Got shocked yesterday - not quite sure how

[glebert]

So this is kind of strange. Got a zap yesterday and now can't figure out what happened so I am freaked that it will happen again.

I was working with a new to me project amp, a homebrew Champ(ish) build. Was sold to me as "powers on, no sound". Had two safety issues I should have addressed right away but didn't (I know, I know) 1.) Grounded prong on three wire power cord snapped off and 2.) Death cap from fuse to ground (who puts that on a new build? I guess some idiot following the schematic). Got the amp making sound by replacing V1. If I know an amp isn't grounded I'm usually super cautious about touching anything conductive.

Hooked it up to my pedal board and was playing through the Champ. Seven pedals on the board, and the 2nd one in (a vintage Maxon real tube overdrive) isn't passing sound when engaged, but fine in buffered bypass mode. Getting frustrated with that I go to turn on my regular amp and get a shock when I touch metal on that amp (I was presumably touching guitar strings with my other hand). Freaked me out, I powered down everything and did a quick assessment and I seemed fine. Disconnected the Champ and turned on the the other amp, Maxon overdrive still not working.

FF to today when I went back to diagnose what happened. I was expecting a shorted death cap and hot chassis, but it wasn't testing as shorted. Putting on a new 3 prong plug and powering on LBL and variac it wasn't making the light glow, amp worked. Reversing the hot and neutral wires at the plug to check that and it was the same. It didn't trip a GFI outlet in either polarity either. I measured the ground pin to chassis to make sure there was good contact.

Testing on the pedalboard I didn't see any voltage on guitar cable to the ground of my regular amp. Maxon pedal now working as before with my regular amp. Have not hooked Champ up to pedalboard again.

So I don't know if the Champ shocked me (though seven pedals) or if it was somehow the Maxon (which runs off a 9V supply) or if I somehow imagined the whole thing. It just seems strange.

I did go the ER afterward to get checked out, everything looked fine. I did have pain across my upper back like a muscle spasm. Still have some of that.

[pdf64]

Some SMPS will pass a few mA of leakage current to earth, with quite a voltage behind it.

[Helmholtz]

In case of questionable safety grounding I use a neon mains tester (called "phase tester" over here) before touching anything.
E.g.: https://www.amazon.de/-/en/710-031UK.../dp/B000K0J8WG

Can save lives.

A "death cap" doesn't need to be shorted to give you some shock via the chassis ground.

[Greg Robinson]

It was almost certainly this Champ which shocked you, death caps do not need to be short circuit to do so.

Presumably your regular amp is a class I appliance, and properly grounded. This means that when you touched its chassis, which is at "earth" potential, your body was between earth potential and whatever the potential the Champ was at.
Now you say that the cap is between "fuse" and chassis, you don't say whether the live or neutral/return is fused, or the value of the cap, but either way you can still receive a shock, just a matter of degree.
Assuming a typical value of 0.05µF from live to chassis, that still allows a "relatively safe" short-circuit current of ~2.26mA on 120V 60Hz, not enough to trip a GFI, but enough that the average person should feel it.

Even if the cap was to neutral, you might have a few volts between neutral and ground (you'll always have SOME potential between neutral and earth unless you have nothing powered by that circuit - and even with nothing, impedances are non-zero, so can still get "ghost" voltages). This reduces the short circuit current available, but sensitive individuals might still feel it with a sweaty hand across the strings.

You can measure this, set your meter for AC mA, and measure between the Champ chassis and an outlet earth.

I presume that even with the earth pin snapped off you observed correct plug polarization, but you also can't assume that sockets are wired correclty unless you've confirmed. I've checked every outlet in my house, and check outlets before I play or work out anywhere (takes a few minutes).

[The Dude]

......A "death cap" doesn't need to be shorted to give you some shock via the chassis ground.

Yes, that! Clip that thing out of there, pronto!

[g1]

When you went back to find out what happened, you say you had already replaced the 3 prong plug. That will have 'fixed' the issue so you would not have been able to reproduce it.
As others mentioned, good cap from line to chassis will still allow shock (if chassis not earthed). Presumably the cap was supposed to limit the shock to less than lethal levels, at least that is why they used to get away with doing it like that.

[glebert]

When you went back to find out what happened, you say you had already replaced the 3 prong plug. That will have 'fixed' the issue so you would not have been able to reproduce it.

I wasn't really trying to reproduce the scenario, I was expecting for the cap to be dumping a fair amount of current to ground which I would see on my LBL. I wasn't testing for "normal operation" or expecting normal operation to be "less than lethal but two day back spasm inducing shock." Man, life back in the 50's must have been quite exciting!

[The Dude]

I wasn't really trying to reproduce the scenario,......

Where's the fun in that?

[g1]

I think what was supposed to be 'less than lethal' turned out to be lethal on occasion, which is why we have much higher standards today.
Hope your back recovers soon.
In younger more stupid days, I zapped myself pretty good a couple times playing barefeet and putting my foot on the steam radiator, which of course got grounded through the water pipes. I didn't really feel like less than the full 120V to me either.

[Mick Bailey]

We have a nominal 240v over here - sometimes much higher. I've had a few shocks directly off the mains - the worst one was when I was a kid, from a radio that was standing on top of a washing machine. The aerial had come disconnected and I reached round the back to plug it in and I had a massive jolt which knocked me out. The next thing I remember was an ambulance driver and his colleague propping me up and asking my name. My parents didn't know anything about CPR - they just went round next door to use their phone (only two people in our street had a telephone back then) and rang the emergency services. I was dazed for a fair while I came round. I still have a radio of the same model and the chassis is wired directly to neutral - no earth. If the mains lead is connected the wrong way, the chassis, pot shafts and screws are live.

Another time was when we had an electric fire with exposed heating elements - nichrome wire wound around ceramic rods. There was just a chrome plated wire grille over the front which unclipped. I wondered how long I could stand my finger on it before it got too hot to bear, so plugged it in, put my finger in the middle of the element and switched it on. Zap!