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Thread: Can a 3way switch short?

  1. #16
    Electrical Contractor/Instructor jwelectric's Avatar
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    Short in electrical terms means to bridge or close. Ground plays no role in a short.

    In the field we become lazy and misuse terms. A good one to look at is the battery in our cars. From birth we are taught to remove the ground before removing the positive terminal of the battery in order to not shunt the positive to the frame of the car with the wrench we are using to remove the positive terminal.

    Well my friends there is no such thing as ground in a car as defined in the NEC or should an electrician ever refer to the negative terminal of a battery as ground.

    What was being talked about in the original post was a ground fault if the equipment grounding conductor was touching one of the travelers, not a short. The switch shorted the circuit and should the light bulb have been replaced with an Edison base fuse the fuse would have explained that the switch was shorting the circuit. But instead we have installed a load, the bulb, which consumes the energy of the shorted circuit so instead of tripping the overcurrent device we have light.

    Understanding terms makes discussing electrical theory much easier.

  2. #17
    DIY Senior Member bluebinky's Avatar
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    Quote Originally Posted by jwelectric View Post
    Short in electrical terms means to bridge or close. Ground plays no role in a short.

    In the field we become lazy and misuse terms. A good one to look at is the battery in our cars. From birth we are taught to remove the ground before removing the positive terminal of the battery in order to not shunt the positive to the frame of the car with the wrench we are using to remove the positive terminal.

    Well my friends there is no such thing as ground in a car as defined in the NEC or should an electrician ever refer to the negative terminal of a battery as ground.

    What was being talked about in the original post was a ground fault if the equipment grounding conductor was touching one of the travelers, not a short. The switch shorted the circuit and should the light bulb have been replaced with an Edison base fuse the fuse would have explained that the switch was shorting the circuit. But instead we have installed a load, the bulb, which consumes the energy of the shorted circuit so instead of tripping the overcurrent device we have light.

    Understanding terms makes discussing electrical theory much easier.
    There are lots of ways to use the term "short", but I don't agree that saying "The switch shorted the circuit..." is a valid one. Oh well, I guess I'm outnumbered on this one by some people I've grown to respect.

    I'm still wondering, JW, have you ever seen a switch fail internally and cause a ground fault? The few that I've broken apart make me believe that there would have to be obvious visible damage.

  3. #18
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by bluebinky View Post
    There are lots of ways to use the term "short",
    But few are correct
    Quote Originally Posted by bluebinky View Post
    but I don't agree that saying "The switch shorted the circuit..." is a valid one.
    If the switch doesn’t short the circuit the light will never come on.
    Quote Originally Posted by bluebinky View Post
    I'm still wondering, JW, have you ever seen a switch fail internally and cause a ground fault?
    Yes

    Quote Originally Posted by bluebinky View Post
    The few that I've broken apart make me believe that there would have to be obvious visible damage.
    The yes answer left just what you are describing here

    Below is what the NEC describes as a fault.
    110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics.
    The overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the equipment grounding conductor(s) permitted in 250.118. Listed equipment applied in accordance with their listing shall be considered to meet the requirements of this section.

    Now going back to that car battery, if I asked you to short out the battery how would you accomplish this? Could the same thing be accomplished by installing a conductor with a switch and simply closing the switch?

    How do we discharge a capacitor? In many cases in industrial establishments this is done by using a switch with a current limiting resistor in series across the capacitor.

    460.6 Discharge of Stored Energy.
    Capacitors shall be provided with a means of discharging stored energy.
    (A) Time of Discharge. The residual voltage of a capacitor shall be reduced to 50 volts, nominal, or less within 1 minute after the capacitor is disconnected from the source of supply.
    (B) Means of Discharge. The discharge circuit shall be either permanently connected to the terminals of the capacitor or capacitor bank or provided with automatic means of connecting it to the terminals of the capacitor bank on removal of voltage from the line.

  4. #19
    DIY Senior Member bluebinky's Avatar
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    JW, I agree that closing a switch connected to wires across a car battery would cause a short. However, turning on the headlights by closing that switch would not -- just like connecting the terminals of a battery with a piece of wire would cause a short whereas the 2/0 or so wire going to the starter is not considered to be a short. Anyway, our disagreement is pretty small, and we're both too old and stubborn to change. I concede that your usage is "valid", just slightly different than what I was taught. Maybe I'm stuck in the past and need to get out more...

    I think you answered the OPs question in that it was probably not the switch itself, unless as DonL interpreted, the problem was really an open. Too bad he didn't stick around.

  5. #20
    DIY Senior Member Murphy625's Avatar
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    The term short is always used to describe an unintended electrical pathway. In the overwhelming majority of its use, the term describes a malfunction in which the current finds a less resistive path to ground potential than what was intended. While much rarer, the term could also describe the unintended pathway between two conductors of the same electrical potential.

    Per the battery and switch argument, the closing of a switch is never described as a short because a switch is designed and intended to be closed. That is just called a "closed" circuit. (remember, the term short is always used to describe an unintended pathway that creates a short(er) route)

    One could change the argument perspective and say that a "Closed circuit" is the opposite of an "Open circuit" and that a "short circuit" has no opposite "long circuit" description. Its a silly argument but would seem to create a marriage between the terms "Short" and "Closed" even though the two are used to describe two different intentions.

    Once again, if you're closing a switch that allows current to flow to ground, its not a short.. its just a closed circuit.
    Replacing the light bulb with a fuse only changes the circuit resistance to such a low value that the flow of current becomes too high for the fuse to handle. If you re-routed the wire that connects the switch to the fuse and ran it through Chicago, then Denver, and then back to Chicago, the fuse would probably not blow at all since enough resistance has now been added to slow the current flow. (basic ohms law).

  6. #21
    Retired Defense Industry Engineer jadnashua's Avatar
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    The simplistic answer is that a switch in a completed circuit IS a short - just like a piece of wire. If you look at it in the context of an ohmmeter, a closed switch looks exactly like a short between two terminals. Now, if it is 'shorted' to a place you didn't intend it, then you have problems. Bantering about context and common usage doesn't necessarily help with the underlying understanding. If you have a good grasp of that, this bickering is kind of funny.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #22
    Electrical Contractor/Instructor jwelectric's Avatar
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    Yes I too find it funny that on a DIY web site we will have folks from different walks of life that knows more about electrical theory than someone who has paper hanging on the wall from different colleges, licenses, certificates, and is even hired by a college to instruct electrical theory.

    I also find it sad that these fine folks are too dense to learn.
    short′ cir′cuit
    n.
    an abnormal condition of relatively low resistance between two points of differing potential in a circuit, resulting in a flow of excess current.

    With a circuit of a 120 volt source, a 100 watt light bulb, and a switch, if the circuit is not shorted the light will never come on.
    Now let’s take a moment and think a little. Let’s take a piece of wire and connect it to the two terminals of the light bulb. With the switch in the off (open) position does anything happen? Turn on (close) the switch and describe the circuit.
    If you say the circuit is shorted then the closing of the switch is what caused the short plain and simple.

  8. #23
    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by jadnashua View Post
    this bickering is kind of funny.
    I would hesitate to call this bickering.. at least not from my perspective. I rather enjoy nit picking out the finer details of conversations like these as it is entertaining and frequently educational. The trick is not to hold a difference of opinion against anyone and/or let it get personal in any way. If one can master that, the debates are a very good thing in almost all respects.

  9. #24
    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by jwelectric View Post
    Yes I too find it funny that on a DIY web site we will have folks from different walks of life that knows more about electrical theory than someone who has paper hanging on the wall from different colleges, licenses, certificates, and is even hired by a college to instruct electrical theory.
    That's impressive! Congrats on all your dedication and hard work. It takes real focus to achieve a career with that prestige.
    I don't have all that paper.. I'm just a lowly engineer from Northern Michigan University Electrical Engineering program who spends most of his time designing specialized processing machinery for the automotive industry rather than any real engineering.


    I also find it sad that these fine folks are too dense to learn.
    short′ cir′cuit
    n.
    an abnormal condition of relatively low resistance between two points of differing potential in a circuit, resulting in a flow of excess current.

    With a circuit of a 120 volt source, a 100 watt light bulb, and a switch, if the circuit is not shorted the light will never come on.
    Now let’s take a moment and think a little. Let’s take a piece of wire and connect it to the two terminals of the light bulb. With the switch in the off (open) position does anything happen? Turn on (close) the switch and describe the circuit.
    If you say the circuit is shorted then the closing of the switch is what caused the short plain and simple.
    The dictionary definition you provided and the example you provided contradict each other. The key word is ABNORMAL.
    Throwing a switch is not an abnormal function.

    In your example, the switch doesn't create the short, the abnormal wire connecting the two terminals of the light bulb is what created the short. Closing the switch just energized the already shorted circuit.

    Just because a circuit is not currently energized does not mean its not yet shorted via an abnormal condition.

    Also, as the dictionary definition you provided is mostly correct, consider the following:
    If two different conductors from two different circuit breakers originating from the same phase leg come into contact with each other (lets say via a break in the insulation of both wires running next to each other), what condition would you call that? In such a situation, that condition may go unnoticed for a very long time until special circumstances occurred between the two circuits.

  10. #25
    Jack of all trades DonL's Avatar
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    Quote Originally Posted by Murphy625 View Post
    If two different conductors from two different circuit breakers originating from the same phase leg come into contact with each other (lets say via a break in the insulation of both wires running next to each other)what condition would you call that? In such a situation, that condition may go unnoticed for a very long time until special circumstances occurred between the two circuits.

    I would call that cheap ass wire.
    Theory only works perfect in a vacuum.

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  11. #26
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by Murphy625 View Post
    That's impressive! I'm just a lowly engineer from Northern Michigan University Electrical Engineering program .
    This is good as we both know what a simple circuit is, a source, a path, and a load. We both now understand that in this simple circuit the load is a short of the source or if you would rather the difference of potential.

    What we have come accustom to calling any circuit that doesn’t work a shorted circuit when in fact a shorted circuit is what is needed in order for current to flow. We have become accustom to calling a faulted circuit a short circuit when the fact is the circuit is faulted.

    In the scenario you posted it would be a parallel circuit that is protected by one or the other overcurrent device. There would be no danger except the fact it would take both devices to turn off either circuit.

    When teaching the NEC the word “fault” is used to define both a fault to ground and a fault between two opposing legs of a system.

  12. #27
    Jack of all trades DonL's Avatar
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    It seems that Many People think that if anything using electricity does not work, "It must have a Short".

    Most of the time it is because it has a Long.
    Theory only works perfect in a vacuum.

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  13. #28
    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by jwelectric View Post
    This is good as we both know what a simple circuit is, a source, a path, and a load. We both now understand that in this simple circuit the load is a short of the source or if you would rather the difference of potential.
    Did you just call the load an abnormal condition per the definition you posted?


    In the scenario you posted it would be a parallel circuit that is protected by one or the other overcurrent device. There would be no danger except the fact it would take both devices to turn off either circuit.
    Two circuit breakers feeding a single conductor will create a situation that could involve the fire department, the insurance company, the hospital, and possibly the morgue.

  14. #29
    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by Murphy625 View Post
    Did you just call the load an abnormal condition per the definition you posted?
    Yes it could be abnormal should it start to fail and if it burned into a low resistance load it would still be the load none the less.
    Should the source be shorted with nothing but a conductor then the conductor becomes the load one of the three fundamentals of a simple circuit, source, load, and a complete path.

    A simple battery with nothing attached does not conduct current. Short the positive to negative and current will flow. The amount of resistance in this short will determine how much current flows, Ohm’s Law hard at work.

    The path cannot be shorted but maybe shortened, the load cannot be shorted, but maybe lowered, but alas the source can be shorted as it is the only place the energy comes from.



    Quote Originally Posted by Murphy625 View Post
    Two circuit breakers feeding a single conductor will create a situation that could involve the fire department, the insurance company, the hospital, and possibly the morgue.
    If both breakers are the same size and the conductor is properly sized then the only problem is there are two breakers to trip instead on one but they both open under the same load.

    No fire department needed nor is there any need to get in touch with the morgue or the insurance or even the hospital.

    Are you thinking that somehow the two breakers would add up to a higher current or something like that?

    The term as used improperly does not constitute something else.

  15. #30
    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by jwelectric View Post
    Are you thinking that somehow the two breakers would add up to a higher current or something like that?
    You think that two breakers feeding one line wouldn't???

    15 amp breaker + 15 amp breaker feeding 14ga wire = big trouble... (think toaster element)

    Think parallel circuit.

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