120kV OCB Contact resistance

[Zogzog]

Anyone have some expected ranges of contact resisnces for a 1200A 120kV OCB? I am seeing some lower than expected values, verified with 2 different microhmeters. Breakers were cycled 10 times before tests.

 

[oldfieldguy]

Lower values? How much lower? Did you try reversing the leads on your microohmmeter? Don't ask me why? but I've seen a difference a time or two.

OCB's can do some strange things with the low voltages used in microohmmeters.

old field guy

 

[Zogzog]

5 microhms. Did not reverse the leads but tried 2 different recently calibrated test sets. I would expect 30 or so microhms, seemed low to me. I am more of an air and vacuum guy, OCB's are not too common for me.

We cycled the breaker 10 times before the test so that could be a factor, still seems a little low to what I have seen before. All other tests, PF, Time travel, etc... were all good. This is pretty new for an oil breaker 1985 vintage.

 

[Gianoli]

Those values do seem extremely low. We maintain many types of transmission system breakers and even the 4000 Amp breakers had higher contact resistances than the numbers you recorded.

The closest match we have in our system would be General Electric type KSO breakers. They are 1200 Amp, 138 kV, bulk oil circuit breakers with three phases in one tank, two breaks per phase. I checked a number of test records for these breakers and typical contact resistances measured bushing to bushing with a 100 AMP instrument were 450-550 microhms.

A typical 4000 Amp, 345 kV, air blast breaker had contact resistances in the range of 100-175 microhms bushing to bushing. Not sure what to think about your readings?