Calculating voltage across a line break

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Broken line, Broken heart

While a broken line may not be a common occurrence out in the field it is still very important for an electrician to be able to understand what can happen.

It is very similar to determining the voltage across a neutral break, it is all about redrawing your circuit, determining the new voltages and then putting them back into the original drawing.

In my years of teaching I see students struggle with this until they understand two things:

  1. The importance of redrawing the circuit.
  2. The importance of understanding that the polarity is going to change across some of the loads.

Let’s take a look shall we?

DC, three wire,line voltage

There you go, the line is broken.  This basically makes the first battery non existent.


Now if you look at the circuit, R4 will maintain the battery voltage across is.  R1, R2 and R3 however will have their world rocked.  R1 and R2 used to be connected to a battery.  R3 used to be connected across both batteries.  Now that the line is broken the circuit has changed.


R4 is happy, R1, R2 and R3 now have to have their voltages recalculated.

Once you have recalculated the new voltages place them back in the original drawing.

The next step is to realize that the polarities have changed states for some of the resistors.

In the original:

polarity of resistors

Once the line breaks the polarities of R1 and R2 has to change as they are no longer using the top battery.

Polarity, direction

To see how this fully plays out make sure you watch the video at the top of the post.

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