Test Question

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fmtjfw said:
0, 104, and 120 are equally valid answers. I can devise a circuit using resistive loads that produces any voltage between 0 and 120 v with C-phase disconnected from the source.
I might be able to produce other voltages using capacitors and inductors?

...
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Really...?!!!

And these would be normal loads even when C phase was good?
 
Following most everyone's elses leads, charlieb gets the brass ring (post 13): 60V Line C to Neutral

Requires balance 3ph load. Note 1ph loads connected line to neutral need not be balanced. They don't change anything.

The load between B and C is in series with the load between C and A. Since they are balanced, each load has 104V across it. But the mid-point, C, is elevated 60V, 104/(sqrt(3)), So, "none of the above".

This is not true if the 3ph loading is motor loads. Dropping a phase to motor loads, the voltage on the dropped phase really looks screwy. Could even read normal.

But the answer could also be "zero", if it is all single phase L-N loads or no loads at all

I think everyone is correct - very poor question.

This is one of those where I get my answer marked wrong, challenge the instructor. pointing out my assunmptions and that the test writer's assumptions are no more valid - it is a poor question. Instructor :rant: gets red in the face, tells me he is the writer and his answer is right and I :dunce: got it wrong. The conversation deteriorates further. :slaphead: Which is odd, cause I thought it had deteriorated about as far as it could.

and now I am known as

The Worm :angel:
 
badly worded question.

if you have any 3 phase loads actually on it won't be zero though.

it will be less than 208V and that is about all you can say with any certainty.

if they are all L-N loads that are on, the voltage will be zero unless you use a high impedance meter.
 
Smart $ said:
Really...?!!!

And these would be normal loads even when C phase was good?
Click to expand...

Sure:

for zero voltage use a 60 watt bulb N-C with no loads (infinite resistance) between A-C and B-C

Simple voltage dividers:

for N voltage use load A between N-C and load B between B-C: load A + load B == 1000Ohms
for 1 volt use loadA == 10Ohms, load B == 990Ohms;
for 3.14 volts use loadA == 31.4Ohms, load B == 968.6Ohms
etc.


for 120 volts use 1500W 240V resistance heater between B-C and no C-N load
 
fmtjfw said:
Sure:

for zero voltage use a 60 watt bulb N-C with no loads (infinite resistance) between A-C and B-C

Simple voltage dividers:

for N voltage use load A between N-C and load B between B-C: load A + load B == 1000Ohms
for 1 volt use loadA == 10Ohms, load B == 990Ohms;
for 3.14 volts use loadA == 31.4Ohms, load B == 968.6Ohms
etc.


for 120 volts use 1500W 240V resistance heater between B-C and no C-N load
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Which question are you replying to???

I am assuming my revised one in Post #17. If so, please re-think your answer.

Ohh... and for clarity, this is one of those questions that use the term "phase" loosely to mean "line" conductor rather than actual IEEE definition of phase.
 
Smart $ said:
OR ???

1. You are called to a shop building on a trouble call and the occupants of the building are saying they think there is one open phase on the incoming 208Y/120 service. You check the voltage at the service disconnect terminals from each phase to neutral. What would you expect the voltage to be on the intact phases to be if one phase was in fact open?

a. Zero volts
b. About 104 volts
c. About 120 volts
d. Voltage will be unstable

I'm leaving the answer as an exercise for participants... :p
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Besoeker said:
The occupants could make that judgement?
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Darn, I was going to say something along those lines.

GoldDigger said:
They probably could not make that judgement, but they certainly could make that statement, for a wide variety of reasons not necessarily relevant to your diagnosis. :)
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Seems like most of them just tell me "there must be a short somewhere" no matter what the symptoms are when something isn't working.

petersonra said:
badly worded question.

if you have any 3 phase loads actually on it won't be zero though.

it will be less than 208V and that is about all you can say with any certainty.

if they are all L-N loads that are on, the voltage will be zero unless you use a high impedance meter.
Click to expand...

If you only have single phase 208 volt load that happens to be "connected" or "on" at the time but one phase is open from the source, you will get 120 volts to neutral from both lines but the reading from the open line is really just the good line feeding through the connected load. Connect your meter from line to line and you will get zero or really close to zero any way.
 
GoldDigger said:
Are we going to have to adopt tables of standard allowed verbacity for feeder and branch posts?
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Open to suggestions... :lol:

Really though, I prefer the poster quote what he is responding to. At the very least direct it to the poster by username. But if in doubt, I use Hybrid mode to see who replied to what.

His initial reply was to post #18 by RichB, which was a reply to my reworded question of post #15. There's no way his replies jive with question as posed in post #15 (i.e. service disconnect open and measurement taken from line side terminals). :happyno:
 
Confusion, havoc, chaos

Confusion, havoc, chaos

Ok I posit that I can give you any voltage on C-N if it is not connected to source up to the highest voltage on another hot wire.:)
Then I state that is why the question doesn't have a single answer (at least 3 according to all the answer groups presented.):D
Then I show you how to do it.:bye:
Then someone asks which posting I'm answering (which parent posting it relates to). :weeping:

Well, when I'm answering, the review of previous posts available DOES NOT HAVE NUMBERS.:happyyes:
 
fmtjfw said:
Well, when I'm answering, the review of previous posts available DOES NOT HAVE NUMBERS.:happyyes:
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When I find myself in that situation, I have four choices:

1. Use multiquote.
2. Start over and write down the post numbers before I start to reply. I can save my partial work if necessary.
3. Finish the post and then go back and edit it.
4. Open a second browser window to find the numbers.

Or, my standard approach:
5. Not worry about it. :thumbsup:
 
GoldDigger said:
They probably could not make that judgement, but they certainly could make that statement, for a wide variety of reasons not necessarily relevant to your diagnosis. :)
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And therefore all of the answers are equally right (and equally wrong) because you have no idea what caused the person to make that statement.
 
ok folks,

lots of great info and insight!1 Thanks again for all the responses.

Final outcome--when I showed the others how they were all right and all wrong--that a, b and d could be correct depending on loads and methods of t/s--they were sorta perplexed but then came around. we agreed to use both questions that Smart came up with--one on the test and the other in the pool.

Thanks!!!!

Rich
 
umm

umm

Missing "C" phase = zero volts. nothing connected upstream hence, "missing"
RichB said:
Hi all,
In reviewing a test we have come across one question that no one can agree on.
In a 120/208Y system You have a report of what appears to be a missing C Phase. You test at the Main Panel OCPD--What voltage would you expect to see when testing from C Phase to Neutral
A. Zero Voltage
B. 104 Volts
c. 330 Volts
d. Voltage will be unstable

I am saying that it will be D--- not really unstable but more likely variable and less then 120 volts.
I'm going to try using drawings and pics to see if I can get the others to see it my way:thumbsup: tomorrow.

I'll let you all know what happens with that, but any info you can give would sure help!

Thanks

Rich
Click to expand...
 
michaelcarter1979 said:
Missing "C" phase = zero volts. nothing connected upstream hence, "missing"
Click to expand...
The original question as stated says nothing about what is connected "downstream", whether measurement is taken on line or load side of Main OCPD, and whether disconnects or breakers are open or closed... nor does it in fact say C phase is faulting open upstream of the MAIN OCPD. Given these uncertain parameters, the actual measurement could be A, B, or D. To choose any one answer would require you to make one or more assumptions.
 
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