Phase to phase and phase to neutral shorts

[Patch2002]

I am currently in school and would like to get a better understanding on this topic (Phase to phase and phase to neutral shorts).

What are the reactions and why?

I believe a fellow student and I came across this the other day.

 

[iceworm]

patch -
This is a really broad subject information request. There are literally thousands of papers/books on the subject. The one I happen to be currently using is, Arc Flash Hazard Analysis and Mittigation, JC Das - not necessarily the best, just the one I recently picked up.

Here is a place to start:
Consider a 480V, 3phase grounded Wye system. Faults generally start phase to ground. As fault currents go, the current is relatively low, the protective relaying takes a long time to trip - may not trip until ---> the fault finally goes phase to phase. The current tkaes a huge jump, protective relaying trips quickly. Interestingly the majority of the damage occurs under the phase to ground portion of the fault.

Good luck with your studies

ice

 

[Patch2002]

Some of my classmates and I were doing some 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.

I'm just trying to get a better understanding, for my [FONT=verdana, geneva, lucida, lucida grande, arial, helvetica, sans-serif]future career.

Thanks for the info.[/FONT]

 

[Smart $]

It should go without saying, but switches are connected in series with the load, not in parallel. The guy that wired the switch connected it in parallel.

When closed (switched ON), that created a line to neutral fault. The conductors in the fault path experience current well in excess of normal current. The magnetic field around the circuit conductors are in opposite directions, and thus repel each other. The fields change intensity and direction relative to the 60Hz sine wave of AC. That's the vibrating that you heard.

The circuit breaker was likely thermal magnetic (inverse-time opposed to instantaneous), so it didn't trip until enough heat was generated in the trip mechanism.

 

[Patch2002]

It should go without saying, but switches are connected in series with the load, not in parallel. The guy that wired the switch connected it in parallel.

When closed (switched ON), that created a line to neutral fault. The conductors in the fault path experience current well in excess of normal current. The magnetic field around the circuit conductors are in opposite directions, and thus repel each other. The fields change intensity and direction relative to the 60Hz sine wave of AC. That's the vibrating that you heard.

The circuit breaker was likely thermal magnetic (inverse-time opposed to instantaneous), so it didn't trip until enough heat was generated in the trip mechanism.

Thanks, that is so far the best explanation ,that I have read over a couple of different forums, that I can really understand. I'm looking forward to monday to see if the breakers are thermal magnetic.

Thanks again for the post.

 

[J.P.]

Think of a circuit as a circuit ( or a circle ) One circuit for Single Phase starts with a hot at the panel and returns on the neutral.

You can have 10 switches/receps/lights/ whatever in the circuit or one. It doesn't matter, just keep you circuits straight. Use colored wire and correspondingly striped neutrals if you have to.

Always switch the hot. Make the neutral and ground continuous.

It sounds like you wired one phase to the top of the switch and the other phase to the bottom of the switch. It happens, especially if nobody marks anything.

Phase to phase will trip all breakers involved in your misadventure. Makes these easy to identify, unless you somehow have phase to phase on a muliti pole breaker.

Phase to neutral will just trip the one breaker the hot is coming from. Screw through your wire from the drywallers is a common cause or a bad makeup from a colorblind text monkey.

 

[Patch2002]

Thanks for the info JP!