Ground faults and AC motors? How does this work?

[hillbilly willie]

I'm an Electrical Engineering student, and working as an Electrician intern for a drilling company this summer. The rigs are run primarily on 600 volt and 480 volt AC power. Just the other day, we had a ground fault on the rig, and got readings of 0-480-480 volts on the phases between the phases and ground. I understand that, because the first phase is going to ground, you sense the first phase added to the second two phases when reading voltage from phase to ground. I have two different questions, that had mixed answers from some of the electricians:

Question 1: If you reached out and touched the phase with 0 volts, (the phase that has the ground fault), would it kill you? Or even shock you at all? The electricians said that it would definitely kill you, but I don't understand how this is possible. If the multimeter is sensing the voltage from the power lug to ground, and it reads 0 volts, if we touch the power lug, and our feet are grounded on the rig floor, which is grounded, shouldn't we have 0 volts across our body, and not be hurt in the least?

Question 2: With this ground fault, and readings of 0-480-480 volts on the phases, the 3 phase motors on the shakers and pumps ran perfectly fine! Not a problem at all. How can a 3 phase motor run on power that only has 2 phases? I understand how AC motors work, and I would understand it if the motors ran a little bit, but were bogged down and slow, but these motors run 100% fine.

For goodness sake, this is driving me crazy. Someone please give me some insight on this! Thanks!

 

[GoldDigger]

I'm an Electrical Engineering student, and working as an Electrician intern for a drilling company this summer. The rigs are run primarily on 600 volt and 480 volt AC power. Just the other day, we had a ground fault on the rig, and got readings of 0-480-480 volts on the phases between the phases and ground. I understand that, because the first phase is going to ground, you sense the first phase added to the second two phases when reading voltage from phase to ground. I have two different questions, that had mixed answers from some of the electricians:

Question 1: If you reached out and touched the phase with 0 volts, (the phase that has the ground fault), would it kill you? Or even shock you at all? The electricians said that it would definitely kill you, but I don't understand how this is possible. If the multimeter is sensing the voltage from the power lug to ground, and it reads 0 volts, if we touch the power lug, and our feet are grounded on the rig floor, which is grounded, shouldn't we have 0 volts across our body, and not be hurt in the least?

Question 2: With this ground fault, and readings of 0-480-480 volts on the phases, the 3 phase motors on the shakers and pumps ran perfectly fine! Not a problem at all. How can a 3 phase motor run on power that only has 2 phases? I understand how AC motors work, and I would understand it if the motors ran a little bit, but were bogged down and slow, but these motors run 100% fine.

For goodness sake, this is driving me crazy. Someone please give me some insight on this! Thanks!

The way you talk about the phases being added to each other mixes the idea of a phase conductor voltage being a phase and a line to line connection being a phase. In any case it is the vector difference among voltages that will drive current to flow. What happened was that one line conductor got grounded. Not one "phase" getting grounded if you consider a phase as being line-to-line.

Answer 1: If you are at most touching ground before you touch the grounded wire, then when you touch that wire no current will flow through you because there is no voltage difference across you. Your understanding is correct.

Answer 2: As far as the motor goes, since there are three windings or sets of windings, one from line A to B, one from line B to C and one from line C to A, the motor will not care what any of the voltages to ground are. It will only notice that there are three different 480 volt line-to-line phases and the current will flow in the windings accordingly.
To go on further, if the motor were wound in wye rather than delta, there would have been nothing to connect the center point of the wye to, since you started with an ungrounded delta and all of the voltage with respect to ground are unpredictable. Instead the center point terminal of the motor would left open, with one end of each of the three windings connected to it, and the motor will still see three equal phases.
Looking at the actual vector diagrams instead of just the magnitude numbers may help to clear this up for you.

 

[Jraef]

...
Answer 1: If you are at most touching ground before you touch the grounded wire, then when you touch that wire no current will flow through you because there is no voltage difference across you. Your understanding is correct.
...

Just a cautionary note to add to this. It is true IF the point at which you are connected to ground has the EXACT same ground potential as the point at which the ground fault is taking place. If it is a high resistance ground fault, and you are offering a lower resistance path, YOU become the new path. ...For a moment, until your cremains become a carbon pile resistor... Such has been the death of many a copper thief who ASSumes that it is "safe" to cut ground and neutral conductors out of equipment.

The comments from your electricians at the sight on it being dangerous are the result of years of training telling them NOT to tempt fate in that regard. Heed it.