At what low mains voltages do devices stop working?

[Jeff Layman]

We were told last week that a temporary generator would be installed because the substation next door needed some attention. It went in this morning, and by late afternoon was having problems. The resulting effects were interesting.

The newish Panasonic TV went off. After a few seconds it went on again, then off, etc. I disconnected it from the mains. As far as I could tell, everything else seemed OK. I turned on our elderly Roberts DAB radio, and although it came on it wouldn't tune any station. It was then I noticed the house lighting didn't seem its normal brightness. All the lights - LED, compact fluorescents, and one tungsten (over the hob) were working, but dimmer than usual. A couple of old T5 fluorescents would not turn on when I tried them. NB - none of the lamps are dimmable.

A digital voltmeter showed the mains voltage was 160V. I turned on our old Sharp 22" TV and it worked without problem. After a few minutes, the Sharp went off. I checked the voltage; it was now 140V. Some of the LEDs no longer worked. These were Wilko ones with a semicircular design of light emitter (they no longer supply them, and I haven't been able to find a picture on the internet). The other G9, COB-type design, continued to work.

Then the remaining LED lighting went off. A quick check showed the mains was now 80V. Amazingly, the compact fluorescents were still working. A couple of other devices working off of wall warts still operated, but that is not too surprising. I was going to investigate further, but the power failed completely.

I find it extraordinary that the compact fluorescents stayed on at 80V.

[Michael Chare]

The PSU for my laptop says it should work at 100v but if the voltage dropped that low we might not be able to cook the dinner.

[John Rumm]

We have a HV cable fault a couple of weeks back. It caused a power cut for a couple of hours in the morning. Then came back when they "fixed" it (turns out they just routed round it and stuck in some temporary gennys - because it was underground and worse than they expected).

We then got another power cut a few hours later during the early evening, while they actually fixed it. (had to cook dinner on the two burner gas ring set I normally drag out to sit beside the BBQ!)

When they fixed it for real, and the power did come back it was clear not all was well. The voltage was sat about 200V. Most things worked ok, although lamps were dimmer - even LEDs. The most notable exception (which drew my attention to the voltage) was the microwave turntable motor sound "weaker", and there was pretty much no heating effect at all for anything you put in the oven.

[Mark Carver]

I remember I was sat one summer evening watching the TV (this was 1998 ish, so a CRT telly). I thought, but wasn't sure that the scan momentarily collapsed and restored. Didn't think any more of it, but then a few mins later the phone/answer phone that was powered by a wall wart started bleeping and clicking. The LED call indicator was randomly flashing digits, and they were dim. I pulled the connector out of the unit, and measured the DC volts from the wall wart, it was only 4 or 5 volts (should have been 12 I think) At that point the doorbell rang, it was my neighbour. She asked whether I had any power. Well yes, I said, and went to switch on the hall light to prove it. The bulb just gave a tiny glow. All the pennies dropped at that point !  I hit the main breaker on the CU, and then just energised the lighting circuit alone. Measured the voltage, it was 80 volts.

The remarkable thing was the TV had been perfectly happy working at 80 volts. That's SMPSUs for you I suppose. I bet its mains flex was warm though ?

[Brian Gaff (Sofa]

I find it amazing that the company have a generator for 230v which actually runs at such a low voltage without simply stopping.

Many devices these days are happy on very low voltages but they do not like variable waveforms and instability of voltage over time. Many go into self protection.

I'd have thought that a power company should understand about loads and capacities of their generators and what will happen if lots of power is taken off a single phase. I mean sounds like a third world country story where the generator runs off an old VW Beetles back wheel and a belt. Brian

[NY]

A few years ago, the electricity company was doing a lot of work on its overhead mains-distribution cabling in various villages - maybe they were replacing four separate wires (three phases plus neutral) with a single fatter cable. This meant that they planned to disconnect each village in turn from the mains and install temporary diesel generators. The one that served our group of 20 or so houses was within earshot of our house.

It was surprising how quickly I got used to the constant low-frequency throbbing. However what was very noticeable was the *variation* in engine note. I could actually hear the legendary "Coronation Street advert break effect": at about the same time every evening, the engine note changed and the engine sounded to be working harder. I put the telly on the following evening and the change happened about 30 seconds after Coronation Street went to a commercial break - presumably as everyone switched their kettles on for a cuppa. I knew it happened and that power stations were geared up for suddenly having to supply extra power according to surges in demand during advert breaks, but I'd never witnessed it in action.

I'm not sure how good the frequency regulation was, but we were warned by the electricity company that mains-powered clocks might not be as accurate while our power was coming from the generators.

Where we live now, the mains voltage varies quite a bit. I've got an energy monitoring plug and when I remember I look at it. The frequency is always between about 49.8 Hz and 50.2, but the voltage varies between 227 V (the lowest I've seen) and about 265.

[Mark Carver]

Oh, it won't be. Our bit of our housing estate spent about 10 days on a genny. The clocks gained about 90 seconds a day, after the first week our two lads noticed they were arriving  early at school :-)

[Martin Brown]

LED ones ought to be able to maintain most of their brightness.

Filament ones would be quite dim on such a low voltage and kettles take an age to boil. We sometimes end up with about 200v if a single phase (not ours) croaks and the village is left with 2 live phases and neutral. I can tell if this happens by looking to see which emergency lights come on in the village hall. My PC ploughs on quite happily.

I'm surprised that a modern TV doesn't have a PSU good for 100-240vac. It hardly costs them any more to do that as the main PSU expense is mostly providing insulation good for 240 mains.

Once the arc has struck the voltage needed to maintain a current flowing through the plasma can be very low indeed.

They should have been able to maintain something nearer 210+v if the generator had been properly sized for the job. Unless the fault is that one mains phase is down and they are trying to fix it.

[John Rumm]

Yup SMPSUs will quite often have a wide operating voltage range.

With a SMPSU they typically draw current only for a short section of the mains input waveform. So with a lower voltage it would draw less peak current, but for a larger proportion of each mains cycle. So the average power would remain much the same...

Some induction motors can be harmed by low voltage though - since it will draw more current when under load at lower voltage, and so will get hotter. You might exceed the maximum current rating for the windings. You also lose starting torque - so there is a chance of stalling it at startup. So fridges / freezers are probably the most likely things to get killed by low voltage in a domestic environment.

[Peter Burke]

Jeff Layman [e-mail] wrote in news:rv9vgu$q0s$1@dont- email.me:

If this is still the case then I'd have an urgent word with them about the out of spec undervoltage.

Fridges and freezers are particularly susceptible to undervoltage leading the compressor to stall and burn out in short order.

In the developing world where brownouts are not uncommon it's common to connect voltage monitor ahead of these items to disconect low voltages automatically and only reconnect when the voltage recovers fully.

[John Rumm]

The voltage drop may not be at the generator itself - it might just be injecting power at a different location that makes the voltage drop where it reaches your property more noticeable.

[Martin Brown]

To be that far out it must be either vastly overloaded or not driving all three phases. Worst I have seen on a generator set is about 210v.

The default assumption on UK mains supply three phase wiring is that the loads on all three phases are approximately in balance.

[charles]

when we had a local under road cable fault - we had a genny to ourselves. Mind you it was installed at about 2am, failure had been about 1pm. But, the stuff in the freezer was fine. We had the genny for 4 days.

[The Natural Philosopher]

Mine did that every time I tried to use its Ethernet interface. I took it back and got it replaced. Its a feature of those beasts. Dont like low voltage AT ALL

I turned on our elderly Roberts DAB radio,

[The Natural Philosopher]

I worked once on a scope design that was happy from 36V DC (what an aircraft raising its undercarriage drops to) to 250VAC. all on the same PSU with no taps changed.

It can be done if people want to do it, but the grid is usually much more stable than that so why bother?

[The Natural Philosopher]

When they cut the 11KV ring to get it from over my house to under the road, two of us had a genny for a week.

Not very stable. Things tripped. But the microwave didn't dim the lights as much as the pole mounted transformer had done!

When they finished I had my own personal substation. That didn't dim at ALL.

[Jeff Layman]

I would tend to agree. However, both the Panasonic (58") and the Sharp (22") are specced at 220 - 240V. Why the Sharp worked ok at 160 and the Panny didn't suggests that Sharp may have been designed for perhaps 150

- 240V, but the Panny wasn't. Perhaps pennies count; the average power consumption is no longer stated in the manual, but I guess it is around

120W. In that case, the "front end" of the SMPS would need diodes, and maybe other components, rated at 0.5A. For the 115V equivalent, they would need to be rated at 1A. Perhaps the bean counters specify 0.5A components for TVs destined for Europe, and 1A components for the USA and other 115V markets if the 230V ones are cheaper. I would have thought that a single-specced 115 - 240V SMPS would be cheaper in quantity than two 100 - 120V and 220 - 240V supplies, but maybe not.

Yes, but 80V seems extremely low; the lights didn't even flicker.

I don't know. The generator size looks to be about 300kVA. I guess that would only do a single phase for our 250-dwelling village. According to the guy I spoke to at SSE (he installed the generator), when the power went off completely he said it was probably a main fuse failing further back down the line. I later heard that it wasn't just our village, but around 2000 homes within a mile or so who had no power. It seems too much of a coincidence that the outage occurred while the temporary generator was running, as the power here is very stable - we haven't had an outage in years.

[Jeff Layman]

The low voltage was around for about half an hour or so, when the power finally failed. It came back on a couple of hours later, when it was 242V.

[Peter Able]

John Rumm has summarised the situation very effectively:

1. Items including switched-mode power supplies work perfectly over a wide range of supply voltage.
2. The dangerous aspect of supply voltage variation is motor stalling.

One of the problem nowadays is that so much of our stuff is SMPSU-based that most householders are unaware of supply voltage variations.

In this part of Buckinghamshire, "brown-outs" are quite common - but I seem to be the only one who calls out the cavalry.

And when I call the emergency line there is no-one who understands John's second point.

PA

[Chris J Dixon]

My rechargeable razor is ok from 12 Vdc to 240 Vac, which is really handy when spending time in my touring caravan.

It is so useful, but no longer available, that when the battery lost capacity, I soldered in a new one.

Chris