No Voltage from Phase A to Phase C

The only things I can think of (in order of most to least likely):

1) faulty measurement, i.e. you didn't have good contact with your probe.

2) you have an open-delta corner gounded system, AND you have a ground fault on Phase A. Open delta corner grounded systems usually ground out Phase C, so if you have a ground fault to Phase A, there will be no potential difference between them.

3) you have a 2 phase system!

Thank you very much,

Can you represent this in schematic form so I can visualize what you are saying? Also, can you schematically represent a corner grounded Delta circuit?

Your 3rd point is valid if he has no 3Ph load running

Connections from a sngle point are made to A & C

Faulty measurement is causing that

If this truely what you are reading, then Phase A and Phase C have the same potential. Is there anyway that you have a open fuse in the circuit. I would start at the source and isolate components and verify.

As JRaef has explained I add:

Is any 3phase load running or just house-holds are on load?

If no 3ph load then it seems that you have the same phase on A & C & you are thinking of a 3rd ph

Sorry for Rating *****.

Actually I thought it will go to "JRaef" 's reply. The ***** are not for the Topic.

Any how Question is Valid & quite a number of people get confused like it. It is natural.

dear roydaniel

its really nice ques. you had put here.

dear plz measure voltage at all 3 (A, B & C) point with respect to ground also, record data seperately and respond me.

i will wait for datas.

Since you state there is no neutral and it's connected in a delta configuration then I'm assuming you have a three phase load on it. If the load is a motor it may actually work like this all be it a considerably reduced efficiency.

Just because you have a 3 wire delta connected load doesn't mean you cant check the voltage to ground. The system will at some point be referenced to ground in some way otherwise all sorts of nasty things can happen. First check each phase has a potential when referenced to ground.

If all three phases have similar potentials when compared to ground then I agree with Haaje that it sounds like somebody has stuffed up the wiring and Phase C and Phase A are actually connected to the same phase somewhere prior to the terminals you are taking the readings from.

This raises the following questions:

Since everything is working what led you to check the phase to phase voltage?

Do you know if anybody has actually checked the phase to phase voltage before or could the problem have been there all the time and till now went unnoticed?

Have there been any electricians working in the area of recent and did they replace anything?

My suggestion is that you go and trace the wiring back and check the voltage at every junction all the way back to the incoming supply.

shut it down!

In a 480 volt system you should not read 480 volts between phases since they are 120 degrees out of phase.

If you are reading 480 volts than one of your coils is open and the motor needs to be rewired or replaced. the result of an open phase is that the resulting voltage is higher and the motor will run faster though not as smooth.

the sum or average of all three phases should be zero. if one phase is missing the sum is about double the input power.

Eh? Sorry, can you run that by me again? How does the motor run faster? Presuming this is a 60Hz system, is the no load speed not fixed? Or am I just being dense!

not to get into the physics of it bot the speed of an electric motor is controlled by the configuration of the coils, both field and armature. In school we built motors from scratch, It was alwas fun when someone wired the motor wrong,(in series) and the motor speed kept increasing until it self destructed and then proceeded to run around the room. a short or open in one phase of a 3 phase motor changes its configuration.

For the science part

- a series motor produces no Back Emf and speed is accumulative

- electromagnetic force (emf) is derived from the current which is 90 degrees out of phase with the voltage.

- in a parallel wired 3 phase motor most of the back emf comes from the upcoming phase.

Phase A is active, the back emf comes from phase B. If phase B is not present the motor will tend to increase in speed at that point.

Phase B is not active, it produces no back emf from phase C. It will also not increase in speed. simply momentum takes over

Phase C is active and has normal back emf from phase A, so the motor will try to run at the correct speed.

Total is the average speed : phase B (speed+) plus phase C (momentum) plus phase A (normal) The typical is a 25 to 30 % increase depending on the load.

Thus the reason to shut it down... the increase in speed and vibration will invariably increase the heat and potential for a fire!

Just because you have no neutral does not make it delta, you can run wye connections without a neutral.

Somewhere in your system there is a neutral. It is just not extented to where you are taking measurements.

There are grounded and ungrounded neutral systems.

With a grounded neutral then you can read phase to ground (270 volts), with a ungrounded neutral you will read 0 volts to ground.

On an ungrounded system a phase can go to ground and there will not be a big bank so to speak.

I believe that you have an ungrounded system and either 'A' or 'C' is grounded. When I have ran a three-phase grounded system I install a light bulb between all phases A-B-C-A. When a bulb goes out you then check for a grounded phase.

With an ungrounded neutral system, the load will not notice the difference and the motor or what ever will continue to work.

Jmart23 stated

"With a grounded neutral then you can read phase to ground (270 volts), with a ungrounded neutral you will read 0 volts to ground. "

With an ungrounded system, in other words no earth neutral link, the voltage to ground could be anything. It can even be as high as the primary voltage on the last transformer. Un-referenced power distribution systems are really dangerous and shouldn't exist. If they do somebody isn't doing their job. If you have a transformer that is isolating your load from the mains it needs to be referenced to ground somewhere, not to do so is dangerous and should be frowned upon.

My company did a Steam Generator change out at Mill Storm Nuclear Plant - All our equipment was set up for a grounded neutral, GFI, Data acquisition unit, etc. We had to almost complety rebuild our equipment to get to work with Mill Storm's ungrounded neutral system.

We have since ran into the same thing in other power plants!

What's the voltage to ground from each leg?

Could it be open delta with an A - B Primary
and Secondaries C & A grounded?

Is this a rural system? Take a look at the pole.

I have 277 volts to ground on all three legs. This circuit goes to different outdoor lights. A and C go to the primary on a transformer and the secondary, 120 volts, goes to a coil on a starter. In order to make the circuit work, I changed the primary to phases A and B. The starter actually controls the lights. I initially checked all fuses and the transformer because the lights quit working. After changing to the other phase the lights now work. By the way, there have been some real intelligent answers sent by you guys. Thanks for all of the hints but what would all of you do to test in order to solve the problem? Also, do any of you know a method for testing phase in order to tell which way a motor will turn instead of having to swap two phases to change rotation?

The easiest way is to use a dual trace oscilloscope to look at the two waveforms simultaneously. If the two waveforms look the same and are in phase then they are defiantly both coming off the same phase somewhere prior to the junction you are testing. If on the other hand they look similar but are out of phase then there is some other sort of strange fault.

If you are unfamiliar with what you should see and how to connect the CRO post a query and I will write up how to set the CRO up and draw what you should see.

I can absolutely use a dual trace O'scope to test phase but what is the CRO? Does this pertain to finding out which way a motor will rotate due to phase? Thanks for taking the time to help.

My apologies Roydaniel for using the acronym CRO without explaining it, I jump on others for not explaining acronyms so I should have known better. CRO stands for Cathode Ray Oscilloscope and should probably be dropped since an ever increasing number of oscilloscopes use liquid crystal displays.

Haajee as for using a dual trace CRO all you do is pick one phase as you reference phase and compare the other two to that. You will find one is lagging behind your reference phase by 120º and the other will be leading by 120º (or lagging by 240º). The direction of rotation will be from the phase you are on to the lagging phase then the leading phase.

As for the problem we are investigating the idea is to ascertain why there is no apparent voltage between phases A and C when checked with an RMS volt meter. Since all three phases are showing 277V to ground then the most likely answer is that somewhere prior to where the testing is being done somebody has connected the C phase to the A phase. The idea was to use the dual trace CRO to look at the A and C phases to either confirm that the problem was indeed what we suspected or whether something more complex and sinister was going on.

Just as an aside I have seen a 3 channel digital oscilloscope, not a lot larger than a digital millimeter, that had a display mode with a small circle that would rotate to indicate the direction of rotation of the phases. With the prevalence of computers there are some nice USB interfaces that can be used with software to turn you laptop into a 16 or 32 channel crow with mind blowing bandwidths. I however still like the simplicity of the good old dual channel, dual time base, delayed trigger, analog variety oscilloscope though because it makes you think of elegant ways to measure some fairly complex problems.

Dual-Ch CRO will not serve but 3Ch [No three Ch is normally available], so 4Ch is needed + 3ch Isolator to Isolate Mains from Test set-up.

This is quite a Luxurios setup for a non-professional.

Please advise simple way to resolve.

Now the discussion has risen to at least **.

Reply to:

masu Post #23

1.)"As for the problem we are investigating the idea is to ascertain why there is no apparent voltage between phases A and C when checked with an RMS volt meter". Since all three phases are showing 277V to ground then the most likely answer is that somewhere prior to where the testing is being done somebody has connected the C phase to the A phase."

This is the point I brought to notice:

[#In reply to #1 Your 3rd point is valid if he has no 3Ph load running. Connections from a sngle point are made to A & C ]

# RMS meter will show between 220 and 230V. It is the Peek Meter to show 277V. Even a normal meter [Average measuring] will read nearly 220V.

2.) "Since you state there is no neutral and it's connected in a delta configuration then I'm assuming you have a three phase load on it. "

Please also see my reply:

[As JRaef has explained I add: Is any 3phase load running or just house-holds are on load? If no 3ph load then it seems that you have the same phase on A & C & you are thinking of a 3rd ph]

******************

3.) "Jmart23 stated "With a grounded neutral then you can read phase to ground (270 volts), with a ungrounded neutral you will read 0 volts to ground. "

I think that there is NO country in the world now DISTRIBUTING WITHOPUT A NEUTRAL TO EARTH OR UNGROUNDED SUPPLY

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Reply to: roydaniel

4.) "In a 480 volt system I have 480 volts from phase A to phase B and 480 volts from phase B to phase C. However, I measure no voltage from phase A to phase C. Oddly, everything is working normally. This is a three wire delta system as there is no neutral. Anyone have ideas as to what is causing this? "

Reply to: 4.) "I have 277 volts to ground on all three legs."

If all the 3 points [to whom you call 3Phases] are connected to a single point say A, B or C, any Multimeter will indicate 220/230V to ground. [not 277V as there is no peak Voltmeter used commonly].

It seems that there is something not considered of.

Reply to: 5.) "This is a three wire delta system as there is no neutral. "

Missing of a N point only does not make it a Delta. In 3Ph Delta supply you need dedicated-Supply system, Delta is only for Long-distance distribution systems, like 11KV [In asian countries. May be universal] for House-hold & small Industrial Consumers. Large Industrial consumers have their Grid-Stations of EHT.

Reply to: Post #19

6.) "A and C go to the primary on a transformer and the secondary, 120 volts, goes to a coil on a starter. In order to make the circuit work, I changed the primary to phases A and B. The starter actually controls the lights.

.. transformer because the lights quit working. After changing to the other phase the lights now work. By the way, there have been some real intelligent answers sent by you guys. "

After all you got the point. Good. Well done !!!!!

Now reply to "Also, do any of you know a method for testing phase in order to tell which way a motor will turn instead of having to swap two phases to change rotation? "

There are Small 3phase motor having a dial on top, showing direction of rotation visually,

running at almost 5/6 RPM, Connection points Marked R, S, T [UVW may be considered by Eu people] coloured accordingly. Look for "Siemens". I had once using in my work place.

Good luck !!!!!

"I think that there is NO country in the world now DISTRIBUTING WITHOPUT A NEUTRAL TO EARTH OR UNGROUNDED SUPPLY"

One would hope but even so I have, on numerous ocasions, come across isolation transformers with an unreferenced secondary side. I remember spending a whole day explaining, to a licensed electrician, why it was necessary to install a neutral earth link on the secondary side of a transformer that was being used to isolate the power supply to a computer room. He quoted a myriad of rules and regulations that didn't apply and it eventually took a phone call, to an engineer at the electricity supply authority, to convince him to put in the neutral earth link. The thing that worried me is, he had been working as an electrician for some 25 years and had installed many isolation transformers prior to this one and stuffed up the lot. It also worried me that nobody to date had ever picked up on his frequent mistake.

Masu,

I nearly jumped in on this a few posts ago. Transmission is usually done with non referenced HV which is achieved by the station transformer being 11kv star/ 132 or 275 kv delta. Earth references at transmission levels are a nuisance. When transmission levels are returned to distribution levels the configuration is delta / star.

The risk you refer to is when a star secondary is non referenced. The star point does not have to be hard grounded and is frequently earth fault current limited via resistors, chokes or transformers. Special applications such as underground coal mines limit earth fault current to 5 amps in this way and of course detect earth faults via one of the core balance methods or direct return current for intertripping.

I have not had a great deal of experience with high tension transmission lines but from memory the ones I have worked on in the mines they star configured secondary at each transformer with a delta primary. Something like this

The earth neutral link could either be at the star point or some remote location. The configuration meant that each level was referenced to ground a single location thus eliminating the ground loop problem. I also seem to remember somebody saying this configuration was more immune to induced voltages and currents from outside sources but never looked into whether or not this was true.

Mind you all this is from memory over twenty years ago and since I was only involved it the control system and we were only monitoring things like current and voltage I never bothered to pay a great deal of attention to the details of the intermediate stages. However this configuration made some sort of sense from a theory viewpoint and it stuck in my head. Mind you once you start talking about transmission line voltages I don't know it matters a great deal if it is or isn't referenced to ground, its only really a problem at the source and final destinations. I can however see that a lightning strike on a transmission line would result in it going to earth through the star point of the transformer and this could be a problem. Without an earth at the star point a lightning strike would raise the potential of the entire transmission level till something either limited it or something broke down.

Yeah Masu,

The drawing is about right from right to left, except expect a 132 / 66 stage in the middle and going further right is the station TX 132 delta / 11 Star.

The overhead earth gives a 45 degree chevron of protection to the conductors below. At the switchgear there will be lightning arrestors and CTs and VTs for protection (differential relays, inst overcurrent, timed overcurrent and distance relay etc.).

I think that line capacitance will bleed a fair amount of the surge and inductance will limit current flow along, given the length of transmission lines and the rise time of lightning surges. If you've ever been around Middlemount there was at one stage about 50 km of blown out poles on the SWER line, kinda the opposite issue I suppose.

To the original issue, using a tong tester on A and C phase will determine if they are the same phase. Measure A then measure C then measure both simultaneously. If the currents are additive, same phase. If somewhat subtractive, not same phase.

Hi Emjay4119,

"To the original issue, using a tong tester on A and C phase will determine if they are the same phase. Measure A then measure C then measure both simultaneously. If the currents are additive, same phase. If somewhat subtractive, not same phase."

Good thinking. Sounds like the best idea to date.

With regard to lightning strikes on high tension transmission lines. I don't know if they do this all over the world but with many of the transmission lines in Australia they extend the towers an extra 5m or so and run an extra cable directly above each of the transmission lines. I believe the idea is that it is bonded to ground at each tower and acts as a lightning rod. It works on the premise that lightning will look for the shortest path to ground therefore it will go for the extra conductor rather than the ones that are actually carrying the power.

Of cource this is a simple-way to resolve the problem.

"With regard to lightning strikes on high tension transmission lines...."

Exactlythis is also in Pakistan & may be in rest of the world.

If as you say the voltage on phases A and C is apparently identical I suggest you borrow an ohm meter from some minion and check the resistance between them, you may well find some incompetent person has made a wiring error.

With regard to lightning strikes on high tension transmission lines. I don't know if they do this all over the world but with many of the transmission lines in Australia they extend the towers an extra 5m or so and run an extra cable directly above each of the transmission lines. I believe the idea is that it is bonded to ground at each tower and acts as a lightning rod. It works on the premise that lightning will look for the shortest path to ground therefore it will go for the extra conductor rather than the ones that are actually carrying the power.

That is the overhead earth I mentioned in the previous post.

Seems to me that if the ground reference of the newly installed equipment was not referenced in the same place, i.e old equipment on one phase (delta configuration) and the new equipment on a different phase. The two points would become the same potential and virtually short out a phase. The potential difference between the two points would only be the difference in ground potential (for long distances). Thus all the equipment on a supply should be referenced on the same phase of a delta configuration. If the people installing the new equipment did not inspect the old equipment already on the circuit, and wire the new equipment the same way the problem stated would result..this problem is not apparent on Y (or star)configuration.

"The two points would become the same potential and virtually short out a phase."

There is no virtual about it, the would short out the two phases and there would be a big flash as the second earth link vaporized giving your face a nice copper colour.

If the 2 ground points are just connected to one ground-point it will short-circuit.

If these are apart enogh then the ground between those 2 spots will be dangerous & pedesrians may get a taste of Electroluting from there legs in wet season; at least pets will get in all seasons.

one thing there no 135KV system but 132KV. Now latest in world is going to 500KV.

Secondly 132KV to 11KV is delta to delta so there cannot be a Neutrl-ground.