[RE-wrenches] Independent vs common trip breakers

[William Miller]

August:

It's all about safety:  If one has a 230V circuit and you are incorrectly 
using non-handle-tied breakers, when an overcurrent event occurs on only 
one phase, only one breaker trips.  The circuit stops working but the 
appliance is still energized with the other phase of AC.  A repair person 
may inadvertently assume the circuit is off based on the non-operational 
status of the appliance and, during repair, come in contact with conductors 
or equipment energized by the breaker that is still on.

In a correct installation (handle-tied) an overcurrent on one phase 
de-energizes both phases.

A similar scenario applies in situations with a "multi-wire branch 
circuit," aka a shared neutral.  It is less likely that a dangerous 
situation will occur because one circuit will still work.  Opening the 
neutral can result in over-voltaging one circuit (been there, bought that 
replacement cash register).  I am not a house electrician, but I think 
dishwasher / garbage disposal multi-wire circuits require handle 
ties.  Anyone else want to clarify this point?

This is a subtle but important point.  This is an example of knowledge that 
a non-licensed or under-experienced electrician may not posses, hence the 
desirability of requiring meaningful licensure.

Sincerely,

William Miller



At 03:06 PM 9/25/2012, you wrote:
>Hi Wrenches,
>
>Do any of you know the reason why we might use an independent trip versus 
>a common trip two pole breaker for a solar inverter? My understanding is 
>that the independent trip will only trip the individual line effected by 
>the overcurrent condition compared to the common trip which will trip both 
>poles. Both are tied together by a handle tie and both lines must be 
>flipped on at the same time. Cutler hammer is a good example of a company 
>who makes both types.
>
>It seems like supply houses sell both types willy nilly.
>
>Best,
>
>August
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Miller Solar
Voice :805-438-5600
email: william at millersolar.com
http://millersolar.com
License No. C-10-773985