# Unbalanced three phase system - Neutral fails/opens/disconnects - consequence?

I have a very basic question about three phase system. I am not an electrical engineer hence I might not have a good understanding of the subject. So, please be bit lenient in answering and asking clarifications.

My home receives a 220v (between one phase and neutral) 3 phase connection from grid. From the post comes 4 wires to our home. Each phase is connected to various loads in my house and since not all loads will be switched on, the three phase circuit will be unbalanced.

So, if I draw a very crude schematic diagram with text:

R--------L1---+
|
Y-----L2---+  |
|  |
B---L3--+  |  |
|  |  |
N-------+--+--+


R, Y, B are the three phases, N is the neutral and L1, L2 and L3 are the loads (unequal resistive and inductive combined).

What happens when the connection to neutral fails as shown by the following diagram?

R--------L1---+
|
Y-----L2---+  |
|  |
B---L3--+  |  |
|  |  |
N--  ---+--+--+
^
|
+------- Neutral disconnection


Some say that the loads L1, L2 and L3 experience high voltages as they will be connected between phases rather than between phase and neutral.

While some say that the entire circuit will self adjust and achieve a balance.

Either way I am not clear. Could you just let me know which version is correct?

Thanks.

I had searched for this Google and could not come up with any suitable answer. Also this question also didn't help.

• Well they'll be both at higher voltage and the circuit will self-adjust. Jul 16, 2012 at 10:38
• Thanks, if you had given it as an answer instead of a comment, I would have accepted it. :) Jul 16, 2012 at 10:43
• I'm writing an answer. Jul 16, 2012 at 10:44

The circuit will both be at higher voltage (well the parts of it) and will self-adjust. The reasoning behind this is pretty simple: First, the phases are out of phase, or to explain it simply, their voltage peaks happen at different times, so while one is at its maximum voltage, the two other phases will be at lower voltage than the one at maximum is, so basically, the load will be at the voltage between phases. Since we don't have the neutral conductor, the current which was going through it will have to go through one of the phases and that's how the circuit will "self-adjust". You can see that from the simulation here.

When the switch is closed, all 3 loads are at the same voltage of 5 V compared to the ground. Once the switch is open, the voltages at the loads will change. So the lightest load will drop the least voltage and the greatest load will drop the most voltage.

• Is there any protection device which can be installed to prevent damage to household items. A lot of appliances got damaged in my home because we had a 3 phase connection and the neutral wire of main distributuion line broke and fell on ground.
– Bot
Jun 17, 2015 at 12:51
• @Bot I'm not aware of any protection devices that could help. Do keep in mind that this doesn't mean that there aren't any. In my country, we usually prevent the problem with correct wiring practices inside of the household. In normal operation, it should be pretty much impossible for the phase line to be connected through a low-impedance load to the neutral line and the 3-phase triangle devices are connected to all phases at the same time, so no damage can happen to them. Jun 19, 2015 at 12:37
• This leaves devices which are to be connected as a 3-sided star. We usually dimension them so they can survive the loss of neutral. For more specific advice, I'd need real info about what exactly happened. Jun 19, 2015 at 12:38
• Thank you so much Andreja. Here i had added a question with details of my prob electronics.stackexchange.com/questions/175959/…
– Bot
Jun 20, 2015 at 15:15
• There are over voltage and phase failure relays and shunt trip breakers available, but that is mostly industrial kit, the usual thing for domestic is just to take the view that that sort of problem is what insurance is for. Jun 18, 2018 at 10:34

In case of breaking of neutral wire in 3 phase if there is unbalance load, voltage will high in your home where the load is lowest.Thus your equipment got damaged.....Jagat Singh Bisht

Meaning your incoming voltage supply is 380v at 3 phase with neutral wire because you said that from phase to neutral you'd measure 220v. Example, if you have bulb with voltage rated 220v, you should connect the L1 into the (R) phase and the L2 into the Neutral wire. Very simple, if the Neutral wire open, your bulb will not be glow and there will be no flow of current and so the phase voltage will remain the same.

The series will be develop & the max current will flow at load having low wattage hence equipment may be damage a k bansal

In 3 phase circuits the phases may appear balanced but when you will turn on load one phase will start dimming and others will show very high voltage thus resulting in damage to your electrical equipment.

If the loads are equal on all the three phases then even if the neutral breaks it will not shift. If the loads are unequal then the voltage will also change with respect to the loads.

Some say that the loads L1, L2 and L3 experience high voltages as they will be connected between phases rather than between phase and neutral. While some say that the entire circuit will self adjust and achieve a balance.

It will find a balance, but the balance it finds may well be destructive to equipment.

The best case is that the loads are roughly equal, and so the voltages stay roughly equal.

The worst case is that one phase has a low impedance load and the other two have high impedance loads. The voltage across the high impedance loads will rise to a voltage close to the phase to phase voltage.

The trouble is with loads coming and going and with some types of load having a much lower impedance at initial startup than when running normally, you have a substantial chance of hitting that worst case sooner or later.