I just experienced a big bang caused apparently by a short through my oscilloscope's ground lead, and wanted to see if you could shed light on the why this happened.
Yes I have watched Dave's video on the subject of scope ground leads, and was trying to be very careful. But obviously not careful enough. :-/
I was diagnosing a washing machine control board. I wanted to view the tachometer output pin's status while in operation to see if it was working properly.
The tachometer (optical sensor) is powered by 13VDC, provided by an on-board power supply.
I connected the oscilloscope ground clip to the 13VDC ground. This is indicated in the schematic below. I connected the oscilloscope lead to the tachometer input pin. I did not disconnect the connector from the control board, but instead piggybacked the scope probe and ground clip into the back of the connector.
When I plugged in the machine's 120VAC power cord, BANG went the washing machine's on-board power supply. The bang was definitely more powerful than I would expect from 13VDC, and let the magic smoke out of several SMD components around the PSU. The oscilloscope was not damaged.
I simply cannot figure out why this connection created a short circuit. Even if the 13VDC ground was isolated from the chassis ground, why would it be at a high potential difference from chassis ground? That would seem dangerous.
I tested the electrical outlet to ensure it's wired correctly.
Thanks for shedding any light on this.
Schematic: (full manual here)