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Nuisance tripping of circuit breakers is a common problem in many commercial and industrial installations. This Application Note explains the need to use ‘True RMS’ measurement instruments when troubleshooting and analyzing the performance of a power system.
Nuisance tripping is often caused by the load current being distorted by the presence of harmonic currents drawn by non-linear loads. Harmonic currents distort the current waveform and increase the load current required to deliver energy to the load. Many measurement instruments, even quite modern ones, use an averaging measurement technique that does not measure harmonic currents correctly.
‘True RMS’ meters take the complete distorted waveform into account. If averaging meters are used to measure distorted current, the readings may be as much as 40% too low. Circuit breakers and cable sizes may be underrated as a result.