120710-1324 EDT
don:
I do not know, but switching power supplies are a possibility. Also maybe my fluorescent lamp noise filters.
The Fluke 27 is fairly good up to about 30 kHz. A Simpson 260 is much better, maybe to 500 kHz. When you get to the higher frequencies the impedance is much higher than at 60 Hz. Data flow to a printer by a parallel port might be in the 100 kHz range, and a USB is probably at least 1 MHz.
Use of an Ethernet connection to a printer is better because it provides DC isolation.
When we discuss power line shorts to the EGC then it is 60 Hz, but if it originates from lightning, we are in the MHz range.
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