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In the aftermath of the recent storms in the UK, one of my neighbours reported a flash and loud bang from his local transformer station, and the power briefly cut out.

After that, several households reported their appliances malfunctioning, including a hair-dryer getting dangerously hot, the smell of burning from sockets and boilers and Xboxes no longer working. The power company cut off power while they investigated and are now repairing/replacing everyone’s appliances at their expense.

So what would have caused this to happen? I can only imagine a spike in voltage as I understand that breakers and fuses blow on too much current, but I can’t believe there aren’t some safety mechanisms to prevent this from happening.

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    \$\begingroup\$ It was an electrical surge. A surge protector could have (as the name suggests) protected the items. There are safety mechanisms in place to help, but these cannot remove, only reduce, the risk. \$\endgroup\$ – Puffafish Feb 20 at 11:10
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    \$\begingroup\$ If you remember the storm "kyrill" from a few years back .. it managed to blow the wires of the 3 phases "together" on overland lines ... thus even after the transformers you had 415 Volts on a wire normally carrying 240 V - most appliances are not hardened for that much voltage- Many tv-sets, gaming consoles and "always on" devices were damaged. Other devices such as freezers / refrigerators "survived" because they simply "wait" most of the time .. and use current only during the short times when the compressor runs \$\endgroup\$ – eagle275 Feb 21 at 12:43
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    \$\begingroup\$ @Puffafish If it was 'just a surge' then I seriously doubt that the power company would have admitted any liability, eagle275's explanation seems rather more likely. \$\endgroup\$ – Mike Brockington Feb 21 at 15:21
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A broken neutral can cause serious over-voltages on an electricity supply.

UK supplies are usually three-phase and neutral, with approximately 415V between phases, and 240V between any one phase and neutral. The secondary of the substation is "star" wound, with the neutral being the common point. Most UK houses are supplied from one of the phases, plus the neutral.

If the neutral becomes disconnected at the transformer, then there is nothing to keep the phase-neutral voltage at 240V. Instead, it can drift wildly, depending on what appliances are plugged in in each house. The worst case is that one house may see the phase-neutral voltage collapsing to near 0V, while another sees anything up to 415V.

The houses are effectively connected in series between the phases, so the voltage varies with the resistances of whichever appliances are plugged in at the time.

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  • \$\begingroup\$ Something like this was my first guess too. \$\endgroup\$ – Darren Feb 20 at 14:35
  • \$\begingroup\$ One possibility that springs to mind is that the neutral was dodgy to start with, then a short-circuit happened which blew out said dodgy neutral connection. \$\endgroup\$ – Peter Green Feb 20 at 16:12
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    \$\begingroup\$ Is the suggested surge protector going to help, or just burn up first? \$\endgroup\$ – rackandboneman Feb 20 at 21:41
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    \$\begingroup\$ @rackandboneman, sounds like a great separate question. I'd recommend asking it and maybe linking to this question for background. \$\endgroup\$ – BowlOfRed Feb 20 at 23:33
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    \$\begingroup\$ @Turksarama: Because nothing drew too much current for long enough, just either overheated or failed catastrophically and stopped drawing current. Breakers only trip on current; they're in series with appliances so they can't tell the difference between 20A @ 240V and 20A at 415V. \$\endgroup\$ – Peter Cordes Feb 21 at 11:02
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We have a HNQ question on that right now over on diy.stackexchange.

That problem is a little different since that person gets 2 phases (hot-neutral-hot), and most UK homes get only one (hot-neutral). But your situation is absolutely identical to my own situation last year -- we're an outlier for the US, but our units get single phase just like you (of course, it's 120V), but our estate is served by split-phase (2 phases of 120V each). Half the flats are one one phase, the other half are on the other phase.

Split phase power is a line - 240V "long" with neutral in the middle. So it's very easy to see the behavior of a lost neutral; each phase is higher or lower than 120V, but the dead giveaway is, they add up to 240V.

We lost our neutral wire. So our flat was (tending to) see voltages go down to 80 volts, but simultaneously, half our neighbors were seeing 160 volts. So they blew a bunch of appliances. I figured this out because my storeroom is on the other "leg" so I was able to run over there and measure 145V. (uh oh!) Our voltage was lower because our "team" was in a tug of war: by pulling more current, we were pulling neutral closer to us, lowering our voltage and raising theirs.

It's your exact same situation with the "tug-of-war", except 3 teams instead of 2, and your playfield is a triangle. You occupy one corner of the triangle. The triangle is 416 volts on a side, with 240V to the dead center of the triangle. That dead center - neutral - has become un-pegged. If your team uses more current, you are pulling neutral toward your corner, raising voltage for the other teams. The highest possible voltage is 416V.

If the transformer is wired correctly, current will also try to return through your earthing system (the grounding electrode system in North American parlance) which will tend to pull neutral back toward the middle. That's how the linked question had less extreme voltage shifts than you might have expected, and why there was current moving on the grounding electrode. If your transformer's connection between its neutral and its earthing system is broken, that won't apply, and voltage shifts can run the full gamut clear up to 415V.

In the UK and the five continents which are not North America, most consumer power is wired so each flat gets neutral plus one of the three phases of power.

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  • \$\begingroup\$ ... but some countries, like Australia, have distributed earthing, with an earth point at every house, and mostly don't have this problem. \$\endgroup\$ – david Feb 21 at 3:08
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    \$\begingroup\$ Not sure about all the other continents, but in continental (western/central) Europe the most common setup is a full three-phase feed to every home, mainly for water heaters and cooktops. Single-phase service is mainly a thing in cheapo condos, causing general unhappiness when residents discover that they can't even get an ordinary electric stove. \$\endgroup\$ – TooTea Feb 21 at 20:18
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Sounds like either a step down transformer failed and you all got a seriously high voltage and current spike, or it was a lightning strike on the power station.

In either case the surge would likely have shorted out insulation on things like motors. The voltage might well have jumped into the tens of kV briefly. However unlike "normal" surges this would have been accompanied by very high currents.

So, all the breakers fuses etc might well blow, but too late to save some bits of connected equipment.

You could try buying surge protectors, but domestic ones are only rated for quite low energy events. Sticking 10kV at a few hundred amps across it for more than a few milliseconds would have destroyed that as well.

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    \$\begingroup\$ I have seen this happen to a residence which was struck by lightning. The owner showed me a surge protector strip which was twisted and melted. Bottom line is, nothing would have helped and this is a remote possibility. Except perhaps, personal property insurance. \$\endgroup\$ – rdtsc Feb 20 at 13:04
  • \$\begingroup\$ @rdtsc one thing I don’t understand here. If a transformer had failed, how would it be passing any current at all? \$\endgroup\$ – Darren Feb 20 at 14:38
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    \$\begingroup\$ @Darren If the insulation had failed between primary (high voltage) and secondary resulting in the primary voltage appearing momentarily on the domestic mains secondary \$\endgroup\$ – Dirk Bruere Feb 20 at 15:44
  • \$\begingroup\$ @rdtsc It's important to note that there are whole-house surge protection systems meant to attempt to protect the house from such things. Surge-protection strips are (to my understand) mainly meant to protect from surges within the home's electric system: georgebrazilplumbingelectrical.com/… \$\endgroup\$ – JimmyJames Feb 21 at 17:42

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