Strange electrical problem in the office

I am currently at an office that has three electrical appliances, a PC, a small refrigerator and a recently installed air conditioning unit.

There is some problem with the electrical installation in the building. For example, whenever the refrigerator compressor turns on, the voltage drop causes the computer to turn off. Other offices in the building experience similar problems.

But whenever the AC is turned on, I noticed that the computer won’t turn off as the fridge turns on.

Any guesses why this happens?

The AC is probably on a separate circuit from the other appliances.

This wouldn’t explain what the OP is describing which is that the PC shuts off whenever the fridge compressor starts except when the AC is running and the fridge compressor starting no longer causes enough voltage drop to cause the PC to shut down.

I don’t have a decent guess about what is causing the issue you describe except the possibility that the AC compressor motor, being an inductive load, is affecting the phase difference between the start and run windings of the fridge compressor so that it doesn’t draw as much amperage when it starts up, just a WAG.

My compleatly wild ass guess is that the AC is acting like a capacitor or battery. When the fridge turns on…um I can’t really think of a good way to explain it. It’s kinda like a flywheel. yeah, that’s the ticket.

Does the AC unit require 240 V, by any chance? See, if it does then here’s what I think is happening: 240 V requires two 120 V circuits, which is what most residences and small commercial buildings are fed. When the AC is on, it’s “bridging” the two 120 V circuits so that the voltage sag from the fridge isn’t as large as usual, so the computer isn’t affected. When the AC is off, this “bridge” doesn’t exist.

Same fridge? WAG: the compressor transients are introducing noise on the power line.

The OP’s location is shown as being in Greece, I know they use 220V there as the standard voltage, I’ve never heard whether or not they center-tap to use 110V circuits as well. They might do that in commercial installations.

Ah, well, I wouldn’t know about his electrical installation then. It’s all Greek to me.

Nice! :smiley:

If this was in the US on the sort of 120/240 volt system I’m used to, I’d be taking a close look at the neutral, and wondering if it was “loose”

In really simple terms, a loose neutral could cause an imbalance like 100 volts and 140 volts instead of 120 and 120. As Q.E.D. said, the air conditioner would be connected to both phases when it’s running, and it may help alleviate that imbalance.

By any chance, are there ever times when the lights get bright when a large appliance starts up? If so, you certainly have a loose neutral, and need to call an electrician to investigate and repair it.

My first thought reading the OP was of a loose neutral, they cause so many ‘strange’ electrical problems. The office in question here may not actually be in Greece so it’s worth considering. I thought of the power factor though because underloaded motors also cause a lot of strange problems to pop up, it’s a fairly common issue in industrial facilities though it doesn’t always get noticed because there are so many motors running. We’ve had problems with pumps bogging down where the mechanics just decide to throw on a larger horsepower motor only to find the problem gets worse because the motor is underloaded so much.

Sorry I didn’t mention it earlier. The office is indeed in Greece so we have single phase 220V AC power supply. And since there is only a single wall socket in the office, all three aforementioned devices run from the same multi-socket strip.

Also another strange thing: I once connected a multimeter on an empty socket on the strip and when the refrigerator motor started, the voltage jumped to 280V and stayed there for several seconds before dropping back to 220.

I’m agreeing with JoeyP
When an AC unit cycles on it usually also introduces a capacitor to the circuit which has the job of holding up the voltage as the compressor draws a lot of current to start. That same capacitor is probably still in the circuit and now adds more stability to the voltage so that the frig startup current doesn’t cause the voltage to dip below the threshold of the computer tolerance.

Sounds like a classic loose neutral.

Your office building is probably fed with “three-phase” power, most likely “240-wye” There are effectively three 240-volt circuits feeding the building, all sharing one neutral. when the fridge starts and the voltage went to 280, someone else’s power dropped.

On re-read… the AC, fridge and computer are all on the same outlet strip? It’s sounding like the computer’s power supply is simply shutting down when it gets hit with the excessive voltage. When it’s running, the AC probably pulls enough amps to “absorb” much of the extra voltage in the form of a voltage drop across a bad connection somewhere in the wiring.

You really need to call an electrician to address this. The excess voltage is a safety issue, and the voltage drop is a fire hazard - pull enough amps through a bad connection and it can heat up enough to start its surroundings on fire.