformer is upstream from the meter. The meter is upstream from the board that caught on fire. The outgoing breakers in said board are downstream from the main incomer switch.

Since the incoming main cable was red hot as you described I figured the arc or loose connection was on the upstream side of this main switch. If the arc had been on the downstream (outgoing) side of the switch chances are the cable would not get as hot without burning the fuse.

But I may be wrong. If, as you say, inspection reveals that the side of the isolator that is burnt is the outgoing side then I’m a bit clueless as to how the incoming cable could be glowing hot when you entered the room. Are you sure the flames did not rise up and burn the outgoing side of the main switch?

I oversimplified a bit with the description. The wire that burned seems to have been the short length downstream of the main breaker rather than the one coming into it. Would that suggest that the fault lay in that breaker itself?

Well if the cable that was burning was on the outgoing side of your main incomer then you would have needed arc fault protection on the main incomer device. This is not the common installation at present, but I guess it could be done. That explains better why the fuse didn’t burn since it probably had the RCD between it and the heat source (is it a fuse, then?). If an AFCI on the outgoing side of the board tripped you would think the fault is somewhere in the house’s wiring, not the leads feeding all of the outgoing breakers. You’d go mad searching for that fault.

Oh, and when I say check your fuseboard now I mean do it in a safe manner. I don’t know what allowances Irish regulations give you for working in your own home. Here in the UK you’d not be allowed to do this yourself. The general public can work on final outlets but they can’t touch the distribution board.

I think we have a right to bugger around with this board. IIRC after 25-odd years my father put it in originally, though it’s been done over by qualified electricians several times since. The Electricity Supply Board only fitted the box outside which we later connected to this one, and as the building was then at the earliest stages of renovation the outer one contains little except the meters, a very rudimentary distribution board and one huge breaker.

That then is the original – or master master – breaker, and fortunately allows me to isolate the inside board for maintenance.

You sound like a woman with a plan and a good one!The year that Dan and I got married my house brenud to the ground. I was 24 years old. I recently wrote about it on my blog. Needless to say, our house is fire safe right down to the dishwasher that does not have a heating element ;-D In New Mexico where this happened the fire department would come out to your house and walk through with you and give you tips about making your house fire safe. Also lots of fire departments have these travel houses they come to your kids school and walk them through what to do. Let me know if you need more tips I could go on -Ladybug hugs,;D

Thank you. This was over a year ago now, and a lot has been done to improve the electrics since. It must have been terrible to lose your entire house; l imagine that put a lot of things into perspective.

I just wonder why I sound like a woman! 🙂

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