# Three Phase problem



## wildleg (Apr 12, 2009)

if there is a partial fault to ground (damage to insulation, machine still hooked up with shorted coils, nicked conductor, etc) then you have a difference in potential, hence the voltage. It is also possible, although unlikely, that it is ghost voltage. Hopefully you can isolate the cause meggar conductors. Hopefully you are using a cat III or cat IV meter. If you are not qualified to troubleshoot the cause, please get someone who is.


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## glen1971 (Oct 10, 2012)

Is there a transformer inside the machine? Somewhere to knock the voltage down from 480 to 230? If there is and you have one blown fuse that would explain the off voltages, since the voltage you would be reading would be from gnd to L1 (before the xfmr) and L2 (after a xfmr being fed only from L1).. 
Think of it like power going in on one side (the one with the fuse), going through windings, controls, etc, then coming back to the Load side of the disconnect. With the fuse blown you are reading L1 voltage-resistive loads that are in the circuit.

Not the best way of explaining it, but hope it helped...


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## wendon (Sep 27, 2010)

tbartek said:


> In my school's welding shop they installed a new welder that would not work. The welder was set up to operate on 230V single phase. I assumed they were using a 230V disconnect. I checked the incoming voltage and it read 237V but when I connected the machine the voltage reading dropped to milli volts. I traced the disconnect back to the equipment panel and found that it was coming out of a 480/277 three phase panel. It used two legs and one of the fuses was blown. My question is why do I get a reading of 480V phase to phase on the line side of the fuses and with one fuse removed am getting a reading of 235V on the load side of the fuse holders with the one phase fuse removed from the disconnect? It's driving me nuts. Any help would be greatly appreciated.


So are you saying that you've got 480 hooked to a 230 volt single phase welder? Something's going to burn!! Where did you check the voltage? Was it phase to phase, or phase to ground?


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## tbartek (Nov 30, 2012)

glen1971 said:


> Is there a transformer inside the machine? Somewhere to knock the voltage down from 480 to 230? If there is and you have one blown fuse that would explain the off voltages, since the voltage you would be reading would be from gnd to L1 (before the xfmr) and L2 (after a xfmr being fed only from L1)..
> Think of it like power going in on one side (the one with the fuse), going through windings, controls, etc, then coming back to the Load side of the disconnect. With the fuse blown you are reading L1 voltage-resistive loads that are in the circuit.
> 
> Not the best way of explaining it, but hope it helped...


No, I thought of that. There was nothing connected to the circuit except the feed wires from the main disconnect to the machines disconnect.


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## tbartek (Nov 30, 2012)

wendon said:


> So are you saying that you've got 480 hooked to a 230 volt single phase welder? Something's going to burn!! Where did you check the voltage? Was it phase to phase, or phase to ground?


I checked the voltage at the machines disconnect and it read 237V phase to phase. When I connected the machine to what I thought was a 230V line the voltage reading dropped to milli volts. Only after I disconnected the machine would the voltage reading go back up to 237V. That's why I went back to the main disconnect and then found that it was actually 480V phase to phase with the burned out fuse. With the burned out fuse removed I checked the line side of the main and got 480V but just for the heck of it I checked the load side with the fuse still removed and am getting 237V on the load side. That is what has me concerned.


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## Bbsound (Dec 16, 2011)

You are reading the potential with a high impedence meter, just like the false readings you can get through conductor induction. 
The only problem here is that somebody is hooking a 230 volt welder to a 480 panel. Dont let meter readings confuse the situation.


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## Spark Master (Jul 3, 2012)

You have 2 problems .
a) bad circuit.
b) wrong welder for the electrical service. or wrong electrical service for the welder.


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## ChrisH8802 (Feb 26, 2012)

just out of curiosity, if this is at your school, why don't you ask your instructor. Having someone there that can actually see what is going on should have a better shot at this


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## tbartek (Nov 30, 2012)

ChrisH8802 said:


> just out of curiosity, if this is at your school, why don't you ask your instructor. Having someone there that can actually see what is going on should have a better shot at this


One problem. I am the instructor. That's why I'm here picking your brains.


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## tbartek (Nov 30, 2012)

Spark Master said:


> You have 2 problems .
> a) bad circuit.
> b) wrong welder for the electrical service. or wrong electrical service for the welder.


Look. The welding shop got a new welder. The welding instructor hooked the welder up to a disconnect in an existing welding booth. When it didn't work he called me (the electrical instructor), over to see if the disconnect had voltage. I checked the disconnect and it read 235 VAC. However when I turned the disconnect on to try the machine the voltage reading dropped to milli volts. Essentially zero. This is what started me back tracking the feed and discovering the 480 V main with one fuse burned out. I'm just trying to understand why I get a 235 V reading phase to phase with one phase having a burned out fuse.


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## Spark Master (Jul 3, 2012)

tbartek said:


> Look. The welding shop got a new welder. The welding instructor hooked the welder up to a disconnect in an existing welding booth. When it didn't work he called me (the electrical instructor), over to see if the disconnect had voltage. I checked the disconnect and it read 235 VAC. However when I turned the disconnect on to try the machine the voltage reading dropped to milli volts. Essentially zero. This is what started me back tracking the feed and discovering the 480 V main with one fuse burned out. I'm just trying to understand why I get a 235 V reading phase to phase with one phase having a burned out fuse.


It's a partial back feed reading across another piece of equipment.


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## wendon (Sep 27, 2010)

tbartek said:


> Look. The welding shop got a new welder. The welding instructor hooked the welder up to a disconnect in an existing welding booth. When it didn't work he called me (the electrical instructor), over to see if the disconnect had voltage. I checked the disconnect and it read 235 VAC. However when I turned the disconnect on to try the machine the voltage reading dropped to milli volts. Essentially zero. This is what started me back tracking the feed and discovering the 480 V main with one fuse burned out. I'm just trying to understand why I get a 235 V reading phase to phase with one phase having a burned out fuse.


Are you getting this reading with the welder unplugged? What is your reading phase to ground? Check it on both legs with the welder disconnected. Do you have the bad fuse removed? If not, remove it and then see if you get the same reading. Shut off the power to the disconnect, remove the fuses, make sure nothing is plugged into the receptacle, then check for resistance to ground and between the phase conductors.Make sure you have a good quality meter when you check the voltage. Whatever you do, don't replace the fuse and plug that welder back in or you may release the smoke.:no:


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## Fluke (Dec 7, 2012)

Sounds like that leg is reading thru the welder and should be 277 but it's not enough to give you 277. Try it straight to ground and with the welder disconnected.


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## LogixElectrician84 (Dec 7, 2012)

tbartek said:


> In my school's welding shop they installed a new welder that would not work. The welder was set up to operate on 230V single phase. I assumed they were using a 230V disconnect. I checked the incoming voltage and it read 237V but when I connected the machine the voltage reading dropped to milli volts. I traced the disconnect back to the equipment panel and found that it was coming out of a 480/277 three phase panel. It used two legs and one of the fuses was blown. My question is why do I get a reading of 480V phase to phase on the line side of the fuses and with one fuse removed am getting a reading of 235V on the load side of the fuse holders with the one phase fuse removed from the disconnect? It's driving me nuts. Any help would be greatly appreciated.


To me, this sounds like an issue of stray capacitance, aka a "ghost voltage" as another member stated in this thread. The longer the conductors are, and closer they are in a run, the more capacitance they build from line to line, and line to ground. You are seeing a buildup of this capacitance when reading your meter (which also picks up ghost readings.)

If you have an anolog meter, that is not digital and high impedance, try that on the circuit doing the same procedure (line to line, with one fuse missing) and see what you read, I but you'll notice a difference.

I would see 40V, 80V, 120V, etc when switching off disconnects/motor protectors in control panels to motors for conveyors with long distances of wire runs, measuring at the motor disonnect switch. It is a ghost voltage but can really annoy you at times because you're not sure if it's dead. It can pick up voltages from quite a few sources. If you pull the fuse on one phase, and swap it with the other one, and then measure the voltage, I'm sure you will get a very similar reading.

If you want a really good example of a ghost voltage, take your digital meter, and put the leads close to a lighting fixture with a ballast (hold them up to it like an antena) and you will see up to 1,000V picked up on your meter. This is a true ghost voltage, the meter can be so sensitive at times. Try an analog meter with the same experiment, and you won't pick any thing up at all.

You are puzzled because you are seing 237V on a circuit that should be dead, I know your frustration!

Why did the fuse blow? Welder overloaded the circuit, or there's a short?


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## 360max (Jun 10, 2011)

.....tbartek, *STOP*, now go and read the nameplate on the welder. What does it read?


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## tbartek (Nov 30, 2012)

LogixElectrician84 said:


> To me, this sounds like an issue of stray capacitance, aka a "ghost voltage" as another member stated in this thread. The longer the conductors are, and closer they are in a run, the more capacitance they build from line to line, and line to ground. You are seeing a buildup of this capacitance when reading your meter (which also picks up ghost readings.)
> 
> If you have an anolog meter, that is not digital and high impedance, try that on the circuit doing the same procedure (line to line, with one fuse missing) and see what you read, I but you'll notice a difference.
> 
> ...


The welding booth that had the blown fuse had not been used since before the current welding instructor took over the program. They have such a demand for the program that they bought more welders. So why it was blown is impossible to say. The whole thing started last year when the welding teacher asked me to come over and check the voltage in the booth so he knew what voltage to order the new welder with. The shop has 480V 3-phase welders, 480V single phase welders and 230V single phase welders. The school was built in 1971 so I don't know why the mix of voltages. Thank you very much for the answer. On Monday I'm gonna break out my Simpson 260 and do your experiment. I hope the other members watching this thread finally realize that I'm not going to try to connect the 230V welder to a 480V line.:icon_rolleyes:


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## tbartek (Nov 30, 2012)

tbartek said:


> The welding booth that had the blown fuse had not been used since before the current welding instructor took over the program. They have such a demand for the program that they bought more welders. So why it was blown is impossible to say. The whole thing started last year when the welding teacher asked me to come over and check the voltage in the booth so he knew what voltage to order the new welder with. The shop has 480V 3-phase welders, 480V single phase welders and 230V single phase welders. The school was built in 1971 so I don't know why the mix of voltages. Thank you very much for the answer. On Monday I'm gonna break out my Simpson 260 and do your experiment. I hope the other members watching this thread finally realize that I'm not going to try to connect the 230V welder to a 480V line.:icon_rolleyes:


Well, the simpson 260 shows 140V Line to no-fuse line. I concede to the "ghost voltage"


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## wildleg (Apr 12, 2009)

seems to me, that if you are unfortunate and get injured using that meter, questions will be raised.


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