Phase Loss -- Explanation Needed -- Story within

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lakee911

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Columbus, OH
Here's one for ya'll that I can't explain. Can ya help?

Only a portion of the power went out in our commercial office space. I'm assuming that we lost a phase or two as some lights and some receptacles were working and some were not. All HVAC equipment stopped working so I'm assuming it's three phase.

A receptacle at my desk was still powered. My PC, monitor, heater, powered speakers, and fluorescent lamp were plugged in to it. When we lost the phase(s), the light did flicker a bit and then it slightly dimmed and remained that way. The PC worked, the monitor worked and the heater worked. The speakers did not work! They showed power but no sound came out! Speaker Amp seems like it got fried, right?

When the power was restored to the phase(s) that was/were lost, the light shined brightly again and the speakers started humming and they working again! I'm assuming that somehow on the remaining phase(s), we had a brownout condition and everything still functioned at the lower voltaged except the speakers. My question is why would a brownout like that happen with the phase(s) loss?

Thanks,
Jason
 
With three phase service and one line open rather than grounded there will be current flowing through any connected line to line loads onto that open phase wire and the return current will go through the corresponding wye loads to the neutral, but at a lower voltage because of the series impedance.
If you lost two pages lines then you would affect two instead of three of the wye load sets.
Chances are that the speaker circuitry simply will not operate below a particular DC voltage and the power supply could not reach that minimum level.
The computer supply has more tolerance, possibly because it was designed for 100-240VAC input instead of just 120.
 
bob said:
This is a good explanation. Check the drawing on page 3 and 5.

https://www.progress-energy.com/assets/www/docs/business/motor-protection-voltage-unbalance.pdf
Click to expand...

This provides a handy reference in terms of the basic pictures, but does not really address the issues of the voltages and currents caused on the house wiring by non-motor loads on single phases both line to neutral and line to line. If there were three phase motors running at the time of the failure and they did not have single phasing protection, they could have kept running and doing a poor impression of a rotary phase converter. That too is not considered in the Cooper Bussmann PDF.
 
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