Transient Voltages

Power readings can fluctuate for a time after or before a power outage...I would wait for the readings to stabilize before taking any notice....

GQ -- Good Questions. I will be interested to hear the responses.

1) - the voltages are direct reading from the UPS. I did not put a meter on them

2) loads for both circuits are identical

3) to the best of my knowledge (I didn't wire them in) they are not connected

4) Again, not sure but the circuits were installed by a licensed electrician so they should be to code

5)The high voltages have not repeated themselves. the measurements I took with the meter were always the same(within normal fluctuation)

6)voltage measured from neutral to ground were less that 2, hot measured 117.9 as all other readings

7) the same arrangement has been in use for a few years and this is the first time this has happened.

8) I will have to measure the voltages. Unit is an APC 1200. No readouts on it, but now on the other isolated circuit everything is working.

9) the higher voltages were only read for a short time period, until we changed circuits

Hope this answers your questions with enough information.

Yeah, I always hate it when something doesn't stay 'broken'.

I'm going to guess that the circuits feeding the UPS were 'a little on the high side' at the time of the observation.

I believe the APS 1200 is an off-line UPS meaning that the inverter in the unit only operates if you lose input power. Otherwise the unit just 'passes through' the utility input power to the outputs.

An on-line UPS takes the input power and uses that to charge the battery and the battery is always supplying power to the inverter that powers the loads. In that way, the load always receives 'conditioned' power from the UPS.

Hope this helps.

BSR

<...voltage measured from neutral to ground were less that 2...> This is a function of the current on the neutral conductor and the earth loop impedance value. One wouldn't expect it to be zero while there is a current. One might be tempted to discard this as a possibility.

It wouldn't be the first time a loose neutral connection upstream on the incoming supply caused a problem like this...

I've had loose neutrals before, hence my reference. I have checked and all were fine.

Thanx for the reminder.

You are using an isolated ground. The UPS must be specifically rated for this. Under certain power loss scenarios, the UPS comes on providing voltage, if there was only a partial power failure 1/2 the line voltage can be applied to the UPS voltage.

Military ships have isolated grounds and use UPS' specifically certified for this environment. APC is what we used exclusively

When I was faced with these types of transient power line problems, I would use a scope to capture both the supplied line voltage and the output of the UPS. This could be triggered with a voltage level, but a more powerful trigger is to use a NOTCH filter with 60 Hz center, whose output is fed to the trigger circuit. If ANY strange waveform hits the filter, it will create a voltage surge to trigger the scope. This would include distortion, amplitude + or -, or complete drop out.

This scope information could be used to get the power company to fix the problem, or the UPS supplier to solve it.

Thank you all for your suggestions. I have change nothing in the system and the problem has not occurred again.

Indeed transient. Hopefully never to occur again.