Once upon a time, in the nuclear submarine navy, my shipmate tags out the ship's battery breaker. As part of the tagout, he checks that the ship's DC bus is deenergized - within the battery breaker box itself. During the check, he finds some voltage, which isn't entirely unexpected, since there is enough capacitance on the DC bus to hold it charged for a while. So he uses a specialized grounding probe - about 1/8" diameter copper rod with a reinforced handle - to discharge it. Unfortunately, he has misunderstood what was inside the breaker box and is actually shorting out the ship's main battery! This is a couple hundred volts DC, and a few thousand amp-hours of capacity, and it delivers enough current to instantly vaporized the grounding rod. He's knocked on his ass by the arc blast. Fortunately, he was correctly considering the box to be energized gear and was wearing a full set of PPE, suffering only minor flash damage to his eyes.
Lucky guy, lucky too he used a relatively thin rod, a thicker one and it would have been quite a different ending.
I did a similarly stupid thing one day. While repairing a clock for a display on a trade show I hooked the ground crocodile clamp of my scope to what I thought was the ground but which in fact was V- of the DC supply voltage of the neon light installation that was use to drive the display. I had a blind spot in the middle of my eyes for many days after that (and took out a fairly large chunk of the mains supply for the Eastern district of Amsterdam). The crocodile clamp totally disappeared, the scope, miraculously survived and wasn't even out of spec.
Every time I use a crocodile clamp to hook it on to something I think back to that moment when the clamp makes contact and some part of me expects it to happen again.
> I hooked the ground crocodile clamp of my scope to what I thought was the ground but which in fact was V- of the DC supply voltage of the neon light installation that was use to drive the display.
Yeao! I own a couple of high-voltage differential probes for this very purpose. Even though most single-ended scope probes are rated to 300V Cat II, I won't go anywhere near main's power with them.
