Electronic – Oscilloscope ground lead – BANG!

groundoscilloscopeshort-circuit

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)
enter image description here

Best Answer

Note that the 'ground' symbol on that 13V rail is NOT the same as the main safety ground symbol by the power cord.

Look at the whole wiring diagram and notice the two triacs a little lower down, if you are designing this thing you can save a couple of opto triacs by referencing the 13V line to the incoming live....

Get paranoid, a multimeter check would have saved you some SMPSU repair work.