Test Question

[RichB]

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

 

[Besoeker]

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

"Appears to be missing" is rather too vague.
Missing from where exactly?
If it is missing at the main panel I might expect to measure zero volts.
But how else would you decide that it was missing other than by measuring zero volts there in the first place?

Seems to be a bit of a daft question.

 

[charlie b]

The question has no meaning. Everything depends on what is meant by "missing C phase," on whether there are any loads connected downstream, and (if so) whether any of those loads are connected to phase C. Let's say that "missing C phase" means that in the 480-120Y/208 transformer upstream of the panel, the C phase leg of the transformer is not connected at the neutral point within the transformer. I get different answers for the C-N voltage in these situations:

• No loads are connected at the panel (or all of the panel breakers are open).
• The only connected load is a single phase 120V load load from C to N.
• The only connected load is a single phase 208 V load from C to B.
• There are two loads connected: a single phase 120V load load from C to N and a single phase 208 V load from C to B.
• There are two loads connected: a single phase 120V load load from C to N and a three phase 208 V load.

 

[Smart $]

The problem text itself is a problem... :blink:

There is no indication of test parameters and conditions (OCPD line or load side terminals, open or closed), nor does it state whether issue seems to be before or after the Main OCPD (it would be an assumption that it is before).

Given the preceding, all but C are reasonably correct. Being it is best to choose one answer rather than none, I too would go with D.

 

[RichB]

I found the exact question--here it is:

1. Youare called to a shop building on a trouble call and the occupants of thebuilding are saying they think there is one open phase on the incoming 208Y/120service. When you check from phase to neutral, what would you expect to be thevoltage on the bad phase?

a. Zero volts
b. About 104 volts
c. About 330 volts
d. Voltage will be unstable

And that's the problem--we can't come up with a way to word it or agree on the answer--4 people and the answers vary between a and d with everyone changing their answer as they think on it---I read it as they think they are missing a phase, i.e., the POCO fuse is open, so then when checking bad phase to neutral you would read something less than 120 volts but it could be anything less than 120 volts because of any 208/240 volt equipment or any 3 phase equipment--as in:
Mister electron comes down thru A phase on the transformer, thru the POCO fuse thru the Main breaker down to the branch circuit breaker, out to the equipment back up the conductor to the breaker on C Phase thru the OCPD up to POCO fuse, which is blown--now put a meter across C Phase to neutral-- I think you should read 120 volts minus whatever the Voltage drop is across the load.

Whhheeeeww---What say you?

 

[Gregg Harris]

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

Would you not expect to see 120 volts from each phase to neutral?

 

[Smart $]

...Whhheeeeww---What say you?

Well if you did lose a phase on a 3? service, which has 3? loads, the first thing I would do is open the service disconnect, then measure voltages on the line side of the disconnect. Should be zero volts to neutral if on a [completely] lost line.

 

[GoldDigger]

Would you not expect to see 120 volts from each phase to neutral?

I would expect to see that if there were no problems reported.
However we know that is not the situation we are given.
So what is actually measured will depend on just what the failure and load condtions are and, as Smart $ said, could reasonably be anything except C.
If there is a three phase motor load which is acting as a poor rotary phase converter, then the voltage will be unstable and vary with other loads.
If there is just a feed through from A and B to C via loads and no POCO connection at all on C, the voltage might be around 100V. Or that may be a reading caused by capacitive coupling read with a high impedance meter.
If there are only single phase line to neutral loads and you use a low impedance meter and the C phase supply is open, then zero volts looks good.

 

[RichB]

The question has no meaning. Everything depends on what is meant by "missing C phase," on whether there are any loads connected downstream, and (if so) whether any of those loads are connected to phase C. Let's say that "missing C phase" means that in the 480-120Y/208 transformer upstream of the panel, the C phase leg of the transformer is not connected at the neutral point within the transformer. I get different answers for the C-N voltage in these situations:

• No loads are connected at the panel (or all of the panel breakers are open).
• The only connected load is a single phase 120V load load from C to N.
• The only connected load is a single phase 208 V load from C to B.
• There are two loads connected: a single phase 120V load load from C to N and a single phase 208 V load from C to B.
• There are two loads connected: a single phase 120V load load from C to N and a three phase 208 V load.

Correct--In the other post I just put up gives the parameters--it is in a shop environment utilizing 3 phase 120/208Y--I think and assumed the writer meant that to mean there are various single phase, phase to phase and 3 phase loads connected. Just what you would expect when walking into someone's shop--then read bad phase to neutral--what would you read---My thoughts again are something less than 120 volts dependent on the various connected loads (Phase to Bad Phase) acting as a voltage drop.

Tha's what qwe are tying to fix both the question and the answer

 

[Smart $]

... the POCO fuse is open, so then when checking bad phase to neutral you would read something less than 120 volts but it could be anything less than 120 volts because of any 208/240 volt equipment or any 3 phase equipment--as in:
Mister electron comes down thru A phase on the transformer, thru the POCO fuse thru the Main breaker down to the branch circuit breaker, out to the equipment back up the conductor to the breaker on C Phase thru the OCPD up to POCO fuse, which is blown--now put a meter across C Phase to neutral-- I think you should read 120 volts minus whatever the Voltage drop is across the load. ....

POCO fuses are typically on the primary side of a service transformer. That overly complicates the matter, not knowing the transformer's configuration. I would classify this as overthinking the problem... but it does have technical merit.

 

[GoldDigger]

Correct--In the other post I just put up gives the parameters--it is in a shop environment utilizing 3 phase 120/208Y--I think and assumed the writer meant that to mean there are various single phase, phase to phase and 3 phase loads connected. Just what you would expect when walking into someone's shop--then read bad phase to neutral--what would you read---My thoughts again are something less than 120 volts dependent on the various connected loads (Phase to Bad Phase) acting as a voltage drop.

Tha's what qwe are tying to fix both the question and the answer

Given my immediately previous comment, you can choose one scenario and try to make the question specific to that.
"Bad phase" symptoms could also result from a high impedance in the POCO C or a low C phase voltage caused by transformer or tap selection problems.
Only single phase loads and those loads only A to C would give you the same voltage exactly from A-N as from C-N since no current would be flowing through the loads.
Only a balanced set of A-C and B-C loads will give you a lower voltage on C-N and poor performance on the other two sets of loads.
A balanced set of A-C, B-C and A-B loads will be left for somebody else to calculate.

But to get anything concrete your question will have to eliminate the option of already running 3-phase motor loads.

 

[fmtjfw]

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 no loads currently connected between A-phase or B-phase to C-phase, no potential on C-phase from source.
B. 104 Volts loads connected between C-N & B-C; or C-N & A-C; or B-C & A-C; or C-N & B-C & A-C; AND feed to C-phase open
c. 330 Volts
d. Voltage will be unstable B. above with some loads trying to start but failing due to low/high voltages.

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

.

 

[charlie b]

Mister electron comes down thru A phase on the transformer, thru the POCO fuse thru the Main breaker down to the branch circuit breaker, out to the equipment back up the conductor to the breaker on C Phase thru the OCPD up to POCO fuse, which is blown--now put a meter across C Phase to neutral-- I think you should read 120 volts minus whatever the Voltage drop is across the load.

In your scenario there is not a complete circuit. So no current would flow, and no voltage would be dropped across the load. That is why I stated earlier that there would be a different answer in the various loading scenarios that I listed.

Consider for example the fourth scenario in my earlier post. "Mister electron" would travel down the B phase on the transformer, through the POCO fuse, through the main breaker, through the B phase branch circuit breaker, and through the B-C load. At that point, it cannot travel (as it normally would) back up the C phase branch circuit breaker, because that path is blocked by the open C phase POCO fuse. Instead, it must travel through the single phase load from phase C, and from there via the neutral wire back to the transformer. That puts the two loads, one single phase 120 volt and the other single phase 208 volt, in series, with only one transformer leg supplying the energy. Assuming the two loads have the same impedance, the voltage difference between the phase C OCPD point and the transformer's neutral point would be 60 volts. In any other connection scenario, including the four others I listed, and with any changes in the impedances of the various loads connected at the time of the measurement, the resulting measurement would be different. It would not necessarily be "variable," in the sense that it would change from second to second as you continue to measure the voltage. But it would vary as you turn on and off the downstream loads.

 

[RichB]

YAY!!!

to all who have responded--THANK YOU!!!

This should easily get the others to let us rewrite the question.

Also--It could depend on your T/S shooting technique Like Smart $--open the disconnect first--Me First thing would be to go Phase to Phase first then Phase to Neutral--just to see if there really was something wrong--then I'd open the service disconnect and go on from there.

In my way I would, again, expect to see something weird on the open Phase to neutral.

Thanks again folks and as always--I learned and appreciate the time and answers.

Rich

 

[Smart $]

...

That's what ..we are tying to fix, both the question and the answer

....

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. After opening the service disconnect, you check the line side terminals from each phase to neutral. What would you expect the voltage to be for an open phase?

a. Zero volts
b. About 104 volts
c. About 330 volts
d. Voltage will be unstable

 

[RichB]

In your scenario there is not a complete circuit. So no current would flow, and no voltage would be dropped across the load. That is why I stated earlier that there would be a different answer in the various loading scenarios that I listed.

Consider for example the fourth scenario in my earlier post. "Mister electron" would travel down the B phase on the transformer, through the POCO fuse, through the main breaker, through the B phase branch circuit breaker, and through the B-C load. At that point, it cannot travel (as it normally would) back up the C phase branch circuit breaker, because that path is blocked by the open C phase POCO fuse. Instead, it must travel through the single phase load from phase C, and from there via the neutral wire back to the transformer. That puts the two loads, one single phase 120 volt and the other single phase 208 volt, in series, with only one transformer leg supplying the energy. Assuming the two loads have the same impedance, the voltage difference between the phase C OCPD point and the transformer's neutral point would be 60 volts. In any other connection scenario, including the four others I listed, and with any changes in the impedances of the various loads connected at the time of the measurement, the resulting measurement would be different. It would not necessarily be "variable," in the sense that it would change from second to second as you continue to measure the voltage. But it would vary as you turn on and off the downstream loads.

That's what I meant by variable--
:slaphead:OOOOOOohhh Correct on this I forgot all about that--That puts the two loads, one single phase 120 volt and the other single phase 208 volt, in series, with only one transformer leg supplying the energy. Assuming the two loads have the same impedance, the voltage difference between the phase C OCPD point and the transformer's neutral point would be 60 volts


I'll let you all know what we come up with for another question cuz this one is gone!

 

[Smart $]

....

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. After opening the service disconnect, you check the line side terminals from each phase to neutral. What would you expect the voltage to be for an open phase?

a. Zero volts
b. About 104 volts
c. About 330 volts
d. Voltage will be unstable

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...

 

[RichB]

....

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. After opening the service disconnect, you check the line side terminals from each phase to neutral. What would you expect the voltage to be for an open phase?

a. Zero volts
b. About 104 volts
c. About 330 volts
d. Voltage will be unstable

Typed tooo soon--Thanks Smart $--That is EXACTLY what we need! And will use it if you don't mind!

Thanks guys-- I need to hit the road and so some inspector stuff.-It has been almost 13 years since I did any serious troubleshooting and I find little things are disappearing--Must be OTD (Old Timers Disease) like my kids say--that or CRS:angel::lol:-I'll get back tomorrow or Wed with an answer as to what we do!~

 

[fmtjfw]

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?

It is a "fatally flawed" question.

 

[Besoeker]

OR ???

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.

The occupants could make that judgement?