7 Facts You Need to Know About Neutral Wire in a 3 Phase Circuit
If you are training electrical apprentices or are an electrotechnology professional, you would’ve heard of the terms neutral wire, neutral conductor or neutral current when discussing 3 phase circuits. In this blog, I will discuss 7 facts that you need to know or explain to your learners about the neutral wire in a 3 phase circuit. This list is by no means exhaustive but covers some of the most critical aspects.
Let's get the basics out of the way
A three-phase system has three branches, one for each phase. If it’s a 3 phase supply or a transformer, these three branches will be windings of the alternator or the transformer.
The load side can have more variations, for example, if the system is a balanced electrical load, then the three branches will be each phase of the load, for instance, windings of a motor. And if the system is an unbalanced electrical load, each phase could be multiple loads.
There are different ways of identifying the phases, and the most popular ways in Australia are A, B and C or U, V and W. Each of these phases have two ends, and they numbered labels, for example, the ends of phase U are U1 and U2
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Star of the show
You can connect 3 phase systems in two ways – Star and Delta. We will only discuss star connection in this blog post because delta connection doesn’t require a neutral wire.
Star connection is when one side of each phase is connected to create a star point, and the other side connects to the lines. The sides at the star point should be consistent; that is, they should all be 2 or 1 but never mixed.
Image of the motor terminal block and winding circuit.
Okay. Let’s get the 7 facts straight
Fact 1: You get two voltages from the 3 phase supply because of the neutral wire
In Australia, the most popular 3 phase supply voltages are 400V and 230V. You may have also seen 415V/240V, that’s a different way of saying the same thing. The 400V is the line voltage, and 230V is the phase voltage.
This setup powerful because it allows you to connect a three-phase load in a way that each phase will have either 230V or 400V across them or connect a single-phase load that needs 230V across one phase and neutral
If there were no neutral wire, only the first scenario would have been possible. Another point to remember here is that the line and phase currents in star connected systems are the same
Fact 2: You don't need a neutral wire for balanced loads
Balanced loads are electrical loads with 3 phases, like a 3 phase motor or a 3 phase water heater. These loads are designed in a way that each phase has the same resistance or impedance, so if they have the same voltage across each phase, the current will also be the same.
A balanced system satisfies the following criteria
- Current in each line is the same and
- Power factor is consistent, which means the phase angle of each current is consistent with respect to their phase voltages
In single-phase, loads the neutral wire provides the return path for the current, and in balanced 3 phase loads, because they satisfy the above criteria, the currents enter and return through lines creating 0A of out of balance current. So, there is no need for a neutral wire.
Fact 3: The current in the neutral wire is the phasor sum of all the line currents
In a balanced system, when all currents and their power factors are the same, the phasor sum of all line currents is 0A. That’s the reason why there is no need for neutral wire in a balanced system. The mathematical calculation can be quite elaborate, so I’ll discuss that in a different post, but here I will show you a graphical method or vector method.
Let’s say the currents are
IA = 5A, IB = 5A and IC = 5A, and the electrical load has a power factor of 1, which means the phase angles are 0, 120 lagging A and 240 lagging A respectively.
What happens to the neutral current when line currents are different or of different phase angle? As you can imagine, it won’t be 0A so let’s find out neutral current for the following example
IA = 727mA at 0 degrees
IB = 727mA at 120 degrees lagging A
IC = 1.927 A at 240 degrees lagging A
Since the neutral is current is returning to the supply, the phasor will be in the opposite direction. This means that we need to measure the phase angle in the opposite direction.
Fact 4: Neutral wire carries out of balance current in unbalanced loads
A load that doesn’t draw the same current in each line or has a different current phase angle is considered an unbalanced electrical load. This usually happens in all 3 phase installations because there could be several single and three-phase loads in an installation and you can’t control which line draws how much current at a time. In these cases, the currents don’t balance out, and there is some leftover. This is the out of balance current, and it’s one of the purposes of the neutral wire to carry it to the supply.
Without the neutral wire, all sorts of instabilities occur in the system like unstable voltages, unexpected currents and even dangers of electric shock.
Fact 5: A broken neutral wire changes the phase voltages when the electrical load is unbalanced
In an unbalanced 3 phase electrical load, the line currents are different, which causes the neutral current to flow from the star point of the load to the supply star point. If the neutral wire is broken or disconnected, the out of balanced current cannot return to the supply through the star point, but it must return. So, this current takes the path back to the supply through the lines.
Ideally, the star point should be at 0V, and that is the case when the neutral wire is intact but when its broken and because the currents are forced to return via lines, this point shifts to a different voltage.
Since the star point is no longer at 0V, the phase voltages on load change because the line voltages from the supply are still at the same levels as before. For example, if one point of a circuit is at 12V and the other at 0V, then the voltage difference is 12V, but what if the second point was at 4V? Now the voltage difference will be 8V instead of 12V.
Fact 6: A broken neutral wire changes the line currents in unbalanced loads
Current cannot exist without voltage and as the Ohm’s law suggests, as the voltage changes, the current changes with it and proportionally. As mentioned in the previous section, the phase voltages are affected by the broken neutral, and this affects the phase currents as well. Since the line and phase currents in star connected systems are the same, the currents in the lines change as well.
The change in voltage and current causes the change in the electric power, and this the reason why the loads work in unexpected ways. In some cases, this could cause a brown-out or overvoltage; either way, it’s bad for the electrical load and bad for the system.
Fact 7: Unexpected voltage at the point of the broken neutral wire
When the neutral wire breaks, the voltage at the star point is not at 0V but a different number. We may never know what this voltage could be because it would depend on the connected loads at that time, which means that this voltage could be close to 0V or much higher.
The higher this voltage, the higher is the risk of electric shock to a person or animal that completes the broken path, and this can be very dangerous.
There you have it, 7 facts you need to know about neutral wire in a 3 phase circuit. As mentioned earlier, this list isn’t exhaustive, and there could be more things you may need to know, but this is a good start.
I hope this post help you or your apprentices get some ideas or refresh what you may have learned in the past. Please add your thoughts and other facts that I haven’t listed here in the comments.
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