Dbl Pole Ganged Breaker Q

[charlie b]

why do you think shared neutral makes it different?

I don't. That is why I said this:

My inclination is to suspect that the breaker will behave in exactly the same manner in all cases.

 

[FionaZuppa]

I don't. That is why I said this:

but you mention trip characteristics based on shared neutral or not, i was looking at it from balanced/unbalanced load perspective, etc.

in reality, with a common trip dbl-pole, does each pole trip at exactly the same time during balanced overload??? probably not.

 

[GoldDigger]

I don't. That is why I said this:

But for the non-fault disconnect use, trip curves are unimportant but there is a lower level of expertise and caution required when work on just one part of the MWBC with common neutral.
Whether this protection for incompetent electricians merits enshrinement in the NEC will continue to be challenged here.

 

[iwire]

My mind boggles. It could be my natural state or the meds I am on.

Here is how I see it.

Eaton, and all the others that make breakers of that style do not make anymore different trip curves than they have to.

Furthermore I submit that they simply take two single pole units, add a common trip rod and rivet them together to make a two pole or three pole breaker.

To me it is inconceivable that they would use different trip curves in the same multi-pole breaker. It makes no sense to, it would likely be outside the listing, and if there was a reason for such a breaker it would be advertised as something special and with a price to match.

 

[ActionDave]

The code says no such thing! :happyno:

OK, everyone take a few moments to let the shock value of that assertion pass over you.

What the code says is that there must be a means for simultaneously disconnecting all ungrounded conductors at the point at which the circuit originates. We have made the practical choice of achieving this disconnection by one of the two methods you list. But the code does not tell us we must use one of them, nor does it forbid us from finding another method.

From a purely technical perspective, I would want to know what changes in the breaker's trip characteristics when we share a neutral as opposed to providing separate neutrals. Consider, for example, a MWBC that serves two loads, one of which is turned off at the present moment. The other is running, and the shared neutral carries all the current of that load's ungrounded conductor. This is exactly the same situation you get with two completely different circuits, each with its own neutral, each serving one load, and with one of those loads not running at the present moment. Now cause either an overload or a short circuit condition on the one load that is running. How does the breaker behave? How would the breaker behave differently if the second load were running in the MWBC configuration? How would the breaker behave differently if the second load were running in the separate circuit configuration?

My inclination is to suspect that the breaker will behave in exactly the same manner in all cases. But I have no facts to offer. This is the way I would want the issue presented to Eaton.

Charlie, this is fantastic, well reasoned, well thought out, and completely logical post..... the only problem is it does not address the reality in some people's minds that MWBC's are the prime source of all that is wrong or can go wrong in the realm of wiring in the Western Hemisphere.

Voodoo is held in higher esteem than MWBC's.

Edison had his mind fixed on world destruction when he completed his circuit.

 

[GoldDigger]

No, just fixed on world commercial domination because only his employees or licensees could be counted on to use it safely. :angel:

 

[FionaZuppa]

To me it is inconceivable that they would use different trip curves in the same multi-pole breaker. It makes no sense to, it would likely be outside the listing, and if there was a reason for such a breaker it would be advertised as something special and with a price to match.

thats not what i was saying, or what Eaton was saying.

a dbl-pole ganged common trip breaker has its trip profile measured under symmetric loading. ok, so the breaker internals do their thing together, they heat together, etc etc, and finally produce a trip in unison (or very close to it). now, load the same breaker in unbalanced fashion, the internals no longer heat the same nor act together, etc. do you get the same exact trip profile between a balanced trip vs unbalanced trip? i suspect under careful lab viewing you dont get same exact trip profile. does it matter in industry use? dunno.