The broken neutral in a 3 wire circuit.




The dreaded broken neutral

This is a issue that if encountered in the field can mean big problems!

When the neutral breaks you run the risk of having voltage drop below rated voltage on one leg (lights dim).  The danger comes when you have a higher resistance on the second leg.  You could end up with a voltage much higher than rated voltage.

Whatever, prove it!

Fine, I will.  Lets take a look at what we are talking about

3 wire circuit

Here we have a 3 wire circuit, using a neutral.  Three loads use the neutral to achieve 120 volts.  The fourth load uses both batteries (and no neutral) to achieve 240 volts.

The problems occur when the neutral breaks:

Broken neutral

When this happens you basically can remove the neutral from the drawing.

3-wire-dc-broken-nuetral-2

At this point we can also get rid of R4 as it is not affected by the neutral.

3-wire-dc-broken-nuetral-2-2

Now we are starting to see what can happen.  Instead of these three resistors being connected to 120 V each (through the neutral) we have a parallel/series circuit connected across 240 volts.

3-wire-dc-broken-nuetral-2-2-2

From here you will calculate the new line current and then new voltages.  This is done just as you would in a combination circuit.

Once you place the voltages back on the original drawing you will be able to calculate the volt drop across the break.  Don’t believe me?   Just watch the video above for proof.

The video uses an AC system as an example but the concept is the same for DC as well.

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