# Determining phase sequence in software

We have 3-phase electric network. How to determine phase sequence (L123 or L132) from 2 line-to-line voltages U12 and U32? I prefer computationally simple solutions. Is it possible to determine phase sequence without computing phase voltages (which seems to me to complicated if searching generally valid solution for assymetric systems).

The solution looked for needs to be a mathematical algorithm. I need to write computer program for this. Lets say we know amplitudes and phases of vectors U12 and U32:

$U_{12} = |U_{12}| \exp \left[j \left(\omega t + \phi_{12}\right)\right]$

$U_{32} = |U_{32}| \exp \left[j \left(\omega t + \phi_{32}\right)\right]$

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Put a capacitor and resistor in series across each line. Measure voltage from each cap to the other line. The line with the lowest voltage is lagging the other line.

But if @optionparty is right that you only have two wires, there is no way. Maybe you mean two lines, and the neutral?

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Here is a circuit design that displays correct 3 phase order. It uses the SCR to store the memory of state B-C sequence and the polarity of A for each cycle. No neutral is required here.

If you wanted to detect a missing phase, one would need a neutral reference and trigger a one-shot for each phase.

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 Does this circuit work on european 400 V lines? And what about using a capacitor to drop some voltage instead of those huge resistors? – Al Kepp Dec 26 '12 at 17:47