[RE-wrenches] CBI circuit breakers

[Exeltech]

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