Power Cut

[Lawrence]

Last night I had a power cut lasting 2 and half hours. This is not unusual in our village but what was unusual was how it started. Initially the oven switched off and the kitchen ring went off but all the lights stayed on albeit dimmer than normal. Over the course of the next few minutes, various lighting circuits went off and on a few times before eventually all went off. While all this was happening I had enough time to check my CU and pop next door to see if they were affected. We came to the conclusion that a transformer was in the act of going down but can anyone be more specific?

[Dave Liquorice]

What lamps for lighting? CFLs are on or off at more or less the same brightness over a large supply voltage range. Confused the heck out of me at 0300 the other winter when an ice storm meant our volts dropped to not much over 100. CFL lights worked, router, switch, server where on (SMPSU's), other bits of IT kit not (lump of iron PSU), tungsten was very red and dim...

Synchronous on/off switching or different circuits at different times. Synchronous would probably be an auto recloser in the feed some where detecting a fault, tripping then reconnecting a few seconds later. If this happens more then n times in x minutes it locks out in the off state. Saves the DNO having to send an engineer out to reset after transient faults like lightning or a small branch touching the line.

[Graham.]

Nothing in that strikes me as unusual for a severe brown-out.

It is normal for an oven to "fail safe" when the power is interrupted and require you to reset the clock, all that happened was this occurred during the low voltage period of the brown-out rather than when it went out completely.

What technology lamps were they? I would expect individual CFLs and linier florescents to go off at differing voltages.

[GB]

Does the small branch cease to exist after the first touch (what a way to go, in a puff of smoke!), or what stops it touching the line again?

[Dave Liquorice]

transient

As you summise a small branch gets exploded. There is a lot of energy available from an 11kV line, just 100 A fault current @ 11 kV is 1.1 MW of power. A branch doesn't stand a chance...

[Andrew Gabriel]

Do you have a 2- or 3-phase supply?

[Lawrence]

happening I

Just single phase.

[Lawrence]

circuits went

Mixture of filament, CFL and linear fluorescent. I guess that answers my query.

[Martin Brown]

Sounds like the sort of thing that happens when there is an abnormal load on the line which is insufficient to immediately trip out the protection circuit but enough to drop the volts at consumer unit.

We used to have a problem like that when the sewage pumping station down the valley switched on at 6.30pm every night the lights would flicker and dim until the pump got properly up to speed. NEDL replaced that cable a while ago and it has never been any bother since.

CFL and LED will run OK almost as bright on a surprisingly low mains voltage but draw more current to do so.