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I work on a industry as an intern and I'm having a trouble. Clearly there's a fault on the net, but I can't figure out which type it is since I'm having strange readings from a basic multimeter. I'm in Minas Gerais, on Brazil, where each phase have 127 V and here it's a very common practice the neutral grounded (there's no difference between the ground and neutral). So, let's go on.

When I measure each live to neutral it gives me 127 V, also does ground to neutral.

When I measure each live to ground:

A to GND: 220 V

B to GND: 0 V

C to GND: 220 V

Neutral to GND: 127 V

If we change the reference a possible voltage on conductors are as follows:

A: 127 /-120°

B: 127 /0°

C: 127 /120°

Neutral: 0

GND: 127 /0°

Anyone have an idea of what's my problem? What's the simplest/cheapest way to solve it?

Thx very much

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  • \$\begingroup\$ The electrical installment is roughly projected and done. Also we do not have a plant and possible the circuit breakers are badly dimensioned. \$\endgroup\$
    – Pedro Quadros
    Feb 2, 2015 at 9:24
  • \$\begingroup\$ It seems like B and Neutral are flipped. \$\endgroup\$
    – Roland Mieslinger
    Feb 2, 2015 at 9:34

1 Answer 1

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The relationship between phase voltage and line voltage (difference between two phase voltages) is \$\sqrt3\$ = 1.732. If you multiply 127 by 1.732 you get 219.9 volts. This should reliably tell you that line B is grounded instead of neutral.

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  • \$\begingroup\$ but if B is grounded the voltage between B and another live will be 127 and not 220, right? \$\endgroup\$
    – Pedro Quadros
    Feb 2, 2015 at 10:04
  • \$\begingroup\$ No, it will be 1.732 x 127V = 220V \$\endgroup\$
    – Andy aka
    Feb 2, 2015 at 10:10
  • \$\begingroup\$ Can you explain me why? Since 220 V comes from the sum of two waves of 127 V with 120° of difference (if there is no difference it will be 254 V, and if it was 180° it will be 0 V). But if we change one of 127 V by ground (0 volt) the sum will be 127 V and not 220 V. Where is my mistake? \$\endgroup\$
    – Pedro Quadros
    Feb 2, 2015 at 10:19
  • \$\begingroup\$ 3-phase theory: if phase voltage is X then line voltage will be 1.732 times higher and not twice as high. Look it up and you'll see I'm completely correct. \$\endgroup\$
    – Andy aka
    Feb 2, 2015 at 11:26
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    \$\begingroup\$ The 3 ph you are measuring probably comes from an isolation transformer so grounding one of the phase voltages (by mistake) is not going to be self-revealing. You measured 0v between B and ground - doesn't this tell you that the B output is likely shorted to gnd? \$\endgroup\$
    – Andy aka
    Feb 2, 2015 at 14:17

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