Wrenches,
An assembly of thoughts:
Circuit breakers operate on a bi-metallic temperature-related mechanical
action. They are rated to hold a specific current at a specific ambient
temperature. If the ambient temperature exceeds the breaker specification for
a given current, the breaker will open at a lower current than expected.
If breakers mounted on one or both sides of a suspect breaker are themselves
running warm, this can raise the effective ambient temperature for that
adjacent breaker and cause it to open at a current below the trip rating,
especially if it's near its max current spec.
Though crimp connections to stranded wire may be tight in of themselves, how
about the connection between the cable lug and breaker? Breaker hardware has
torque specs that must be observed. If it's not torqued properly, it will run
warm at higher current, and transfer some of that heat to the internal
mechanisms of the circuit breaker, which may cause an early trip.
As a test, use a good DVM set to a millivolt DC scale. With the maximum
current flowing in the circuit, probe across each connection. Check wire to
lug; lug to breaker stud, and even across the breaker itself. You should
measure low millivolts at any connection or location - the lower the better.
Several tenths of a volt drop across any two points with maximum current
flowing in the circuit indicates a bad connection.
If you measure what you feel is an excess voltage drop across a circuit
breaker, and the circuit current isn't excessive, try replacing the breaker
with a new one (preferably from a different batch or mfgr, but of an identical
trip rating) and re-measure the voltage drop across the new breaker. If both
breakers have a substantially similar drop for the same current, it's likely
the first breaker was ok.
Most of you also know the breakers used for PV-side protection must have a DC
voltage rating greater than the VOC rating of all the PV in that circuit. If a
breaker is being used in a circuit that provides more voltage than the breaker
ratings, repeated opening of the breaker will cause damage to the internal
breaker contacts, leading to a premature trip condition.
Once a breaker trips .. it takes fractionally less current the next time it
trips due to stress and wear of the connecting elements inside the breaker. If
a breaker trips often enough, or if the circuit voltage is near (or above) the
breaker rating when it trips .. it damages the breaker contacts each time the
breaker is opened - both "trip" or manual open. When this happens, all bets
are off as to the current at which the breaker will open the next time.
Then again .. there's still a possibility the breakers are defective.
Hope this helps.
Dan
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