Internal trip mechanism required?

[LarryFine]

It's confusing to me, too. I always understood 240.15(B) and (B)(1) to be the entire rule. Now, I'm less sure.

It appears that standard 1p breakers are slash rated.

 

[Another C10]

you guys crack me up .. I mean that in a good way.

My question then would be, so what really is the point of having a breaker tye, if its simply to shut off a second single pole breaker , how would that be cost effective marketing incentive. Is there really a big demand to disable to independent circuit that aren't serving a common load.

 

[Another C10]

One last note of constructive criticism, If internal mechanism are so important why is it that many times I go to a main breaker trip were only 1 phase was disengaged. Seem to me even a internal and external method of disabling 2 poles is not always effective as designed.

 

[GoldDigger]

I guess they DO consider the handle tie adequate to open both/all poles upon one tripping.

No, actually they cannot consider the handle tie adequate to open both poles when only one trips. If nothing else, the trip free feature guarantees that one pole can trip without exerting sufficient force on the other handle, no matter how good the coupling through the handle tie.
The issue is whether or not a breaker is required to simultaneously open both poles on trip when serving only a line to line load. It is not required when serving an three wire single phase MWBC. That case only requires that a manual opening affect all poles.

 

[GoldDigger]

One last note of constructive criticism, If internal mechanism are so important why is it that many times I go to a main breaker trip were only 1 phase was disengaged. Seem to me even a internal and external method of disabling 2 poles is not always effective as designed.

Is the main required to have an internal common trip? (The shorthand of "internal trip" for "internal common trip" is potentially confusing.)
There are also occasions where a defective main will not open all poles even when manually actuated and all levers move.

 

[GoldDigger]

you guys crack me up .. I mean that in a good way.

My question then would be, so what really is the point of having a breaker tye, if its simply to shut off a second single pole breaker , how would that be cost effective marketing incentive. Is there really a big demand to disable to independent circuit that aren't serving a common load.

The point of a breaker tie is that when opening an MWBC with a common neutral or simply disconnecting a line to line load for service it is critical for safety to open all poles. Dislike for handle ties on MWBCs is based on the hope that a competent electrician will realize that opening one pole will allow them to repair a faulted line to neutral circuit only if they are careful not to disconnect a common neutral in the process. For just replacing lamps in a line to neutral luminaire, turning off just a single pole is just fine.

 

[wwhitney]

It is not required when serving an three wire single phase MWBC.

. . . that supplies only L-N loads. If there is a mix of L-L and L-N loads on the MWBC, then common trip is required.

I'm curious about 240.15(B)(3), in the context of a 3-phase system does the phrase "line-to-line loads" imply single phase 2-wire loads, or does it also include 3-wire delta connected loads?

Cheers, Wayne

 

[infinity]

It is not required when serving an three wire single phase MWBC.

. . . that supplies only L-N loads. If there is a mix of L-L and L-N loads on the MWBC, then common trip is required.

Cheers, Wayne

I believe that is if it's supplying one piece of equipment like a clothes dryer. Is there a code section that says otherwise?

 

[wwhitney]

I believe that is if it's supplying one piece of equipment like a clothes dryer. Is there a code section that says otherwise?

(2014) 210.15(B)(1), one of the sections under discussion, uses the language "multiwire branch circuits that serve only single-phase line-to-neutral loads"

Cheers, Wayne

 

[synchro]

The issue is whether or not a breaker is required to simultaneously open both poles on trip when serving only a line to line load. It is not required when serving an three wire single phase MWBC. That case only requires that a manual opening affect all poles.

. . . that supplies only L-N loads. If there is a mix of L-L and L-N loads on the MWBC, then common trip is required.

I think a reason common trip is required for a mix of L-L and L-N loads on a MWBC is that current could still flow between L-L and L-N loads that are connected to the same phase if the line supplying that phase is opened by a breaker but the other line(s) of the L-L loads are still hot.

I'm curious about 240.15(B)(3), in the context of a 3-phase system does the phrase "line-to-line loads" imply single phase 2-wire loads, or does it also include 3-wire delta connected loads?

Good question. It could also be asked whether "open delta" loads between two pairs of phases would be included in 240.15(B)(3).
To consider this let's take an example where there's a 12Ω load between lines LA and LB, and another 12Ω between LB and LC in a 120/208 3-phase system. Then the current through LA and LC will be 208V / 12Ω =17.3A. The current through LB will be 1.732 x 17.3A = 30.0A. Now if there was an individual 20A breaker on each phase, the breaker for LB would blow from the overload but those on LA and LC would not. Then the two 12 loads would now be in series across LA and LC with 208V applied, and so they would still be drawing current (half of their previous current or 17.3 /2 = 8.65A).

I think this simple example shows that an individual single-pole breaker feeding L-L loads connected to more than one other phase would not fully interrupt the current through those loads. So in that case I think a common trip would be needed for the same reason mentioned above for not allowing individual breakers with a mix of L-L and L-N loads on a MWBC. I think 240.15(B)(3) should be more explicit about this if that was the intent.

 

[hillbilly1]

What is a parallel CB?

You have probably seen mains like some GE single phase panels where the use two 100 amp breakers to make a 200 amp main. One common handle tie across both breakers.

https://www.lowes.com/pd/GE-1-in-Main-Breaker-Kit/50253107?cm_mmc=shp-_-c-_-prd-_-elc-_-google-_-pla-_-206-_-switchgear-_-50253107-_-0&placeholder=null&&ds_a_cid=114168065&ds_a_cid=476315417&gclid=EAIaIQobChMI3uOOyO2i6gIVIQiICR28CQKYEAQYBCABEgLwOvD_BwE&gclsrc=aw.ds

 

[wwhitney]

I think a reason common trip is required for a mix of L-L and L-N loads on a MWBC is that current could still flow between L-L and L-N loads that are connected to the same phase if the line supplying that phase is opened by a breaker but the other line(s) of the L-L loads are still hot.

Thanks for the explanation. I see that if L1 trips but L2 does not, the L1-N loads are still energized in series with the L1-L2 loads from the L2-N potential. That's similar to your 3 phase open delta example.

Cheers, Wayne

 

[Little Bill]

It's confusing to me, too. I always understood 240.15(B) and (B)(1) to be the entire rule. Now, I'm less sure.

It appears that standard 1p breakers are slash rated.

I too always thought that (was told that and thought I read it) L-L loads needed internal trip, & MWBC could use the handle ties if no L-L loads. (B)(2) has been hiding from me and apparently some inspectors!