Reduced mains voltage

We had a couple of power cuts early yesterday evening (Wednesday), and for about half an hour were on reduced voltage, down to about 115 volts. Strangely enough, both the TV and computer worked perfectly normally (switched mode power supply?). But the microwave wouldn't work and none of the fluorescent lights would come on, although the politically incorrect filament bulbs were ok, if a bit dim (yet another reason for not abandoning them).

Some questions: Does it do any harm to leave equipment 'on' under such circumstances? I'm thinking of freezers etc that might be drawing current through the compressor motor but not enough to turn it over and might overheat (no back EMF or cooling), or anything else that might be drawing a higher current than normal to achieve a given wattage output, like TV etc.

What would cause such a severe drop in mains voltage for that time? We live in a small country hamlet with a transformer-on-a-pole serving six properties, overhead supply. A massive short somewhere surely wouldn't have gone undetected for that long. A phase gone down on the

11,000v supply to the transformer perhaps?.
Reply to
Chris Hogg
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In article , Chris Hogg scribeth thus

As expected..

Not a good idea to leave motors on under such circumstances..

Usually the loss of a phase on the 11 kV overhead system.

Though sometimes it can be a break in the LV side with a very poor return to the transformer. We had one like this at a remote site where the neutral return line had fallen and was laying across damp grass and making the return to the pole mounted tranny via the earth connection via the grass;!...

Reply to
tony sayer

Most modern switchers, as are used in TV sets etc, are 'universal', and will work from any voltage from around 100v up. They do this by having an additional 'front end' pfc supply in them. So there is actually three (almost) independent switchers on these boards - that's a small standby supply, the pfc supply that generates around 350 to 380 v DC irrespective of the input voltage, and the main 'run' supply which generates all of the rest of the rails needed by the TV when it is actually on.

Running such an item at reduced line voltage like you describe, will not therefore cause it a problem per se. However, if such a thing occurs again, I would certainly switch them right off or unplug from the wall, anyway. The reason that I say this is that when such events occur, the restoration of the full line voltage, or the nature of the fault that's causing the low supply, may result in nasty short duration very high amplitude spikes, or the mains input to your house going up substantially above the nominal 240v (230v) if the fault involves losing the neutral to your pole transformer. Switchers most certainly will not tolerate this, and the cascade failure of the supply which will result, may well be beyond practical repair. Complete replacement supplies are available at very reasonable cost from some manufacturers, but if the set is a no-name Chinese Tesco-Sonic, a catastrophic power supply failure, would likely render the set BER.

You have a good point about compressor motors.

The reason that the microwave wouldn't work is because most of these have a simple linear supply for the control electronics. So half the voltage into the mains tranny, would result in half the voltage out, which would likely produce a DC rail too low for the following regulator to produce an output. Even if you could get the control electronics going, the high voltage supply and filament supply for the magnetron, are both transformer derived, so would be too low under the fault condition, to allow the mag to generate microwaves.

(Most) CFLs won't work under low voltage conditions, because even though they contain what is essentially a switch mode power supply, it is a very crude one, which likely wouldn't run, and even if it did, would probably not produce enough volts across the tube, to strike it. Likewise, a standard (i.e. not electronically ballasted) flourescent fixture will not strike because there will be insufficient voltage to heat the cathodes in the tube enough to get the required amount of emmission, and not enough volts to work the glow starter to get the initial high voltage striking pulse across the tube.

Dimmable CFLs and electronically ballasted flourescents might work in some degree, at a pinch. Regular incandescents will, as you say, continue to work 'normally', allbeit a little dimmer than usual.

Arfa

Reply to
Arfa Daily

I've seen strange behaviour from CFL's on low voltage, including one coming on much brighter than it should have. Old inductive ballasted fluorescent tubes won't stand a chance.

Yes it could be a problem. Your voltage was so low, that an induction motor might manage to be stalled without wrecking itself. Fridge and freezer induction motors have so little spare torque even at their design voltage that they actually routinely fail to start in normal operation, and have circuitry to wait a bit and try again, so they'd probably protect themselves. Other induction motors might fare less well.

Probably. Can also happen if neutral breaks on LV supply, but then some people get high voltage, and others low voltage, and it varies considerably as you turn things on/off if there are only a handful of consumers.

Reply to
Andrew Gabriel

No, the protection will normally detect and isolate faults, although I recollect one case on a very long and very rural network where a conductor had broken mid-span and the conductor was still live on the ground. It was during a long hot dry spell, late summer after harvest, and the ground, stony chalk, was very dry. The fault level (the ability of the network to pass very large currents when a fault occurs) was very low because of the very long lengths of small capacity conductors, and there just wasn't enough fault current to operate the protection.

A phase gone down on the

Quite possible. A jumper, perhaps, on a section pole burned off because of a bad connection. You were probably getting a connection of sorts through other transformers on the system.

Reply to
The Wanderer

Compressors can get hydraulic lock, which will stall a compressor. Most induction motors have a degraded starting torque.

Reply to
Fredxx

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