Mains quality monitoring

[Caecilius]

I've got an office with some servers on a UPS. From time to time, the UPS is reporting issues with the mains power quality and switching to battery. It then typically switches back soon after, but sometimes the RCBO for that circuit trips at the same time.

I'm looking for something that can monitor the power quality, so I can try to isolate the problem. Does anyone know of monitoring equipment that can monitor the quality of mains power over a period of time? I guess something that just measured voltage would be a start.

I've got a multimeter, but it only shows the instant reading and can't record data. And these events only occur once a week or so, and seem to be random.

[Andy Burns]

I've got an old Seaward Mains Disturbance Recorder

You leave it plugged in, it counts the number of transients, and has LEDs to indicate if the voltage has been out of range (mine's the old

240V +/- 6%, presume they are now calibrated for 230V +/- 10%) and if there's been a brownout for more than 1/2 a cycle, or 2 cycles.

[Dave Liquorice]

Does your UPS not measure the volts and allow you to poll it for that information?

Shouldn't be the spec never actually changed to +/- 10%. It's still

230 V +10% -6% (216 - 253 V).

[Andy Burns]

I know te supply was +10%/-6% but I thought it had eventually changed (nominally) to +/-10%, or is it that appliances have to be designed to accept +/-10% now?

[Caecilius]

It probably does. It's got a network card, sends email alerts, and I think it supports SNMP. I'll look into this.

I had an event early Sunday morning, which resulted in the following emails (the contents of the emails doesn't really add anything):

Subject: UPS: On battery power in response to rapid change of input. Date: Sun, 18 Aug 2013 00:27:14

Subject: UPS: No longer on battery power. Date: Sun, 18 Aug 2013 00:27:15

The RCBO didn't trip this time.

One advantage of a monitor is that I could plug it into other outlets and see if it's ocurring on multiple rings/outlets or if it's just one.

[Toby]

In my office, I had to change the 16A MCB for a type C, as that would always trip on mains power restore if there was a power cut, which was kind of annoying! - It had a 3KvA UPS' on it, plus some other small non essential IT equipment we could live without during a power cut, so you might want to look at replacing the RCBO with a type C to stop it happening to yours too.

[Dave Liquorice]

RCBO MCB ...

IT/electronic kit tends to leak rather a lot to earth because of the filters on the mains input to keep all the nasty RF interference off the mains cabling. IIRC it's not recomended to have more than 10 such appliances on a single RCD.

Switching transients may well create enough earth leakage current to push an RCBO that is already near its leakage limit past the leakage trip point.

It could be a switching surge that a type C MCB would help with.

[Dave Liquorice]

There was a date given to change to +/- 10% spec and appliances, a date several years ago but I'm fairly sure that any action on that date got quietly dropped and we (and Europe) have remained on the 230

+10% -6%.

230 +/-10% is 207 to 253 and a heck of range for kit to deal with.

[Caecilius]

That's what I suspect I may be seeing. Perhaps the UPS contains MOVs which snub spikes to earth, so noise on the supply causes earth leakage which trips the RCD.

[MrWeld]

A really DIY approach would be to feed the output of mains transformer, possibly via a potentiometer, into the audio line-in socket of a PC. A wall-wart with an AC output (rare, but they do exist) would probably satisfy most safety concerns. You could record the waveform with audio software of your choice, presumably at a fairly low sampling rate. I'm not sure whether a typical integrated soundcard can go as low as 50Hz though.

[Steve]

the range of human hearing is 20Hz to 20 KHz and many sound cards support LFE (Low frequency extensions) so I'd expect the sound card approach to work.

In addition, subtracting a synchronised 50Hz sinusoidal signal from the recorded signal will then give you a signal containing just the bad quality stuff.

[Caecilius]

That's an interesting thought. Mains transformers are easy enough to find, or as you say an AC PSU. Maybe I can use a low-end CPU device instead of a PC to keep it low-power: the raspberry pi doesn't have audio in, but the pandaboard es does. If I've got time I might give that a go.

Maybe there are cheap ADCs with USB or serial outputs I could use. This is getting interesting. Thanks for the idea.

[Dave Liquorice]

Low Frequency Effects, the bangs and thumps. Don't think it goes any lower than the other five channels that are full bandwidth. The LFE has an uuper frequency limit around a few hundred Hz. Just means you have signal to augment the existing LF in the main channels by giving it a bit of welly into decent sub-woofers.

But any sound card ought to cope with 50 Hz no problem.

[Dave Liquorice]

leakage

MOVs breakdown at quite high voltages so a bit of "noise" won't trigger them.

I'm thinking of a interuption and possible phase change when the UPS switches from it's internal frequency back to mains causing the capacitors in the filters to charge/discharge all at once thus dumping or pulling current via the earth. An RCD is only looking for an imbalance between L and N it doesn't care if that imbalance current leaking to earth or current leaking from earth...

[steve]

I'm not sure how well very fast transient spikes would pass through a mains transformer with its large number of turns and magnetic properties that obviously favour mains frequency.

A divider that consists of resistors only would provide an accurate view of the situation, but safety and isolation could then be a problem.

[The Natural Philosopher]

Arduino has ADC at fairly low quality (8 bit I think)

[F Murtz]

[Caecilius]

Yes, that looks possible. Arduino nano has a 10-bit ADC with a sample time of 100us. More than enough for this task I think. The only poblen I can see is that the arduino doesn't have much storage so it would need to log to something else.

[The Other Mike]

SD card. Libraries to do this are readily available.

[Mike Tomlinson]

En el artículo , Caecilius escribió:

You've got an APC unit.

With a message like that, I'd be looking for a loose connection somewhere between the consumer unit and the socket it's plugged into.