I get three live wires and one neutral from the mains. Different rooms in the house are connected to different circuits which terminate at a changeover panel. The changeover switches allow each circuit to be connected to any one of the live wires at a time. I can even connect all the circuits to a single live wire which puts the entire load on it and no load on the other two.

The power company also randomly switches off one or two of the live wires whenever they feel like it and I must changeover to another live wire.

Is this probably even a three phase system? Are they 120 degrees apart? Could each wire be just one of three independent single phase systems since the power company hase no control over people unbalancing the loads?

I assume it's useless for running three-phase equipment even if it was three-phase since some of the phases are randomly switched off. This is in India.

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    \$\begingroup\$ That sounds like 3-phase; which country is this in? \$\endgroup\$
    – pjc50
    Apr 26, 2013 at 9:19
  • \$\begingroup\$ Where is this? Remember that filling in your profile isn't really for you but a courtesy to us. \$\endgroup\$ Apr 26, 2013 at 12:21
  • \$\begingroup\$ Is that an attempt at a smart grid? Maybe you are supposed to put critical items on one line, interruptible items on the others? \$\endgroup\$ Apr 26, 2013 at 14:53
  • \$\begingroup\$ @Bobbi: Good point, I hadn't thought of that. However, the smart systems I've seen still have a single feed from the power company. The switching is done after that on the customer side. \$\endgroup\$ Apr 26, 2013 at 18:01

2 Answers 2


Wow! What country & area are you in?
It would be interesting to know.

Is this probably even a three phase system?
Are they 120 degrees apart?
Could each wire be just one of three independent single phase systems since the power company has no control over people unbalancing the loads?

You can easily [tm] find out.
See section "Method" below.

3 phase power when 1 or 2 phases is missing is not very good for 3 phase equipment so that does not sound like a very useful choice.

You didn't ask, but you could easily make an "auto-swapper" using relays that would deliver power to the output from one of whatever phases were available. Don't tell your neighbours - if everyone had these they would probably dance together as the power company struggled to balance the system :-).

An auto-swapper could be built with one relay with 3 changeover contacts plus one relay with one changeover contact.

Vout = A + B/A + C/A/B

If A is alive, use it via an A operated contact.
If A is dead the A operated contact blocks A.
If A is dead use B if B is alive. (B contact and /A contact)
If A is dead and B is dead use C. (C contact + /B contact.)

If /A and /B and /C, go for a walk.

C could also have a C operated contact if you don't want C connected when dead.

Some equipment may not like power being disconnected and connected again at relay-operate speeds.



A meter used for the following test MUST be rated for 400V mains if you are on a 230 VAC or similar system. NOT JUST "has a 400V capable range" .

Usually people will say "CAT 3" or "CAT 4" but even that is misleading.
This paper The myths of instruments and safety addresses up a few comminly held and potentially dangerous ideas.

For now, understand that the measurement I am about to suggest involes 400 VAC in a 230 VAC system and 200 VAC in a 110 VAC system. Your meter and leads and anything electrically connected must withstand mains PLUS any spikes that may be present. Given the situation you describe, spikes and worse could be quite likely. Be aware that arc over in an under-rated multimeter can kill you and people have died in such circumstances, even measuring 230 VAC!.

You could also use a resistive divider. They too can have issues.

If proceeding ... :-)

Name you phases A B C.
Measure phase to phase to phase in all combinations. AB BC CA.

If two readings are about 1.7 x Vmains (200V or 400V for 110VAC/230VAC systems) you have two phases of a 3 phase system.

If 3 readings are ~= 1.7 x Vmains you have 3 phases.

If readings are ~= 0V or ~= 2 x Vmains (220 VAC or 460 VAC you have independent single phases from separate transformers.
If they are in phase you get about 0V. This would be usual.
If you get about 2 x maians they are out of phase. This would be unusual due to construction methods.

I said "independant separate phases" meaning that they may have two transformers with 3 phases R G Y phases. If you have R from transformer 1 on your phase A, and R from transformer 2 on your phase B then you will probably get 0V between them.

They COULD even feed you R G Y phases from 3 seperate transformers. You could use that as 3 phase power but the current flows would be "interesting".

  • \$\begingroup\$ even more important is that you MECHANICALLY interlock those A/B/C sourcing relays (there are relays specifically designed with mechanical interlocks for this very purpose) -- you don't want a relay fault to cause a line-to-line short, and even mechanically interlocked you want fusing on each of the phases. If it were me I would also use the relay aux contacts in line with the coils of the other relays to have an electrical interlock as well. \$\endgroup\$
    – akohlsmith
    Apr 26, 2013 at 12:14
  • 1
    \$\begingroup\$ @Kurian You said " ... . The changeover switches allow each circuit to be connected to any one of the live wires at a time. ..." -> If that means that you can have 3 outlets with each running from a different inout circuit, then two "extension test leds" (essentially suitably long wires connected to meter and pluggable into the mains both in some suitably safe manner) would allow you to connect between any two circuits at a time. \$\endgroup\$
    – Russell McMahon
    Apr 27, 2013 at 8:51
  • \$\begingroup\$ Your "The myths of instruments and safety" link is dead. Do you have a replacement by any chance? Also, if 2×Vmains is a possibility (which would be 460 VAC in a 220 VAC based system), isn’t a 400V rated meter a bit underspec’d? \$\endgroup\$ Nov 3, 2016 at 8:05

I expect you don't really have a 3 phase system. If each line were a phase from a 3 phase transformer somewhere, the power company would have a difficult time turning off individual phases. However, what you describe doesn't make much sense either, so I can't really guess.

You can find out whether the phases are in sync, 180° out of phase, or three phase. Measure the voltage with a portable AC voltmeter of each of the three combinations of hot leads. If any two are 180° out of phase, you will be twice the hot-to-neutral voltage. If they are in phase you will get zero. If they are 120° apart, as they would be in a 3 phase system, you will get 1.7 times the hot-to-neutral voltage.

  • \$\begingroup\$ @Russell: Oops. Fixed to say 1.7, which is sqrt(3) to two digits. \$\endgroup\$ Apr 26, 2013 at 14:34

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