Challenge Circuit 7

Finding Voltage Potential Across Broken Neutral in a 3 Phase System

Solution and Practical Results

One of the units of competency of Certificate III in Electrotechnology (Systems Electrician) (UEE30811 or UEE30820) is UEENEEG102A or UEEEL0020, Solve problems in low voltage a.c. circuits, which expects the learners to identify the neutral current and the voltage potential across the broken neutral using Phasor diagrams. The Challenge circuit 7 asks questions related to this topic.

In this post, I’ll discuss one of the ways of solving Challenge Circuit 7. There are 3 questions in this challenge circuit, let’s look at them one by one.

Challenge Circuit 7 details

One function of the neutral conductor In an unbalanced 3 phase system is to carry out-of-balance line currents to maintain the voltage across loads on each phase at equal values. (Please note: yellow colour is used for white phasor). Based on the circuits shown here, we asked 3 questions

Q 1: Assuming the line voltage is 41.5v, which of the following voltage sets would you expect to appear on the four voltmeters?

This question is mainly about understanding how a star connected circuit behaves if the neutral connection is intact. In a star connected 3 phase circuit, the voltage across the phases is less than the line voltage by a fixed ratio. The calculation is shown below. Since the V4 voltmeter is connected across a wire, it should have no voltage drop.

V1, V2, V3 and V4 voltmeter readings be 24V, 24V, 24V and 0V

Q 2 and Q 3: Based on the measured voltmeter values of V1, V2 and V3 shown below, what voltage would you expect to appear on V4 if the neutral conductor was broken? Identify the correct phasor diagram

This is where things get interesting. Since the neutral conductor is supposed to carry out-of-balance line currents and maintains the voltage across loads on each phase when it’s broken or disconnected, the out-of-balance current has to find its way back through the line conductors. This destabilises the voltages across the phases and creates a voltage potential across the break in the neutral.  

One way of finding this voltage potential is by using phasors. To make it easier to explain, I have combined the solutions Q2 and Q3. 

Mick Taylor, an Electrical Trainer and a good friend, made an animated video to demonstrate the solution and the method he uses in his classes. Please see the video below

Voltage potential across the broken neutral is 8.25V

Circuit Connection Demonstration

I made a video to demonstrate what happens in the circuit when the neutral connection is intact and when it breaks. In this video, I start by connecting the circuit in the challenge with a slight change – I connected the 33 and 20 ohm resistors load in phase 3 instead of 1. However, this doesn’t make any difference to the point we are trying to understand here and that is the voltage potential across the neutral.

Once I have connected the circuit, I then measure the currents through each line and neutral. You will notice that when the neutral connection is unbroken, there is a current through it, and that is the out-of-balance current, or the neutral current. 

I then disconnect the neutral to see what happens to the currents in the circuit. As you would expect, the neutral current becomes 0 A because there is no path, but what’s interesting is that this current doesn’t just disappear but tries to return to the supply through the lines.

This creates imbalances in voltages across the phases. In the video, I then show the voltage measurement across phases with the neutral connected and then disconnected, and we find these voltage imbalances. 

We are more interested in the voltage potential across the break in the neutral, so I show that towards the end of the video. We found the measured value to be approximately 8.49V, as compared to 8.25V from our phasor diagram in the video above. 

Here is a video demonstration of the circuit and its measurement.

Conclusion

One of the functions of the neutral conductor is to carry out the out-of-balance current in a 3-phase star connected unbalanced circuit. This is what we call the neutral current. As this current is given a return path, the voltages across the phases are equal and maintained.

If the neutral conductor is disconnected or broken, the neutral current has to return through the lines, which creates imbalances in the phase voltages. More importantly, there is a voltage potential across the break in the neutral connection. This can be quite dangerous because it is waiting for something or someone to pass the current through. 

In the unit UEENEEG102A or UEEEL0020, Solve problems in low voltage a.c. circuits, the learners are expected to identify the neutral current and the voltage potential across the broken neutral using Phasor diagrams. Mick Taylor’s video shows the latter quite well.

If you would like to discuss other relevant points about broken neutral, please share them in the comments so our readers and students can benefit from your input.

Thanks for reading.

Husnen Rupani

Husnen Rupani

I help electrical training organisations increase learner engagement by designing innovative training equipment. I have a saying "Electricity - you cannot see, you cannot hear it, but by the time you feel it, it may be too late." My main aim is to turn this black magic that we call electricity into something that people can understand.