Say, for example, while conducting online testing, a low current is measured on one phase and high current is detected on the remaining phases. A quick assessment of collected data may lead a technician to believe that a connection problem exists in the circuit on the phase with the low current reading.

Subsequent troubleshooting and analysis, though, does not reveal any other problems. As testing continues a pattern develops with low current readings on all motor circuits tested that share a common bus. Now it appears that a bus problem exists.

If this sounds familiar, a defective probe can produce similar results. Certainly if the probe utilizes batteries they may be weak or something may be preventing the jaws on the probe from fully closing. This can be easily prevented or diagnosed by connecting each of the amp probes on a single phase and running a test to verify all phase currents are equal and/or within the accuracy tolerance of the probes. This test should be conducted each day before testing any motors. It should also be performed after identifying a current unbalance to confirm that there is, in fact, a current unbalance and not a problem with one of the amp probes.