Contact resistance tester doesn't display any reading

[lnides]

Hi, I just wanted to know how can I get the contact Resistance of Square D Circuit Breaker? When I test it the contact resistance tester didn't display any reading.

[SecondGen]

Could you provide more information, like the instrument model and troubleshooting steps? I'd hate to state the obvious but was the breaker closed and did you try different measurement scales? Try measuring a small piece of bus or cable to check the instrument.

[lnides]

square d type QOB 40 ampere 10kA 120/240V...Sometimes it gives a very low contact resistance like..0.00001milliohms,not stable..sometimes 19 milliohm..thank you..

[SecondGen]

square d type QOB 40 ampere 10kA 120/240V...Sometimes it gives a very low contact resistance like..0.00001milliohms,not stable..sometimes 19 milliohm..thank you..

For a breaker of that size I would say 19 milliohms is probably the true reading. You might not be getting a stable reading because you are not making good contact with the test leads. All four test probes need to be making good contact with the (P) probes facing eachother.

[lnides]

Ah ok, Sir I Tested one this morning,,it has no continuity and contact resistance,but when I Injected it with 230V, It has an output of 230V..Is it ok?I mean is the Circuit Breaker ok for operation?

[SecondGen]

I can't say whether the breaker is "ok for operation" because there is not enough information. It sounds like if you measure 19 milliohms on each pole the contacts are OK but you will have to inject 300% or 600% of rated current on each pole individually (40 * 3 = 120A / 40 * 6 = 240A) first and see if the breaker trips. Also do insulation resistance at 500V on each pole and make sure you get at least 25 megohms to ground.

[slts1991]

Charge your DLRO fully, check the connections on the test set, verify leads continuity (maybe even try with different leads) and try again.

[lnides]

Thanks Sir.

[robertjgraham22]

Check your leads I have had this happen, and it was because the leads had gone bad, swap yours with another tech and use thiers on your ductor , this may be your issue.

[desilvajp]

Eaton doesn't recognize contact resistance on such small breakers because it is unreliable. Yes I know this is a SqD but the designs are so similar.
Try doing a millivolt drop test when you inject rated current.
If this is a used breaker, open and close the breaker 5-10 times while dead. This smashes any high spots on the contacts to give a better reading. Under load, the high spots melt until the current is more evenly spread.

[Kalbi_Rob]

Eaton doesn't recognize contact resistance on such small breakers because it is unreliable. Yes I know this is a SqD but the designs are so similar.
Try doing a millivolt drop test when you inject rated current.
If this is a used breaker, open and close the breaker 5-10 times while dead. This smashes any high spots on the contacts to give a better reading. Under load, the high spots melt until the current is more evenly spread.

Millivolt drop test is only performed with rated DC current, as discussed in NEMA AB4. Performing millivolt drop test while performing AC testing with load machine is not considered accurate by IEEE, NEMA, or NETA and many white papers have been produced to discuss this subject. I learned this when I changed companies and my old company would just perform it while performing load test and I was proven very wrong at the new company.

NEMA AB4 Excerpt:

6.4.2 Equipment
6.4.2.1 This test should be conducted using a 24 volt, or less, direct current power supply capable of supplying the rated current of the circuit breaker. For circuit breakers rated higher than 500 amperes, the power supply should be capable of delivering no less than 500 amperes.
6.4.2.2 If the above equipment is not available for field tests, a Digital Low Resistance Ohmmeter (DLRO), or 4-point tester, capable of 10 to 100 amperes (DC), may be used.
NOTE—Use of a multimeter or low current ohmmeter in place of the power supply will not provide an accurate or reliable measurement of millivolt drop and should not be used.

[desilvajp]

Millivolt drop test is only performed with rated DC current, as discussed in NEMA AB4. Performing millivolt drop test while performing AC testing with load machine is not considered accurate by IEEE, NEMA, or NETA and many white papers have been produced to discuss this subject. I learned this when I changed companies and my old company would just perform it while performing load test and I was proven very wrong at the new company.

NEMA AB4 Excerpt:

6.4.2 Equipment
6.4.2.1 This test should be conducted using a 24 volt, or less, direct current power supply capable of supplying the rated current of the circuit breaker. For circuit breakers rated higher than 500 amperes, the power supply should be capable of delivering no less than 500 amperes.
6.4.2.2 If the above equipment is not available for field tests, a Digital Low Resistance Ohmmeter (DLRO), or 4-point tester, capable of 10 to 100 amperes (DC), may be used.
NOTE—Use of a multimeter or low current ohmmeter in place of the power supply will not provide an accurate or reliable measurement of millivolt drop and should not be used.

Good to know, I too accepted some verbal data instead of reading the up to date spec when I changed companies. Now you have me wondering if the High Current test sets put out AC or DC. Hmmm. I'll have to check it.
Thanks,
John

[ShinAkuma]

Good to know, I too accepted some verbal data instead of reading the up to date spec when I changed companies. Now you have me wondering if the High Current test sets put out AC or DC. Hmmm. I'll have to check it.
Thanks,
John

Its AC Wave form! It wouldn't make much sense to test LVCB with High current test sets that are rated for AC currents with a DC source, specially when we trying to look at time curves.