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Rockingit

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So we had a powercut at home last night, overnight. Took my tired brain a few minutes to work it out, though, as a whole bunch of downlights, device electronics (washing machine controls, google hub, sky router, oven timer....) etc all stayed on but 'unhappy'. I first thought it might have been a surge from lightning as there was some around but then after flipping some breakers off and checking that the digital meter was actually also dead, I realised that that the incoming L must have still been live but the N broken, the resultant path for enough current to make all these micro-amp devices still 'work' was the PME.

Sod's law says that only hours before I'd left ALL my tools and test gear securely on site for the weekend in London as I couldn't be doing with a 14hr day on a Friday AND three flights of stairs, so I couldn't prove my theory, but I isolated at the main breaker, rang the DNO to report it and went to bed having dug out some candles.
 
I'm interested in what factors specifically alerted you to the fact that the fault was in the neutral, and specifically a PEN, rather than the line., in the absence of a voltmeter. Clearly if the voltage on a single-phase installation rises, that is a strong indication of a broken neutral in a 3-phase section, especially on TN-S where there it can float anywhere the load takes it. I have seen multiple SNE neutral loss incidents with L-N voltage all over the shop, but only one definite PEN loss where abnormal CNE-real earth voltage was the clue (and which the DNO attended within an hour, and fixed the same night)

If the voltage falls, it could be high-resistance in a single-phase line or neutral, or again high resistance or a complete disconnect in a 3-phase neutral. Any high-resistance point serving significant load is likely to dissipate enough heat to self-destruct sooner or later, so if the voltage is reasonably stiff (e.g. low but doesn't drop much if a heater is turned on) which in turn suggests a heavy load already in parallel, then a completely broken 3-phase neutral is more likely to persist than a failing connection in either conductor of a single-phase section.

In summary I guess that an estimate of the source impedance made by changing the load, and the nature of any apparent variations in the voltage that might be characteristic of arcing or of response to changing loads, would be my pointers in trying to understand the root cause without the aid of measurements. And it's the sort of thing where I would immediately try to identify the nature of the fault because of the different possible effects:

Hi-R line or single-phase separate neutral: Connection burns out.
Hi-R or broken single-phase combined neutral/earth: CPC becomes live to touch.
Hi-R or broken 3-phase separate neutral: Equipment can be overvolted.
Hi-R or broken 3-phase combined neutral/earth: CPC becomes live to touch and equipment can be overvolted.
 
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Many appliances turn into relaxation oscillators if you put the right resistance in series.
 
I'm interested in what factors specifically alerted you to the fact that the fault was in the neutral, and specifically a PEN, rather than the line., in the absence of a voltmeter.
It was in fairness just an educated guess. It could have been a very high R live, too. My very bleary-eyed diagnosis started with realising that all the circuits affected were on a sub board off a 1p MCB and for a moment wondering if there was some weird, caused-by-lightning kind of inductance going on, then I realised the whole house was dead. It was just very odd how some appliances sprang to life and some didn't - for example the Google Hub thought it was fully alive and went hunting for wifi, a wifi repeater in the same double socket didn't even blink, and so on. The same three LED GU10's ran at about 50% (out of an identical batch of about 30, etc etc).... Those that ran did also seem to be closer to bonding or sup bonding locations, although of course that could be a red herring. We're right out in the sticks here so I wasn't going to go investigating the neighbours! That being said, there's a supply pole opposite although our incoming is underground. It just seemed to figure that the most obvious energy path was L > N(internal) > PME parallel paths.

But like I said, I was so concerned that I went to bed!
 
What and how did the DNO respond?
Rang them up, spoke to a sleepy sounding engineer at about 1am, explained that we had an outage that didn’t seem to be on their online ‘we know’ system, he said he’d send a team out. About 30 mins later I checked online again and there was a postcode area listed saying 36 properties affected with an expected 2hr fix - woke up at 6am ish and still off, finally got up about 9 when the cats started assaulting me and it was on. Have to say, WPD around here are generally pretty good.
 
WPD always seem good around here, the power was off for a few hours last week, you could see they were trying to get it back on but it was only staying on for a few seconds. Was a good few attempts over 2 hours before it stayed on again!
 
That could have just been an unlimited auto-recloser duking it out with a fault that was not going to blow clear.
 

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