inconsistent inrushmeasurment depending on "power plug"

Question

I'm measuring inrush current with a current probe ( tektronix P6303) that is connected with an amplifier between, which is powered by an AC line before it's connected to an oscilloscope.

I got some struggle with my measurement so I changed oscilloscope to another oscilloscope of the same model, suddenly I realized that despite the setup is the same it gives totally different result in amplitude and shape.

So I changed the power cord between the oscilloscopes with the result that they displayed the same result as the other part did before the change.

After this I rotated the contacts in the AC line with the result that it for one oscilloscope resulted in half the amplitude still same shape and for the other one another totally new shape.

There is only one possible earth loop and it's between the current amplifier and the oscilloscope not against the DUT. I can't really see why it in this case should matter that much.

Have anyone else experienced this before and how did you proceed to get a "true" measurement?

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::Extra

I inserted a isolation transformer to the osilloscope, in this config it seem to give a consistent result, even if I change the power cords rotation and oscilloscope. Maybe I've found a solution.

Current inrush Measurement with isolation transformer

White = with min voltage

Yellow = with max voltage

Answer 1

You obviously have reproducibility issues. Inrush occurs when large capacitors are charged up. After you pull the plug, it may take a surprisingly long time for the caps to discharge. It could be 30 seconds or even more depending on the design of the supply. So if you don't wait long enough, you may get a much smaller inrush waveform.

Another thing that can confuse you when looking at inrush is triggering. There may be more than one inrush pulse. For example, maybe there is an initial large pulse, followed by a smaller pulse with a different shape. If the oscilloscope re-triggers on the second pulse, that is all you will see on the scope.

Finally, when using an external amplifier, you introduce yet another variable which is amplifier setup. It may be a good idea to validate your test setup In order to get a correct result, the amplifier and oscilloscope probe scale have to be set up correctly. It may be a good idea to measure a known current before trying to measure the unknown inrush, just to validate the setup.

Also, current probe amplifiers can clip if the maximum current is exceeded. So if you have a sort of flat-topped pulse, and that is not what you expect, try turning the probe amplifier to a less sensitive setting (don't forget to also change the scale on the oscilloscope).