Weird power quality problem in shop

[Cigaro]

Hello,

Part of my job is to install and maintain computer systems, and one in particular that I look after is a till system in a small shop.

Ever since these machines were installed, they keep powering off and rebooting. Some days they don't do it at all, some days you can't get one to stay on for more than a minute.

The problem is not isolated to one machine - one or both of them can be affected at the same time, but some days there are no problems at all.

We have replaced both machines with others that are known-good and tested, and yet in this shop they keep powering off.

Our first thought was a power quality issue, so we installed UPS's to back up both machines - I think they are Zigor units. Although they have helped with the problem, they have not eliminated it altogether - one or both machines will still power off at random, usually late in the afternoon, and they may or may not struggle to power them back on.

Sometimes when this problem occurs the UPS's will beep to signify that they have gone into battery backup mode, but sometimes they won't. Once the alarm system started beeping at the same time as these events happened, but only once.

The UPS units are supposed to offer surge protection, but how effective this is I don't know - my theory is that there are some kind of large transient spikes on the power line and this is getting through the UPS's surge protection circuitry and causing the machines to shut down.

The customer has phoned the DNO who came and did a voltage measurement and said all was OK - they are supposed to be coming back to fit a voltage monitor but so far no-one has heard anything back from them.

My question is really one of curiosity - I have a keen interest in electricity and I'm working towards a career as an EE at the moment - have any of you sparks ever seen a problem like this? Will this be something that the DNO's voltage monitoring equipment will pick up?

I would really like to get to the bottom of this, both because my customer has reached the end of my tether, and also because I've never seen anything like it!

 

[xr4x4]

My old boss (computers) was convinced that pluging a fridge and or freezer in near or in the same extension lead as a computer would cause power surges and cause the computer problems, as he had experience of this.

I wasn't convinced myself.

 

[Nozzle]

Keep hassling the DNO to get that voltage recorder fitted. I had a similar problem, again at that time of day and it turns out there's some sort of fault on that cable phase. They have now (twice) changed the phase we are on and the problem has now gone.

Get yourself a plug-in power meter from Maplin. They also measure volts, and you can use this to see that the volts are probably going low at that time of day. Unfortunately, the voltage recorder the DNO install will only take a sample every two minutes, but this should be often enough to pick up your potential sustained low-voltage problem.

Nozzle

 

[Jackrae]

If the DNO only offers sampling at once per 2 minutes then either they don't care about their systems or don't know what they are talking about.
They could get/hire a Dranetz unit that can look at the voltage with microsecond resolution.

 

[AdrianUK]

My thoughts:

Most 'cheap' UPS are off-line types, that is they normally pass the mains power straight thru to the load. The load only runs off the batteries when the mains fails. Most surges or transients are far to fast for the UPS to react - they will pass straight thru. Maybe try an on-line UPS - these cost a lot more money but with this type the load is always run off the batteries which are in turn recharged by the mains. This type is so much more immune to dirty mains.

There has been much talk on other forums about whether the presence of third harmonic voltages on the mains can cause rebooting/crashing issues with PCs. Third harmonics are a result of our widespread use of nonlinear loads (switch mode power supplies) and can lean to the neutral/earth voltage fluctuating.

Some of the nasties on the mains can happen extremely quickly......logging the voltage every couple of mins or even every couple of seconds wont see it.

Tell us more about your Customer...... does he have a 3 Phase supply? Is he on the end of a long overhead distribution circuit? Is he a village shop out in the sticks or in the middle of a busy high street? What about his neighbours? (I have known one 'craft' place that shared a building with a woodworking workshop..... everytime the woodworkers started the 5 HP planerthicknesser the craft shops PCs would crash)....

 

[JM2]

Yes. Any UPS worth its salt is a fully on-line type - and this is what you need to eliminate/isolate possible mains supply issues.