In high-voltage testing, common-mode voltage is a common but not negligible risk. It refers to the voltage that appears between the positive and negative input terminals of the differential probe and the ground at the same time, with the same amplitude and phase. If its value exceeds the rated range of the probe, it will not only cause measurement distortion, but also may damage the equipment and even endanger the safety of personnel.

Common-mode voltage mainly brings 3 risks: the 1 is to cause measurement errors, excessive voltage will exceed the common-mode rejection capability of the probe, resulting in waveform distortion; 2, it may break down the internal insulation of the probe and cause permanent damage. The most serious thing is that in the floating ground high-voltage system, electric shock hazard may be formed through the ground circuit.

In order to ensure safe measurement, the probe with common mode voltage rating and isolation withstand voltage rating meeting the requirements should be preferred, and its common mode rejection ratio (CMRR) performance should be paid attention. During operation, it is necessary to reliably connect the grounding wire, shorten the grounding length as much as possible, and avoid touching the metal part. It is also important to calibrate the probe regularly and check the common mode rejection capability.

In short, a correct understanding of common-mode voltage risk, selection of appropriate probes and standardized operation are the basis for ensuring the safety and accuracy of high-voltage differential measurement.