Wye-connected ungrounded System

[robertplant22]

A 208V wye-connected, ungrounded system suffers a ground at phase A. What is most nearly the line voltage at phase B?

The correct answer is:

SQRT(3)*208V = 360V

Can someone shed some light into this problem please? I could probably memorise this, but I want to uderstand why this is the case. Also could you argue that the line voltage between phases A and B is the same as phase B to ground? Which would make the voltage:

208V/SQRT(3) = 120V

Should the question had specified that the line voltage wanted was between phases B and C?

Thanks

 

[stbtigerr]

I have the same question. When they say 208V system, I would assume that's line-line. If that's the case, V-bg would equal V-ba(208V), wouldn't it?

 

[Flyer_PE]

^That would be my assumption also. 208 V is a typical line voltage with a phase voltage of 120 V.

 

[vskneifl]

In a ungrounded wye the neutral is floating and not connected to ground, that the ground is independent so it acts just like a delta. When it ask what is the voltage at phase B it is in reference to ground not the neutral, and since phase A is connected to ground the voltage between phase B and the ground is the same as the line to line voltage between phase A and phase B.

 

[Flyer_PE]

^Agreed. What the problem apparently doesn't tell you is whether 208 is the line voltage or the phase voltage. Given no further details other than it is a 208 volt system, I would assume that 208 is the line voltage since 208/120 is pretty common.

 

[stbtigerr]

^Agreed. What the problem apparently doesn't tell you is whether 208 is the line voltage or the phase voltage. Given no further details other than it is a 208 volt system, I would assume that 208 is the line voltage since 208/120 is pretty common.

Exactly. That's why I was thrown off. I understand the philosophy, but I wouldn't have guessed 208V was the phase voltage.

 

[robertplant22]

This is yet another example where PPI questions are vague and leave too much space for assumptions. I think the test will not have questions where assumptions can be made. Had this question come from the NCEES sample book, it would have probably asked for the line voltage and it would have also specified whether the line voltage wanted was between the A and B phases or the or the B and C phases.