[OT] Power cut and strange noise

[Spike]

A day or two ago, I was in the shower, lights and extractor fan on and window open; SWMBO was on the other side of the house near an open window.

The electricity supply went off - lights dimmed and fan slowed, only for circa one or two seconds later the supply to come back on.

This outage was accompanied by a very loud screeching noise, that sounded to both of us to be outside.

The cooker and microwave clocks were correct, no circuit breakers had been activated, the modem stayed up, the boiler did not trip out; nothing seemed to have been affected.

Any ideas as to what this terrible screeching noise might have been? It was…LOUD…All our nearby neighbours were out so not really worth asking them about it.

TIA

[Tim+]

Extractor fan bearing not liking the lower than normal speed maybe?

Linesman touched something he shouldn’t have? ;-)

Tim

[NY]

Sounds like a brownout: a reduction in the voltage but not a full-blown power-cut where the voltage drops to zero. Interesting that things like the modem, the boiler and the clocks didn't reset themselves.

I wonder if the screeching noise was a symptom of the thing that caused the brownout: maybe something nearby (electric motor) suffered a major mechanical fault such as a seized bearing, and this caused the voltage for everyone round about to drop until the fuse protecting the failed device tripped.

[charles]

Someone's burglar alarm witha flat back-up battery

[Mark Carver]

All those items (well, perhaps not the boiler CPU) normally have switched mode power supplies. They can operate on voltages lower than

100v and in excess (but don't push your luck) of 250v.

I had a 'brown out' that lasted for 30 mins, I was watching the telly unaware of problems until I switched a lamp on. The mains voltage was about 80 volts

[Rob Morley]

We had a blackout years ago that became a brownout before power was fully restored. Telly worked fine, I think 85V is about the cutoff for most multi-voltage PSUs but they might still work at lower voltage. The LED luminaires I just bought quote 85-265 V.

[Brian Gaff]

Some nasty device with a huge current seized up and squealed then blew a breaker somewhere. Are you near an industrial estate or printing works, something like that? Brian

[Brian Gaff]

Unlikely to be the fan I'd imagine, since its low current and would blow its fuse well before it got into supply dropping level. We had one of these strange things last night, but no noise, just things slowing, like my fan for 3 seconds. Only casualty was a crashed DAB radio. Brian

[jon]

Dowty Rotol had a huge DC motor ~2MW in a test rig for propeller blades, although it had its own line from the power station, it caused a local dimming when starting up.

[Spike]

This sounds a likely scenario - a brownout rather than a cut would explain why none of the electronic items reacted and the light dimmed but didn't seem to go out. There's no industrial activity near here, and the local sub-station is about 600yds away.

Being around 7:30pm all was quiet, until the mystery event happened. There's been no kind of activity to suggest urgent repairs to anything, and no-one has commented locally on the noise.

The pylons bringing the power to the area are about 100 yds away, could it be something to do with them, perhaps?

[Martin Brown]

Most things on modern switched mode PSU's will work quite happily through a brownout. My PC will still work and LED lights look normal even when one of the 3 phases is completely off! When that happens we are effectively on 200v effective. The kettle takes forever to boil and incandescent filament lights are visibly dim and orange.

I doubt it. The screeching noise might have been something that really didn't like a reduced voltage though. Brown outs were common round here when high loads switched on like the sewage pumping station until they upgraded the mains feed a few years back.

[NY]

I've not experienced a brownout for ages - but then if LED lights and electronic equipment can tolerate them, maybe they have happened and I've not known. What we get a lot of, every summer when the trees have grown close to the HV feed to the village, is short and total power cuts (*). And a power cut of a second is plenty long enough to cause all electronic equipment to reboot.

(*) Overhanging branch touches HV wire and shorts it to ground. Circuit breaker trips and retries - either from same HV feed or a different one.

[SteveW]

I used to work for a manufacturer of *large*, industrial compressors. The largest electric motor that we had was 8MW, although 3.5MW was more typical. When an idiot, who was supposed to be a professional, turned on the supply to a 3.5MW motor that still had earthing bars on it, it took out the test stand breaker, the distribution room breaker, the electricity company's breaker (in the same room), the site's substation and the one supplying that. Most of the local estate was blacked out for

3 hours!