[Patch2002]

As I am currently in school I'm trying to do some research on Phase Bucking.

Some of my classmates and I were doing some test wiring. We were wiring 4 sets of fluorescent lights (4 T8 bulbs each) and some receptacles. The power is coming from a three phase panel, using two breakers (one for lights, one for receptacles [one on the A phase and one on the C phase]). When we turned on the breaker the lights came on, even though the light switch was off, and when the switch was turned on we heard the wires in the conduit vibrating and then the breaker tripped. The guy that wired the switch said he hooked up the neutral (we only pulled one neutral because the two circuits were on different phases) and one of the power wires.

Do you have any knowledge on this type of problem or can you direct me to some resources where i can find out more information? Any help will be greatly appreciated.

 

[Wyremonkey]

Check your wiring

 

[Patch2002]

Check your wiring

I know that it was wired wrong, What I am trying to understand is phase bucking. Can it happen with the neutral wire or just with the two different power phase sources? Plus, what causes phase bucking and why does it not happen when you make a 240 leg?

 

[bkmichael65]

The term phase bucking generally applies to a phase to phase short, but the trade lingo varies and can be used to describe any kind of short circuit(circuit path bypassing the load). In this case the breaker, if working properly will open immediately, if not you'll have a shower of sparks, fire or explosion until there is no longer a path for electricity to flow

 

[brian john]

First as apprentices you should be using proper terminology

You either have a phase to neutral short
Or a phase to phase short.

Where was your instructor in all this? When I taught classes I checked and tested everything prior to energizing.

 

[Patch2002]

First as apprentices you should be using proper terminology

You either have a phase to neutral short
Or a phase to phase short.

Where was your instructor in all this? When I taught classes I checked and tested everything prior to energizing.

I did ohm the circuits out and didn't find any shorts before we energized them. Also, I am trying to do research to learn what the proper terms are and trying to understand what happen. I do not think that he hooked the neutral wire up to the light switch but i believe that he hooked both hot wires up to the switch. If you know where I might learn more about phase to neutral shorts and phase to phase shorts I would greatly appreciate it.

In addition, there were no sparks just hearing the wires vibrate then the breaker tripped.

As far as my teacher, lets just say that I wish he had better teaching skills too.

 

[Troyboy]

I did ohm the circuits out and didn't find any shorts before we energized them. Also, I am trying to do research to learn what the proper terms are and trying to understand what happen. I do not think that he hooked the neutral wire up to the light switch but i believe that he hooked both hot wires up to the switch. If you know where I might learn more about phase to neutral shorts and phase to phase shorts I would greatly appreciate it.

In addition, there were no sparks just hearing the wires vibrate then the breaker tripped.

As far as my teacher, lets just say that I wish he had better teaching skills too.

There must be a dead short. Maybe he hooked the hot to the neutral? A short is the only way the breaker will trip that fast It's not an overload. That would take time for the thermal part to heat up. The hot could be hitting the emt and going to ground.

 

[Patch2002]

There must be a dead short. Maybe he hooked the hot to the neutral? A short is the only way the breaker will trip that fast It's not an overload. That would take time for the thermal part to heat up. The hot could be hitting the emt and going to ground.

Thats what he said he did, but why did it not show when I ohm'd it out (there was no continuity from either hot to ground)? Also, why would it not have tripped the breaker as soon as I flipped the breaker on (dead short)? Instead it didn't trip until the light switch was turned on (after the lights were turned on it to a few seconds for the breaker to trip and we heard a lot of vibrating noise coming from the conduit).

 

[Troyboy]

Thats what he said he did, but why did it not show when I ohm'd it out (there was no continuity from either hot to ground)? Also, why would it not have tripped the breaker as soon as I flipped the breaker on (dead short)? Instead it didn't trip until the light switch was turned on (after the lights were turned on it to a few seconds for the breaker to trip and we heard a lot of vibrating noise coming from the conduit).

Maybe a bad ballast ? Did you take apart his switch? It's def the switch or lights. As it stays on till it is flipped.

 

[RandyM]

I may be a phase to phase short across the switch. One of the phases is on the load of the switch. When you closed the switch, it went across to the other phase.
Or your wiring is really messed up.

 

[Troyboy]

I may be a phase to phase short across the switch. One of the phases is on the load of the switch. When you closed the switch, it went across to the other phase.
Or your wiring is really messed up.

Either way it should be an easy fix. Take the neutral and one phase to the plugs. Take the same neutral and the other phase to the lights. And just break the hot at the switch. It has to be funny wiring at the switch.

 

[brian john]

I did ohm the circuits out and didn't find any shorts before we energized them. Also, I am trying to do research to learn what the proper terms are and trying to understand what happen. I do not think that he hooked the neutral wire up to the light switch but i believe that he hooked both hot wires up to the switch. If you know where I might learn more about phase to neutral shorts and phase to phase shorts I would greatly appreciate it.

Then you obviously over looked a step in your testing.

In addition, there were no sparks just hearing the wires vibrate then the breaker tripped.

That is what we call a bolted fault.

As far as my teacher, lets just say that I wish he had better teaching skills too.

He is still culpable in this error in energizing a circuit.

 

[Patch2002]

I may be a phase to phase short across the switch. One of the phases is on the load of the switch. When you closed the switch, it went across to the other phase.
Or your wiring is really messed up.

I agree that is probably what happened. What I am really trying to understand more is the what, how, and why ,phase to phase, and phase to neutral short, works...I get very little info when I google them.

 

[Troyboy]

I agree that is probably what happened. What I am really trying to understand more is the what, how, and why ,phase to phase, and phase to neutral short, works...I get very little info when I google them.

Phase to neutral is exactly like phase to ground. There is no resistive load between the hot and neutral thus nothing to limit the current so the current is infinite and trips the breaker as it passes 15 amps with the magnetic feature on the breaker. I dont really know how to explain phase to phase. I would assume its the same as there is no resistive load between the two phases the current is infinite and causes the trip. That is my best explanation, maybe someone with more understand can explain it

 

[Big John]

...What I am trying to understand is phase bucking...

I wouldn't. No kidding. "Phase bucking" isn't an actual description of a fault, and it's not common-enough slang that I'd want to add it to my vocabulary.

Breakers trip on overcurrent. Overcurrent occurs during an overload condition or a short circuit. A "short" is any low unintentional impedance that completes a circuit.

Any time you have voltage between two points you have the potential to drive current. How much current depends on the impedance of the circuit. Phases "A" and "C" are obviously set up with a low circuit impedance because you want them to be able to supply power to a load. When there's no end load and those phases are connected to each other, the only thing limiting that current flow is the impedance in the supplying transformer and cables. That allows thousands of amps to flow, which is obviously dangerous, so the breaker trips.

 

[brian john]

Phase to neutral is exactly like phase to ground. There is no resistive load between the hot and neutral

There is a "resistive load but the resistance is very low, near "0"

thus nothing to limit the current so the current is infinite and trips the breaker as it passes 15 amps with the magnetic feature on the breaker.

That also is not true the current is limited by the impedance of the supply therefore no infinite and the instantaneous trip current (he magnetic portion of a molded case circuit breaker) typically on a 15 amp circuit breaker is 150 amps )10 times the rating of the CB.

Troy Google NEMA circuit breakers, they have a decent manual on circuit breakers, explaining how CB's work. It will be very helpful for you.

http://www.nema.org/pages/default.aspx

 

[Pharon]

It sounds like one side of the switch got wired to a hot and the other side to a neutral. When the switch is open, no problem. But then when it's closed you get a hot to neutral fault.

 

[theloop82]

If you checked for phase to phase or phase to ground with your meter after the circuit blew up, there is a good chance it blew clear and now you probably dont see any short, but nothing works either. Light fixtures are easy to screw up even if the wiring is correct, all you need to do is pinch a wire between the belly pan and cover or something, or have a wirenut slip one of the small #16 wires.

 

[Troyboy]

Yes if you want to call the minuscule amount of impedance that is in the conductor, then yes there is a small load on the circuit.

Theoretically it is infinite the current would surge till the breaker trips or the supply fuse blew from the supply side. If neither of these existed it would keep drawing current till the conductor heated up and burned itself up.

 

[brian john]

Yes if you want to call the minuscule amount of impedance that is in the conductor, then yes there is a small load on the circuit.

Theoretically it is infinite the current would surge till the breaker trips or the supply fuse blew from the supply side. If neither of these existed it would keep drawing current till the conductor heated up and burned itself up.

The MINUSCULE amount of impedance in a conductor is not minuscule it is all part of the circuit and is considered in any arc flash analysis and coordination study.

It is NOT theoretical, the current is limited by the impedance of the supply source and the impedance in the supply side distribution system. We are talking actual facts and how electrical circuits work, now you can make excuses for errors in your post or you can learn.


Your choice, I tried to point you in a direction for you to broaden your knowledge, take it or leave it.