Ill effects from low mains voltage ?

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Old 01-17-15, 02:21 PM
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Ill effects from low mains voltage ?

Hi all,

A small section of my community was hit with a power outage this morning, that lasted only a short time before what seemed to be a full restoration. It was only a "half" restoration to say the least. When the power came back on, many of my appliances were acting funky such as my computer that was blinking a blue power light, and the monitor that refused to turn on at all. My Sony rear projection TV turned on fine, although I never tried any functions to confirm operation.

I went to hit the lights, and was greeted by the fluorescent light bulbs struggling to light up, only managing to flicker. Incandescent bulbs were very dim.

I then hooked up my multimeter to read AC volts, and found that the voltage from the socket was only 66-67v AC, almost half of the 115-120 that it should be. After realizing that voltage was cut in half, I decided to unplug many of my devices to prevent possible damage. Is it possible that household electrical devices could be damaged by having a mains input so low? Here's a video link proving the outlet voltage.
https://www.youtube.com/watch?v=V-HI...tRM8C2KchrLshw

Hydro company is Toronto Hydro, and hopefully they have the problem fixed by now.
 
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Old 01-17-15, 03:20 PM
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I would shut all the breakers leaving the main for last. Then turn the main on again followed by the rest of them. Check the voltage again & then see what works & what doesn't.
 
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Old 01-17-15, 03:35 PM
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If you can (and feel comfortable doing so) remove the breaker panel inner cover so you can see all the wiring inside.

After all the individual breakers are turned off, turn the main breaker all the way off and then all the way on.

Measure the voltage for each breaker between its screw terminal and the panel neutral bus (terminal strip). This is most easily done just after flipping the breaker back on. Start with the "single" breakers. If you measure voltage much different from 120, flip that breaker back off and make a note of which one it was.

Also, after each additional breaker is turned on, measure some of the breakers you turned on earlier again (terminal screw to panel neutral bus). Make note of any changes.

If you measure more than two breakers with voltages well over 120 measuring hot to neutral (breaker terminal to panel neutral bus) then stop and turn everything back off. Call the power company telling them you have a bad neutral.
 
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Old 01-17-15, 04:20 PM
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Has the Hydro checked the supply voltage coming into your service?make sure they are aware of your situation.
Geo
 

Last edited by Geochurchi; 01-17-15 at 04:23 PM. Reason: Correct mistakes
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Old 01-17-15, 09:04 PM
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You should start by measuring your voltage coming into the panel at the main breaker. Hot to hot, one hot to neutral, and other hot to neutral. This should be checked with the main breaker off.
 
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Old 01-17-15, 10:11 PM
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Thanks all for the responses!

The power company was notified and full service was restored in a few hours. I guess the proper terminology to use in a situation like that would be "brownout" rather than blackout. Its my first time encountering such a problem, but the cause was nothing in my control.

I guess a local substation for my block of residences was put into some sort of protection mode to protect against overload? Either way, how does a brownout compared to a complete blackout even benefit the customer? With only half the power being supplied, I'd imagine that many customers were scratching there heads like I was, wondering why the electrics in they're home were acting weird. Industries using large electric motors would be highly at risk, too.
 
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Old 01-18-15, 07:00 AM
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If just half of your circuits (representing one of the two service legs) had low voltage, chances are that the problem was at your pole transformer or was in your service drop.

One possible cause could be one of the hot legs being dead but various 240 volt appliances in the home(s) served by that leg happen to to be switched on. The good leg performs normally but also feeds electricity through 240 volt appliances over to the dead leg and then into 120 volt lights and appliances on that leg (and then back to the neutral). Depending on how many and what kind of 240 volt appliances are switched on and how many and what kind of 120 volt devices on the dead leg are switched on, the voltage measured from the dead leg to neutral will vary down from 120 volts.

This is not a brownout. A brownout is a situation with all wiring normal but the power source (generator, substation transformer, pole transformer) is unable to provide the number of amperes that everyone and everything downstream is trying to use. The result is a drop in voltage. Some makes and models of generating equipment can be deliberately run in an overloaded condition for long periods of time with a lower voltage output causing a prolonged brownout.
 
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Old 01-18-15, 07:04 AM
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That's exactly what I was thinking Allan. I have seen it before.
 
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Old 01-18-15, 07:22 AM
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The problem cannot be upstream of the typical pole transformer. Just one (primary, typically ca 7000 volt to ground) line feeds the pole transformer. A reduction in voltage from this feed will cause both legs of the 120/240 volt (secondary) output of the pole transformer to drop in tandem.
 
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Old 01-18-15, 08:28 AM
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Allan; I think the OP means half restoration to mean half voltage. Unless I missed a key point.

If indeed it was the primary at half voltage, could have been a LTC (load tap changer) failure in a distribution transformer, but who knows.

I recall as a kid at night, our local utility had a very unusual failure. The lights went out to zero, but it took a full minute or so. That is, the voltage slowly reduced over a minute, like the generators were actually left on line as they wound down. Bad for induction motors, for sure!
 
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Old 01-18-15, 05:51 PM
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It is not proper to do a "half restoration" in the form of intentionally supplying half the voltage.

If for some reason a primary line ends up at half voltage then both legs of the 120/240 volt secondary of a pole transformer fed by that primary line will be 60 volts (each with 120 volts between the two hots).
 
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