Nec 240.15(b)

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erickench

erickench

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This section of the NEC describes the use of handle ties for circuit breakers. My question is this: If there is a fault or overload on a multiwire circuit, will two or more single pole circuit breakers with handle ties be able to automatically trip simultaneously or are the handle ties just there to allow for manual common switching only.
 
erickench said:
This section of the NEC describes the use of handle ties for circuit breakers. My question is this: If there is a fault or overload on a multiwire circuit, will two or more single pole circuit breakers with handle ties be able to automatically trip simultaneously or are the handle ties just there to allow for manual common switching only.
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I believe the handle ties will not trip both poles on a fault.
 
They may or may not all trip. My guess is that they would but there's no guarantee.
 
erickench said:
This section of the NEC describes the use of handle ties for circuit breakers. My question is this: If there is a fault or overload on a multiwire circuit, will two or more single pole circuit breakers with handle ties be able to automatically trip simultaneously or are the handle ties just there to allow for manual common switching only.
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Depending on the breaker - often multi pole breakers are two or more single poles with the actual trip mechanisim in side tied together... You can see this with breakers that have a shunt trip accessory available - usually a little lever that sticks inside, sometimes a plastic cam... Two breakers with just a handle tie field installed won't trip all poles - just try to shut the other off with the handle-tie when the faulted one trips...
 
According to Mike Holts book 'Understanding the NEC' single pole circuit breakers with handle ties will not trip automatically. An old NEC handbook from the 1990's say's that the handle ties provide common switching and that they do not cause the circuit breaker to serve as a common trip mechanism. I just asked you guys for clarification on this.
 
They may or may not all trip. My guess is that they would but there's no guarantee.
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I have to agree with infinity here.

To elaborate a bit more, from my understanding the handle tie does not function as a common trip and it's purpose is to allow common operation of the individual breakers as the disconnet means of the multibranch circuit. I believe the thought process behind this protection scheme is that since multibranch circuits can only serve line-to-neutral loads, the individual circuit breakers are mean to protect a single phase fault. Even with quality system analysis software and solid information about the load, I think it would be very hard to predict if all of the breakers would trip. This is, however, irrelevant since the breaker that is protecting the line-to-neutral circuit which is experiencing the fault will definitly trip and the purpose of the device is served. Finally I think that since this kind of multibranch circuit serves only smaller, single-phase loads it's probably not a big deal if the other breakers trip as opposed to a large commercial or industrial situation where it is important to coordinate the protection to minimize downtime.
 
Let's say you have two or more single pole circuit breakers with handle ties. If one of the circuit breakers trips would'nt the handle tie force the other CBs to trip as well? Or would none of the CBs trip?
 
erickench said:
Let's say you have two or more single pole circuit breakers with handle ties. If one of the circuit breakers trips would'nt the handle tie force the other CBs to trip as well?
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Not always - as mentioned (or I tried to) the trip mechanism is more sentitive to this type of trip action - where they are much less so with the handle.
 
Well it seem's that the NEC permits circuit breakers with handle ties to be used for protection. It's just the way it's worded in Mike Holt's book.:cool:
 
erickench said:
Well it seem's that the NEC permits circuit breakers with handle ties to be used for protection. It's just the way it's worded in Mike Holt's book.:cool:
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Yes, the code permits the use of single pole breakers with handle ties to protect circuits. They can be used for multiwire branch circuits that serve only single phase line to neutral loads, single phase circuits with line to line loads and 3 phase circuits with line to line loads. Note that they can't be used for circuits that have both line to line and line to neutral loads...example an electric clothes dryer. 240.15(B)
 
erickench said:
Let's say you have two or more single pole circuit breakers with handle ties. If one of the circuit breakers trips wouldn't the handle tie force the other CBs to trip as well? Or would none of the CBs trip?
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As the others have indicated it might, or it might not. The reason it might not is because breakers are required to be of 'trip free' design. 240.80 This means that a breaker can trip even if the handle is physically held in the closed position.

That being the case, the force the tripping breaker can exert on the other breaker(s) may not be enough to open them.
 
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...or are the handle ties just there to allow for manual common switching only

...or are the handle ties just there to allow for manual common switching only

It was my understanding from the NFPA ROP's that the intent of required use of "Handle ties" for multiwire branch circuits with provisions for "Grouping" in the panel board was for those less informed persons changing lamps and devices. It appears that they were not trained in the FACT the neutral will have a potentioal to ground when the 'A circuit' was off to be worked on, and the 'B circuit' was still energized and in use?

HTML: 
...Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at the point where the branch circuit originates
.

HTML: 
..each multiwire branch circuit shall be grouped by wire ties or similar means..
 
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