Bad AC Power?

[Robert Comber]

Hi,

I have replaced 3 power supplies in a K-360 in less than two months. I think I have found the problem but I can't understand it. I measured the voltage on the power distribution strip and I read 120 volts between the lower pin and either of the top pins but when I measure the voltage between the top pins I read 208 volts. I first thought that my volt meter was bad but another meter reads exactly the same. I then thought that the UPS must be bad but when I read the voltage at the input, I get the same thing. Next I measured a completely different circuit in another part of the building and the same thing happened.

Any clue about what could cause this? Bad building transformer maybe? Should I call the power company?

TIA,
Bob

[Robert Comber]

Hi (again),

I forgot to mention that I repeated these measurements at my home and this time I measured 120 volts and 240 volts just as I expected so I am pretty sure that there are serious electrical problems in our building.

Thanks (again),
Bob

[A. Clay Stephenson · Accepted solution]

Not to worry, Bob; this is perfectly normal. Your building has 3-phase Wye-connected power. You need to think of 3 rotating vectors separated by 120 degrees.

Try this math:
240V * sin 120
or
120V * 3^0.5 (square root of 3)

Both of these are equivalent although the former is probably easier to visualize.

[A. Clay Stephenson]

Oh, I suppose that I should add that your home readings are "normal" because your house is supplied with single-phase power.

[Robert Comber]

Hi,

Thanks Clay. I would have felt really dumb if I had called our power company. I'm sure that I would have given them a chuckle. Since my power is normal, any ideas on why my power supplies are failing.

BTW, you must be an engineer to know that much about power.

Thanks,
Bob

[A. Clay Stephenson]

Well, just because the supply voltage is correct doesn't mean that the power is clean (nasty harmonics, spikes) or stable. While at your time of measurement the voltages were fine, that does not mean the voltages were fine at other times. However, since you mentioned a UPS (I'll assume a quality unit that does filtering and handles under/over voltage conditions), I'll give you something else to consider: temperature. Is your environment clean, cool, and humidity controlled? Is the air-flow within your K-box restricted in any way. All the fans working? If this is a rack-mounted box, are the rack fans all running normally. Finally, there is this little-known thing whose scintific name is "coincidence".

And no, I ain't no engineer.