# When 2 different phases touch?



## JasonCo (Mar 23, 2015)

Lets say you have a 240v Motor hooked up to a 2 pole breaker. Phase A and Phase B somehow get nicked and touch together and cause the 2 pole breaker to trip. I'm just curious how it actually causes the breaker to trip. What happens exactly? 

For example, I understand what happens when there is a ground fault. The current travels through the bonded raceway (EMT) or electrical grounding conductors (whichever has least resistance) and makes its way to the grounding bar and through the bonding jumper, then through the neutral conductor back up the transformer and through the ungrounded conductor to finally trip the breaker.

I just don't know what happens when 2 different phases touch together and how it trips the breaker.


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## RePhase277 (Feb 5, 2008)

A large current travels through the line conductors, through the fault, tripping the breaker.


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## frenchelectrican (Mar 15, 2007)

JasonCo said:


> Lets say you have a 240v Motor hooked up to a 2 pole breaker. Phase A and Phase B somehow get nicked and touch together and cause the 2 pole breaker to trip. I'm just curious how it actually causes the breaker to trip. What happens exactly?
> 
> For example, I understand what happens when there is a ground fault. The current travels through the bonded raceway (EMT) or electrical grounding conductors (whichever has least resistance) and makes its way to the grounding bar and through the bonding jumper, then through the neutral conductor back up the transformer and through the ungrounded conductor to finally trip the breaker.
> 
> I just don't know what happens when 2 different phases touch together and how it trips the breaker.


Very simple you will get either arc fault or bolted fault on this and no question asked you will trip the breaker.


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## Bird dog (Oct 27, 2015)

The same thing as someone taking a straight piece of wire & wiring a breaker with it-a dead short. The wire is zero ohms, so, there is no load to resist current flow. The more resistance, the lower the current flow.


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## JasonCo (Mar 23, 2015)

Thanks for your help, so you're saying a large amount of current will only travel through your ungrounded conductors back to the breaker and trip it? The neutral, raceways, grounding conductors don't come into play at all right? The current will simply flow through the ungrounded conductors back to the breaker?


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## Bird dog (Oct 27, 2015)

Yes.


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## PlugsAndLights (Jan 19, 2016)

I=E/R. 
In this case E=240. 
Replace the motor with a short and R drops dramatically. 
Reducing R results in proportional increase in I.
P&L


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## FF301 (Jan 12, 2014)

Sounds like someone's home work

Basically the Breaker operates the same way weather it's a short or an overload. Either by it's thermal or magnetic trips. 

Now the engineers will over explain inverse time, response, AIC ratings etc, etc,


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## macmikeman (Jan 23, 2007)

Phase to phase short circuits are simply very more dangerous to be near to than phase to ground ones because both of the faulted phases are kabooming simultaneously. Burnt face victims endorse this message.


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## macmikeman (Jan 23, 2007)

Whether. Just sayin.....


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## ChrisHakkaraine (Mar 20, 2017)

In electrical engineering, we deal mostly with phasors, which can be simply explained as rotating vectors. Don't be confused yet, since we often draw phasors with reference to a phasor, which makes it stationary with the frame of reference. Let us look into the basics of vector addition and subtraction. 
We know that we do not compute the arithmetic sum of vectors. We use vector addition. In this case, I'm going to assume that the concept of parallelogram law is already known to you.
Let us represent each phase-to-neutral voltage with the same magnitude, but displaced from the consecutive phase by an angle of 120 degrees. Let this magnitude be represented as |V|=|Vrn|=|Vyn|=|Vbn|, where Vrn, Vyn, Vbn are the phase to neutral voltages of each phase. By rules of vectors, we compute the magnitude of phase to phase voltage using parallelogram law.
Let us consider the voltage difference between the R phase and Y phase.
Let this voltage be represented by Vry, and its magnitude be |Vry|.

We have computed the magnitude |Vry| to be 1.732 times(square root of 3) the phase-to-neutral voltage |V|.
If we take the range of magnitude of phase-to-neutral voltage to be between 230 V and 240 V, the range of phase-to-phase voltage would be 398 V to 415 V. Usually, we consider it to be 415 V.

To answer your question, it would create a short circuit, since you would be creating a low resistance path between two points with a 415 V potential difference.
If you connect two wires of the same phase, nothing would happen, except that the line can be treated as a double circuit line which would now have double the current carrying capacity. Also, when you want to short two lines of the same phase, make sure they are of the same voltage level and frequency. Any mismatch would result in circulating currents between the two lines.


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## micromind (Aug 11, 2007)

Interesting you stated the breaker is connected to a motor. 

If the motor is running when the fault occurs, it will instantly become a generator and contribute to the current across the fault. 

Also note, a basic standard breaker has 2 trip units;

1) Thermal......this is for long term overloads, like 30 amps on a 20 amp breaker.

2) Magnetic.......this is for high current incidents, like short circuits and ground faults. This trip unit will respond MUCH faster than the thermal one. 

The reason for having both is because the magnetic one will not see 30 amps on a 20 amp breaker as a problem so the thermal one is needed. The thermal one will indeed trip under fault or short circuit current but it'll take a few seconds. A fault current incident needs to be de-energized very quickly or bad things will happen. Like explosions and/or fires.......


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## sparkiez (Aug 1, 2015)

This is pretty simple. You have two lines coming off of your transformer. 120V and -120V taps. When you add the voltage (magnitude of the sin wave) together, you have a potential difference of 120V to -120V. This gives you 240V. Ultimately, these are the two connection points for the 240V single phase motor. You are not getting a neutral from the transformer in this case, so there is no "short to ground."

Circuits MUST have a load on them. What is Resistance? It is the opposition to the flow of current, while voltage is the "pressure" that makes the electrons want to move from a "spot" with more electrons to a spot with less electrons. Pretty much the same way if you put a drop of food coloring into a glass of water it will want to evenly distribute throughout the water. We name these properties of electricity and quantify them.

Voltage (aka potential difference, aka potential) - the pressure created to move electrons through a conductor when there is a difference in the electron concentration from one point to another.

Resistance (the opposition to a flow of current. Without getting too involved, electricity flows through the outside edge of atoms, because the atomic structure is rigid and the outside of the atoms are close together, and since the outside areas of the atoms are far away from the center of the atom they require less energy to "jump" to the next atom. Now imagine that we put a material that requires more energy for the electron to jump to the next atom. This extra energy required means that more pressure from voltage is required to push it across the atom AND/OR that less electrons per second can be moved at the same voltage).

Amperage (the actual quantity of electrons that are jumping from one spot on the atom to the next spot on the atom per second. It is a really high number.)

Just to word it another way, voltage being present means that there is a difference in the number of electrons between two points, amperage is the number of electrons passing through a given point in 1 second, resistance is how hard they have to be "pressed" on to move to the next atom.

When you have a complete circuit AND a potential difference (voltage) then current WILL FLOW. This is where your head will hurt a bit. You have to think of this in terms of energy. Heat and light given off by a bulb are energy. A motor TURNING is energy. Electricity is also energy. We convert electrical energy (which in itself isn't all that useful) to a form that is, such as rotational energy like what a motor gives off by turning, which is then transferred to mechanical energy that we use to do work.

We know the voltage given between the two points is 240V because we have -120V + +120V. We also have a resistance on the circuit that LIMITS the amount of energy we can transfer with the voltage we have. We only have so much pressure pushing the electrons forward, so that means only so many electrons per second can pass any given point.

When the circuit is abnormally completed (aka the L1 and L2 touching each other, remember this is single phase, so they are called Line not Phase), then the circuit is "shorter" than normal, hence the name. Because there is very little resistance in the circuit and the same amount of pressure (called voltage from here on in) pushing the electrons forward, they will go really really fast, and since amp means the number of electrons passing a given point in 1 second, there is a LOT of energy there.

Now when an electron gets knocked away from its current "atomic orbit" it takes on more energy, and when it falls into the orbit of the next electron, it gives off this extra energy. This extra energy is given off as heat in the wire. Now, the wire only has so many electrons and so many atoms and when you get a whole bunch of them moving really really fast and giving off a bunch of heat in the same spot at close to the same time, that energy has to go somewhere, which happens to be the area around the short. Air has electrons too, and they don't like to get shuffled around like the ones inside a wire does, but what this does do is give the extra electrons and energy a place to go. That is how arc flashes are born.

Now, back at the breaker, we have the same number of electrons flowing as we do at the spot where L1 and L2 are shorted. This equipment is of a higher rating and can handle more amperage, so most of the time instead of sparking it heats up. This heat allows a mechanical mechanism to move and opens the breaker.

This exact same thing happens with a ground fault, but as you mentioned, the current is going to ground. The fireworks just tend to be a lot larger when dealing with a phase-to-phase or line to line short.

Lots there, but if you get those basic concepts you will start seeing a lot more "ah ha" moments as you do electrical work.


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## dmxtothemax (Jun 15, 2010)

Perhaps he's not getting his head around the fact that electricity doesnt just
Travel from hot to neutral, or plus to minus but it will in fact travel between
Any two pionts with a potentual difference.
Such as three phase
Due to the difference in phase
There is a potentual difference
Hense current flows !


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## splatz (May 23, 2015)

dmxtothemax said:


> Perhaps he's not getting his head around the fact that electricity doesnt just
> Travel from hot to neutral, or plus to minus but it will in fact travel between
> Any two pionts with a potentual difference.
> Such as three phase
> ...


I think that's the short answer, 
if you have two points with an electrical potential between them (aka voltage) 
and you put a conductive path between them, 
current flows. 

How much current depends on the resistance (loads etc.) in the path, the higher the resistance, the lower the current - Ohm's law. 

With shorted wires, it's just wire in the path, no loads - near zero resistance - and the current will be very high. 

You could kind of think of it as voltage is "potential" current flow, short it and you have a LOT of "actual" current flow!


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## Flyingsod (Jul 11, 2013)

sparkiez said:


> This is pretty simple. You have two lines coming off of your transformer. 120V and -120V taps.


Wha?



Sent from my C6725 using Tapatalk


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## JRaef (Mar 23, 2009)

To the earlier points, when you have a "bolted" fault like that, ALL of the AVAILABLE current in the system up stream of the breaker will attempt to flow at the speed of light (which is the speed of electricity), slowed down only by the resistance remaining in the circuit. Wires touching = almost zero resistance. That's what the "AIC" rating of the breaker is all about. In a typical residential panel, the breakers will be rated for 10kAIC. That means they can handle the mechanical stresses of attempting to interrupt 10,000A of current. How much fault current is actually available is not likely that high because it is impeded by the transformers in the utility circuit and the wire resistance all the way from the last transformer to the actual fault, but it's still a LOT of current.

When the breaker senses that fault, it BEGINS to trip, but the actual current flow happens much much faster than the breaker can react to it, plus when the contacts open, there is an arc that keeps it going still. That total time it takes to stop the current flow, called the "clearing time" is essentially an eternity when the current is moving at the speed of light. So when the breaker does finally begin to open, the current is ALREADY flowing at the max available fault current level and the magnetic forces involved in that are a lot more than people think.


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## sparkiez (Aug 1, 2015)

Flyingsod said:


> Wha?


http://www.annsgarden.com/poles/JP0-PS-Schematic.jpg

That image shows the two Line taps on a standard pole transformer. If you look, you will see that one is +120V and the other is -120V relative to the neutral on the transformer's secondary. That is how you get 240V potential between L1 and L2.


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## brian john (Mar 11, 2007)

sparkiez said:


> T
> This exact same thing happens with a ground fault, but as you mentioned, the current is going to ground. The fireworks just tend to be a lot larger when dealing with a phase-to-phase or line to line short.
> 
> .


Depending on the voltage the exact thing does not happen with a ground fault, at higher voltages a ground fault can become an arcing ground fault which will not generate sufficient current to operate SOME Over current protection devices. This fault can be a long term event with devastating results


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## sparkiez (Aug 1, 2015)

brian john said:


> Depending on the voltage the exact thing does not happen with a ground fault, at higher voltages a ground fault can become an arcing ground fault which will not generate sufficient current to operate SOME Over current protection devices. This fault can be a long term event with devastating results


Thank you for the correction on this. You are right about that. I used to work in a packing plant and saw this a few times on motors where it would take a LONG time to trip the overload. We replaced the motor and it happened a week later so our team broke open the conduit and found wet wires with damaged insulation.


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## triden (Jun 13, 2012)

In the simplest terms, you're essentially shorting (bolted fault) the secondary of the transformer together through the breaker. That's why the breaker trips.


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## triden (Jun 13, 2012)

sparkiez said:


> http://www.annsgarden.com/poles/JP0-PS-Schematic.jpg
> 
> That image shows the two Line taps on a standard pole transformer. If you look, you will see that one is +120V and the other is -120V relative to the neutral on the transformer's secondary. That is how you get 240V potential between L1 and L2.


You have a 240v secondary across L1 and L2. Don't worry about the neutral - it confuses most people. It's just a reference.


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## triden (Jun 13, 2012)

brian john said:


> Depending on the voltage the exact thing does not happen with a ground fault, at higher voltages a ground fault can become an arcing ground fault which will not generate sufficient current to operate SOME Over current protection devices. This fault can be a long term event with devastating results


This was today's job. 5kv cable fault. Probably was arcing for a period of time based on the condition of the cable.


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## chicken steve (Mar 22, 2011)

*Theory Query*

If i may, i'd like to throw the theoretical monkey wrench in, a real stumper that an EE presented in an effort to enlighten the great unwashed....

What if  on tied all three legs of a _perfectly balanced_ *3* phase system together? perfectly vectored, if you will....

Think something low enough in terms of *R* value here.

Wouldn't said perfect balance simply _cancel each other out_?

~CS~


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## Flyingsod (Jul 11, 2013)

chicken steve said:


> If i may, i'd like to throw the theoretical monkey wrench in, a real stumper that an EE presented in an effort to enlighten the great unwashed....
> 
> What if  on tied all three legs of a _perfectly balanced_ *3* phase system together? perfectly vectored, if you will....
> 
> ...


I don't see how. Phases are not diametrically opposed. Unless said conductors happened to be laying in a coil I don't think there would be enough induction forces to reread ****** electron flow. Thought experiments rule btw, thanks.

Sent from my C6725 using Tapatalk


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## sparkiez (Aug 1, 2015)

Flyingsod said:


> I don't see how. Phases are not diametrically opposed. Unless said conductors happened to be laying in a coil I don't think there would be enough induction forces to reread ****** electron flow. Thought experiments rule btw, thanks.
> 
> Sent from my C6725 using Tapatalk


This is dead on. If you put the three separate phases completely in phase, their magnitudes would combine.


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## Flyingsod (Jul 11, 2013)

Flyingsod said:


> I don't see how. Phases are not diametrically opposed. Unless said conductors happened to be laying in a coil I don't think there would be enough induction forces to reread ****** electron flow. Thought experiments rule btw, thanks.
> 
> Sent from my C6725 using Tapatalk


Omg political correctness should not override real words. "Reread *******" should of course be re. Tard


Flyingsod said:


> I don't see how. Phases are not diametrically opposed. Unless said conductors happened to be laying in a coil I don't think there would be enough induction forces to reread ****** electron flow. Thought experiments rule btw, thanks.
> 
> Sent from my C6725 using Tapatalk



Sent from my C6725 using Tapatalk


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## splatz (May 23, 2015)

chicken steve said:


> If i may, i'd like to throw the theoretical monkey wrench in, a real stumper that an EE presented in an effort to enlighten the great unwashed....
> 
> What if  on tied all three legs of a _perfectly balanced_ *3* phase system together? perfectly vectored, if you will....
> 
> ...





Flyingsod said:


> I don't see how. Phases are not diametrically opposed. Unless said conductors happened to be laying in a coil I don't think there would be enough induction forces to reread ****** electron flow. Thought experiments rule btw, thanks.


I don't even get it. I thought it would be either tie them together in a star or a triangle.


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## micromind (Aug 11, 2007)

chicken steve said:


> What if  on tied all three legs of a _perfectly balanced_ *3* phase system together? ~CS~


It would blow up.


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## JRaef (Mar 23, 2009)

chicken steve said:


> If i may, i'd like to throw the theoretical monkey wrench in, a real stumper that an EE presented in an effort to enlighten the great unwashed....
> 
> What if  on tied all three legs of a _perfectly balanced_ *3* phase system together? perfectly vectored, if you will....
> 
> ...


I think you are conflating the effect of magnetic fields cancelling each other out, and current flow, which does not. They are apples and tractors (as in not apples and oranges, which at least are both fruits). With current flow, all you need is a path from one source to return to that source or to earth. With your perfect bolted fault on all three phases the paths from one phase to the next are going to exist and current flow in each phase will immediately jump to being all of the available fault current in the system at that point. 

The only difference is that if this bolting of all three phases took place at EXACTLY the same moment and was a PERFECT connection between phases (meaning exactly the same resistance), the fault current wave forms would be symmetrical, not asymmetrical. In the past, things like breakers would have a rating for asymmetrical vs symmetrical fault current because the amount of current relates to the physical stresses exerted on the breaker when trying to open under a fault. The symmetrical rating was always a higher value, but of course was something that never happened in real life. Yet people got confused with that and would use the higher ratings by mistake, so they stopped providing that information. Fault interrupting ratings are now always asymmetrical.


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## cuba_pete (Dec 8, 2011)

*Don't mind me...just trolling for ET junkies*



Flyingsod said:


> Wha?


Yeah…that’s pretty fücking rich.



sparkiez said:


> http://www.annsgarden.com/poles/JP0-PS-Schematic.jpg
> 
> That image shows the two Line taps on a standard pole transformer. If you look, you will see that one is +120V and the other is -120V relative to the neutral on the transformer's secondary. That is how you get 240V potential between L1 and L2.


I like to see when people try to explain something by posting to someone else’s interpretation as well…


...this interpretation brought to you by the Society of Broadcast Engineers, courtesy of the Ann's Garden..._wth??? _What kind of a reference is _that_?



sparkiez said:


> This is dead on. If you put the three separate phases completely in phase, their magnitudes would combine.


oh my...oh my my my my my...

JRaef...you're one of my heroes.


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## brian john (Mar 11, 2007)

triden said:


> In the simplest terms, you're essentially shorting (bolted fault) the secondary of the transformer together through the breaker. That's why the breaker trips.


The OP mentioned touching the conductors together, in terms of fault current this MAY BE somewhat different that a true bolted fault.


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## triden (Jun 13, 2012)

brian john said:


> The OP mentioned touching the conductors together, in terms of fault current this MAY BE somewhat different that a true bolted fault.


I guess it's possible fault current could be worse upstream closer to the transformer. Also, if we are talking arc flash, the location of the actual bolted fault may not have the worst incident energy.


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## chicken steve (Mar 22, 2011)

Perhaps a _lame_ *Q *, but does the definition of '*bolted fault*' mean actual bolted parts, ocpd's, etc....?:001_huh:

~CS~


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## RePhase277 (Feb 5, 2008)

chicken steve said:


> Perhaps a _lame_ *Q *, but does the definition of '*bolted fault*' mean actual bolted parts, ocpd's, etc....?:001_huh:
> 
> ~CS~


Bolted means the conductors in the fault are connected with a mechanical strength greater than the fault's ability to seperate them through explosive or magnetic force.


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