Voltage Dips and Spikes

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mike33

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Irvine, CA
On a service call at a friends home for dimming lights and malfunctioning appliances I found the voltage at the service jumping all over the place. I used a voltage meter from hot to neutral. I found it as high as 125V and as low as 108V. I did not simultaneously measure both hots to neutral, but I think as one went higher the other went lower. When I measured both hots the voltage seemed more stable, staying right around 233V. These measurements were made on the bus after the meter and before the Main breaker. All exposed parts look clean, relatively new and free from corrosion.

I advised the HO to call POCO. From reading other threads here, I suspect a loose connection before entering the house.

Does that make sense that one phase drops while the other spikes?
Is there anything else I could have done for them?
 
If A phase to neutral goes high and B phase goes low, that indicates a open or weak neutral connection. Likely not open neutral, but a high resistance connection. Depending on the circumstances the damage to loads could be severe and because of this it would be my judgement to disconnect all loads until the problem is located and fixed.
 
120827-1138 EDT

Almost certainly there is a high resistance neutral between the power company transformer and the main panel input terminals. Could even be within the transformer.

If there are wires from the meter to the main panel bus bars, then put your meter probes directly on the wires, not the terminals or input bus bars. If there are bus bars directly from the meter to the main breaker, then your measurement between the input bus bars is the best location without actually getting to the meter terminals.

A very easy test for neutral problems is to use two 15 or 25 W incandescent bulbs. One bulb is from phase A to neutral and the other from phase B to neutral. The eye can monitor the two bulbs simultaneously easier than two meters.

When this high vs low reading is occurring at the input to the main panel, then there is a problem preceding the main panel.

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