I got shocked while replacing a fuse... what gives?

[Hardtailed]

Yesterday, I got a nasty electrical shock right across my two arms (thus through my chest) while replacing the mains fuse (which is installed before the power switch as usual). My left hand was holding the chassis since it's not installed in a cab yet, and when I came to push the fuse in by holding the plastic cap (classis Fender style fuse holder), I felt electricity right across me. Not fun!

I almost felt like throwing the amp out the window, but after calming down I tested everything for continuity, made sure the chassis was properly grounded (ironically, that's how I got shocked) then proceeded to install the fuse WITH THE POWER CORD DISCONNECTED and everything was fine (only the filaments are connected right now).

The only way I see that I could have been shocked would be that I was just slightly touching the metal ring of the fuse without noticing, so as soon as it touched the socket, it found a way to ground through my other hand that was touching the chassis.

Thing is: I could swear I was only touching the plastic part, and it almost felt like the shock came from the chassis on both hands. But this makes no sense!

Although everything seems to be working fine now, I'm sort of scared of finishing this project right now...
Anything I should check that could explain the shock? Or should I just accept that I stupidly touched the metal ring and should make sure to always disconnect the power cord before replacing a fuse?

Hey, it's my first shock in 5 projects
(If you're wondering why the fuse was blown in the first place... well it blew with just the indicator lamp installed and it seems I must have used a 6V part even though I specifically ordered a 110V one... the fuse was a quick-blow so it didn't fight for long)

[jrfrond]

Fuse holders should be wired with the tip lug hot, NOT the side lug, to minimize shock potential. This way, if you DO grab the fuse by hand with the power on and it is open, you will not get bit. You WILL if it is not open. Also, always wire a fuse holder AFTER the power switch. If you are fusing the DC mains supply, the best place is in the center-tap of the B+ winding (if it has a CT), where you will often find Standby switches as well. This is very important. AC can hurt you and can be lethal under the wrong conditions, but DC is far worse and will put your lights out. Benn there, done that.

Getting shocked is an occupational hazard in the business of amp building and repair. All you can do is minimize your chances, use your head, and get used to it.

[MWJB]

"Or should I just accept that I stupidly touched the metal ring and should make sure to always disconnect the power cord before replacing a fuse?" Yes, accept that. My favourite trick is to rest my arm/wrist on a tolex Fender chassis (powered down, but still plugged in) & shock myself on the AC connections/fuse holder etc...aaah well, you live and learn, or that's the theory anyway! ALWAYS disconnect from wall AC before handling...especially when handling the things that directly connect to that wall AC.

[Steve Conner]

Fuse holders should be wired with the tip lug hot, NOT the side lug, to minimize shock potential. This way, if you DO grab the fuse by hand with the power on and it is open, you will not get bit.

Modern fuse holders have a springy thing that makes the fuse come out inside the cap, so you're not supposed to get bitten any way.

The B+ center tap is no safer than the cathode of the rectifier tube. When the fuse blows, the B+ sinks to zero and the center tap just goes 400 volts negative, destroying your bias supply into the bargain.

[Hardtailed]

Fuse holders should be wired with the tip lug hot, NOT the side lug, to minimize shock potential. This way, if you DO grab the fuse by hand with the power on and it is open, you will not get bit.

Ahhhhhhhhhhhhh, did not think about that. I did in fact connect the hot to the side lug. I will reverse those when I get home tonight. The simple stuff...
I know that doesn't make the amp foolproof, but every bit counts. Thanks for that tip.
What bothers me is this amp is for someone else, and I can't expect him to disconnect the power cord if he needs to replace the fuse.

My thinking when putting the mains fuse before the power switch is that if the switch shorts, the fuse will go before something melts. Also, with the IEC connector and fuses in the back, while the power switch is in the front (classic head style), it means shorter wires (no need to go back to back panel before going to the PT's primary)

[MWJB]

"What bothers me is this amp is for someone else, and I can't expect him to disconnect the power cord if he needs to replace the fuse." Er... yes you can! If he doesn't then he is just a massochist. Whichever way you wire the fuse holder, if he thinks that it is "safe" to change fuses whilst connected to wall AC, he will still be able to shock himself...if he thinks it's "safe" what is to stop him from pushing the new fuse into the holder with his finger, for instance? Pulling the IEC lead takes a second, it's about as foolproof as you can get - recommend that he does this!

[Hardtailed]

Modern fuse holders have a springy thing that makes the fuse come out inside the cap, so you're not supposed to get bitten any way.

The B+ center tap is no safer than the cathode of the rectifier tube. When the fuse blows, the B+ sinks to zero and the center tap just goes 400 volts negative, destroying your bias supply into the bargain.

I'm using this holder:


It does have the spring. But the only explanation is there...
I'm guessing I may have touched the outer ring of the fuse, so when the inner ring touched the "top" connector of the fuse holder, the fuse conducted for a fraction of a second (the shock was really short, just long enough for me to notice). Had I put the hot wire on the end lug as suggested, this most likely wouldn't have happened unless the amp was switched on.

The whole deal about breaking the connection on the center tap always bothered me too... sure it's at ground potential, but only until you cut that connection.

[Hardtailed]

"What bothers me is this amp is for someone else, and I can't expect him to disconnect the power cord if he needs to replace the fuse." Er... yes you can! If he doesn't then he is just a massochist. Whichever way you wire the fuse holder, if he thinks that it is "safe" to change fuses whilst connected to wall AC, he will still be able to shock himself...if he thinks it's "safe" what is to stop him from pushing the new fuse into the holder with his finger, for instance? Pulling the IEC lead takes a second, it's about as foolproof as you can get - recommend that he does this!

Yeah I get what you mean, and I've already made sure to communicate as much safety information as I can. I'd just hate for someone to get hurt on something I built. I'm not even making real money on this, I'm just happy to help someone.

[Steve Conner]

I once gave an old MIDI master keyboard that I built to my brother.

It quit working, so he thought he would change the fuse, which was buried inside.

So without removing the power cord or turning it off, he unscrewed the lid and tried to change the fuse, which being live with 240 volts, of course shocked him to kingdom come.

I can only suppose that I got all the DNA for electrical aptitude and there was none left over for him.

[Hardtailed]

I once gave an old MIDI master keyboard that I built to my brother.

It quit working, so he thought he would change the fuse, which was buried inside.

So without removing the power cord or turning it off, he unscrewed the lid and tried to change the fuse, which being live with 240 volts, of course shocked him to kingdom come.

I can only suppose that I got all the DNA for electrical aptitude and there was none left over for him.

He's not into tube amp building hopefully?

I did put a "Warning: lethal voltage inside" on the back panel, I think I'll add one over the fuse holders.

[tubeswell]

Naughty boy Hardtailed! Ten to one, that chest pain you felt is the aftershock of muscles in your heart frying. I know what that feels like. You have to live more carefully now.

[jrfrond]

I can only suppose that I got all the DNA for electrical aptitude and there was none left over for him.

OK Steve, but how do you explain it when YOU get your ass kicked, and DON'T tell me it never happens! I have far too many burn scars (as well as one impromptu visit with the paramedics) after 30+ years to believe THAT!!!

[Hardtailed]

Naughty boy Hardtailed! Ten to one, that chest pain you felt is the aftershock of muscles in your heart frying. I know what that feels like. You have to live more carefully now.

I was scared to hell and kept my phone with me in case I had to call 911 (I live alone).

Funny how I build amps with 450V+ in them stored in huge caps and never get shocked, but I get shocked changing a simple fuse in a amp with just the 6.3V filaments wired! Perhaps I get more cautious with the high voltage and forget to keep my head in check with the simple stuff.

That's the explanation I found for the fact that all my DIY pedals are junk while my amps rock: I pay no attention to a simple 9V circuit hehe

[tubeswell]

After I shocked myself (year before last IIRC), I always make sure the amps are unplugged from the wall before doing any work on them. I was also being bone lazy at the time and it was also late at night and I was doing a repetitive voltage checking and biasing exercise with lots of switch on switch of sequences. All the more reason to be careful, as well as not work on amps late at night! (BTW I did end up checking myself into A&E at 2am in the morning for peace of mind). Luckily I didn't get the chest pain at that time tho'.

However I do remember shocking myself one xmas when I was about 10 or 11 on 240VAC mains voltage faulty Xmas tree lights, and I distinctly remember getting a big knock and chest pain with that. I guess my heart recovered somewhat after that episode because I was still young and growing at that stage (but that time of life doesn't last unfortunately - so nowadays you got to be extra careful)

I suppose if you were to be philosophical about it, at least now you have had it drummed into you how to wire a fuse holder! (Its a shame that one has to do that to oneself in order to learn about safety tho). Positive way to look at it is to share your experience with others so that they take more care.

[Steve Conner]

OK Steve, but how do you explain it when YOU get your ass kicked, and DON'T tell me it never happens!

Well, when I was a kid I liked to take apart old tube radios and the like. I got one really good jolt off 400V DC that left me stuck to it for a while, and have been super careful ever since. I was actually scared to mess with anything that had tubes in it for a few years after that. Yes, I've been shocked since, but never as bad as that time.

In my day job, I hardly see anything bigger than 5 volts, and even that is high, now 3.3V is the norm.

I worked at BBC Scotland one summer, and they wouldn't let you work on the guts of high voltage equipment, TVs, video monitors and so on, alone. There always had to be one other person present. They also insisted you wore glasses or goggles when soldering.

I felt bad for a while about my brother getting shocked, because I never put any of the usual "Danger high voltage, no user serviceable parts inside" stickers on things that I built. It's obvious to me, and probably to you guys, which parts of the guts of some machine are live and which aren't, and that it's a good idea to pull the power cord out of the wall before taking it apart.

But he's an artist and knows nothing about electronics. I mention this because if you build amps for sale, you can't assume that your customers know anything, either.