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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