Broken Neutral

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JoeNorm

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I have read on this forum a lot about lost neutrals and the havoc that can be caused as well as experienced it first hand a few times in the trade.

But I will admit I don't really know why the problems occur from a theory perspective. Can someone explain what is happening to say, sensitive electronics, when a neutral is lost?

Thanks
 
JoeNorm said:
I have read on this forum a lot about lost neutrals and the havoc that can be caused as well as experienced it first hand a few times in the trade.

But I will admit I don't really know why the problems occur from a theory perspective. Can someone explain what is happening to say, sensitive electronics, when a neutral is lost?

Thanks
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Simply put, they are exposed to voltages above what they were designed for. Modern power supplies have a much wider range so that damage seems to going down.
 
If you lose a neutral that's being shared by two or more hots, the resistance on the circuit affects the voltage, as @ptonsparky alluded to.

Say, for example you have a multiwire circuit that has 2 hots, and each of those hots are feeding a 65" tv and 20 LED bulbs. Same exact load - then you lose a neutral, it becomes a hot-to-hot circuit. But there's no change in voltage in the equipment because the resistance is the same on both legs. And the voltage of each leg (to ground) stays the same. It would be like wiring two 60w bulbs in series.

But turn off the lights on one leg and the change in load changes the resistance on that leg and the overall 240v circuit. Now the 240v circuit will probably read something like 80 volts to ground on one leg with tv and lights, with 160 volts to ground on the other leg with only the tv.
 
Look up the concept of a 'voltage divider'.
en.wikipedia.org

Voltage divider - Wikipedia

en.wikipedia.org en.wikipedia.org

This is an intentional circuit design used in electronics to reduce the voltage of a signal for further processing.

When the neutral is present in a circuit which uses it, then it forces the voltage across the legs to be balanced at about 120V. But when you lose the neutral, the various loads on on the two legs of the system become an unintentional voltage divider. The more heavily loaded leg (with the lower resistance) will see its voltage drop. The lightly loaded leg will see its voltage climb.

-Jon
 
When you lose the neutral, and any loads on different hots are plugged in/turned on, then those loads are now in series at the line-line voltage.

The voltage drop over loads or components in a series circuit is proportional to their resistance in the circuit. That is the voltage the load 'sees'.

Ohms law: Voltage = Current × Resistance

Say the 120V loads that end up in series at 240V are a 1200 watt heater (12 ohms, normally draws 10amps) and a 12 watt electronic power supply (1200 ohms, 0.1A). Now these are in series at 240V. The total circuit resistance is 1212 ohms, so the circuit draws 0.198A. This is nearly twice as much current as the electronic power supply is designed for so it may fry.

Working backwards with Ohms law, we see that the power supply 'saw' 237.6 volts (1212 ohms × 0.198A). The 1200W heater saw the remaining 2.4V.
 
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