3 phase delta interview question

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rtull01

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I was asked on a 3 phase ungrounded deltas system what would be the voltage from one leg to ground ( i forget 240v or 480v) I was not sure and answered 0 volts for the ungrounded system and 240v or 480v respectively since i don't remember which voltage they asked me. note not an electrican just maintenance. any help would be great and some technical explanations would help too.

Thanks
 
For any ungrounded system, from zero up to phase voltage. Even greater under rare conditions. But essentially zero would be the correct assessment without additional 'posturing'. Greater than zero is common in reality. A result of mutual inductance and/or capacitive coupling between transformers, other inductors (such as motors), wiring, other electrical equipment and grounded conductive parts.

For grounded delta systems, either the midpoint of a winding is grounded, or a line terminal (dubbed corner grounded), so either half or full phase voltage, respectively.
 
Smart $ said:
For any ungrounded system, from zero up to phase voltage. Even greater under rare conditions. But essentially zero would be the correct assessment without additional 'posturing'. Greater than zero is common in reality. A result of mutual inductance and/or capacitive coupling between transformers, other inductors (such as motors), wiring, other electrical equipment and grounded conductive parts.

For grounded delta systems, either the midpoint of a winding is grounded, or a line terminal (dubbed corner grounded), so either half or full phase voltage, respectively.
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On the mid point grounded system two lines are half the full phase voltage to the grounded point, the third and opposite corner of the delta from the grounded point is 1.73 times the half full phase voltage and is frequently called high leg, wild leg, stinger, bastard leg. On a 120/240 system this high leg is 208 volts to ground, on a 240/480 system this high leg is 416 volts to ground.
 
rtull01 said:
I was asked on a 3 phase ungrounded deltas system what would be the voltage from one leg to ground ( i forget 240v or 480v) I was not sure and answered 0 volts for the ungrounded system and 240v or 480v respectively since i don't remember which voltage they asked me. note not an electrican just maintenance. any help would be great and some technical explanations would help too.

Thanks
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For NEC purposes, when the NEC states that you need to do such and such when the voltage to ground exceeds X Volts, the voltage you apply to such an application for an ungrounded system is the same as the line-to-line voltage. An example is the need for bonding bushings when ring knockouts remain, which is required when voltage to ground is greater than 250V.

Even though in a practical sense, it may be considerably less. It likely will end up being such that each line is "equidistant" from ground in terms of voltage. Such that a 480 ungrounded delta will typically be 277V to ground. This will vary with conditions of operation.
 
All very true, but the QUESTION on the exam only wants to know what the voltage will read, and it specifically said "3 phase ungrounded delta(s)". So Smart $'s answer is the right one, but that kind of ambiguity can only be answered correctly in an essay or at least explanatory response format. If it is multiple choice, I'd be interested in seeing what the choices are that they gave you!

If the choice was only between 240 and 480, then the answer would be 240V as the best guess, you can't read 480V on a 240V transformer output. If you had a choice that included Zero or 240, then I don't know how I would answer that. I think I would likely still go with 240V because from a safety standpoint, that's what you would have to ASSUME is the maximum potential reading.
 
Jraef said:
All very true, but the QUESTION on the exam only wants to know what the voltage will read, and it specifically said "3 phase ungrounded delta(s)". So Smart $'s answer is the right one, but that kind of ambiguity can only be answered correctly in an essay or at least explanatory response format. If it is multiple choice, I'd be interested in seeing what the choices are that they gave you!If the choice was only between 240 and 480, then the answer would be 240V as the best guess, you can't read 480V on a 240V transformer output. If you had a choice that included Zero or 240, then I don't know how I would answer that. I think I would likely still go with 240V because from a safety standpoint, that's what you would have to ASSUME is the maximum potential reading.
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I somehow missed that it did say ungrounded, though he later mentioned 240v or 480v respectively, which may have thrown me off a little on what system was the topic of discussion.

My answer for ungrounded system is anywhere from zero to full system voltage.
 
rtull01 said:
I was asked on a 3 phase ungrounded deltas system what would be the voltage from one leg to ground ( i forget 240v or 480v) I was not sure and answered 0 volts for the ungrounded system and 240v or 480v respectively since i don't remember which voltage they asked me. note not an electrican just maintenance. any help would be great and some technical explanations would help too.

Thanks
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I would have answered "indeterminate".
 
Besoeker said:
I think a floating (isolated) system could float to any voltage. I don't see why a 240V limit would apply.
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Me either. And why not 277 or 480 if we are going to use system voltages?

Add: looking back I see he sided with 240 nominal.
 
Smart $ said:
For grounded delta systems, either the midpoint of a winding is grounded, or a line terminal (dubbed corner grounded), so either half or full phase voltage, respectively.
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But the question specifically states ungrounded.
So your point is an irrelevance.
Why do people keep doing that........
 
Besoeker said:
But the question specifically states ungrounded.
So your point is an irrelevance.
Why do people keep doing that........
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Depends on how you read the question. Yes, he only asks regarding UNgrounded specifically. Grounded appears implied to me (it's in the 240 or 480 part).

So if people keep doing so and you continue to point it out, is doing so also an irrelevance........? :D
 
The old "I don't like the topic as-is so I'll change it so I can say what I want" approach. :D
 
I think I like.....if you were using an analog meter or.... if your were using a high impedance digital multimeter.........which would be the right answer? Pretty lame question in my humble opinion. About as useful as.....if you touch line to ground on an ungrounded Delta, will you get the snot knocked out of you? Makes about as much sense. Maybe I'm just having a bad day, though!:)
 
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