Have You Ever Been Shocked Working On An Amp

[dartanion]

Well, from inside the heart at the tip of a pacing lead, it only takes a few mV to capture the heart. One of the first things at implant is to test passing thresholds and map the best placement of the lead(s). Active fixation tips are pretty cool devices. You have a little wrench that you spin from the tip of the lead that sends a tiny corkscrew in the inner wall of the heart which presses the pacing tip against the wall. Passive fixation is like a little plastic anchor that wedges into the chordae tendinae and hopefully against the inner wall. The conduction paths here take very small amounts of energy to capture and pace the heart.

From out side the body, we have much more resistance, thus it takes more energy to capture the heart. What happens with folks how have been shocked across the heart is like using a defibrillator to stop the heart or cause arrhythmias. Ventricular Tachycardia can be fatal and is referred to as sudden death syndrome, where as Ventricular Flutter or Atrial arrhythmias are usually not fatal.

Then there is the permanent damage that can be caused that Dr. H pointed out earlier.

[Phil_S]

One time I grazed the inside of the IEC inlet. It was more than a little tingle. Nothing bad happened, aside from a bit of pain lingering for a day or so. I remember thinking how glad I was that it was AC and not DC.

[krash]

I've been zapped by 120VAC a number of times by touching the heat sink in an open-frame power supply in a router or switch ... why in the heck do they connect those to 120VAC anyway?

[skyboltone]

I've read that it can take as little as 500 ma to stop your heart if you get the current across the chest. Don't know if that is true and probably also depends on the voltage pushing it.
One time in the early 70's in electronics shop at high school I was working on a circuit for flashing neon bulbs and the supply was 500 vDC.
I was sitting on a bar stool type chair at the bench when I touched something I shouldn't have and it catapulted me backwards flat on my back.
Luckily I only got the charge in one hand and wasn't injured but the next class was a lecture on the dangers of electrocution and it's effects on the heart and muscles.
Thinking back I'm a bit surprised we were allowed to work with voltages that high but it was early on for semiconductor experiments so we were still dealing with B+ type voltages.
Of course we had been taught proper safety procedures but my little incident prompted a more in depth discussion......

Way, way less than 500ma. More like 25ma. Just depends on so many things. DrH knows better than I, but I do know that many electro shock victims die of internal burns too.

[gearfixer]

I've been shocked so many times I realy can not count them. My first was when I was a kid and stuck finger my between the terminals on a plug when it was still half pluged in the wall. The worst was when I was working in a poorly run textiles plant. I had shut of the new disconect on the blender but the old disconect was still wired up I shut it off too but it was jumpered and when I went to remove a relay I tuched the old one and 800v in my left hand and out my but into a toledo scail. I yelled so hard I scaired the crap out of the guy in the same room. I could tast metal in my mouth for days. The worst thing is I have no fear because I've been shocked so many times.

[drhulsey]

... I remember thinking how glad I was that it was AC and not DC.

Actually, 120 V 60 Hz is a HEART STOPPER if it crosses the heart

... many electro shock victims die of internal burns too.

That's usually seen with High Voltage injuries, like a kid I remember who was trying to remove glass insulators from a pole he thought was no longer live. WRONG The entry wound in his hand looked like an explosion. He had multiple small exit points on his feet. He didn't even lose consciousness If you act quickly and remove all dead tissue and keep the bad chemicals from dead muscle from killing their kidneys, they can survive, but usually with a few less parts. Of course, someone has to think to do CPR if they don't have a pulse, in order to get them to the hospital for treatment
The worst I've seen was a 19 yo boy in Guatemala who had run over a downed HT line. He was OK until he grabbed the door handle. I don't mean this flippantly when I say he fried 4 extremities and an area on his belly under a BIG metal belt buckle. He was 3 days post injury when I was first asked to see him (We were there operating on kids). He had minimal effect on his kidney, though he had not been treated for that. We amputated four extremities, because they were dead! It's pretty easy to tell the live from the dead tissue. He would have died from tetanus infection in the dead muscle had we not removed it. Some might say it was better to die than live without arms and legs. Maybe, but it wasn't my day to play God so we did what you're supposed to.
Telling him that his arms and legs were gone was the hardest thing I've ever done Just one of the perqs of being a doctor.

[Buschman]

The answer is yes. It was b+ voltage both times. I don't count the little ones. My meter probe was not on all the way leaving a bit of metal exposed-and I grabbed it. It was on the D'Lite. It hurt. Luckily my hand was resting on the chassis, my other hand was at my side, & I was on a rubber mat.
The other time was on an old Marshall & was a lesson on how long those big caps hold a charge.

[Buschman]

woops!

[krash]

Way, way less than 500ma. More like 25ma.

Way, way less than 25mA.

ITE product safety standards require a leakage (touch) current limit of 0.25mA for conductive parts not connected to earth, and 3.5mA for conductive parts that are connected to earth.

There's no real hard limit, it all depends on how you are shocked, your body chemistry, your health, and a zillion other things. As a rule, you probably cannot sense a 1mA electric current, but much over 1mA can be dangerous. I recall being advised by UL that deaths have been reported due to as low of an electric current as 1.25mA. Limits on leakage currents in hospital equipment, for example, are much lower due to the increased susceptibility to adverse effects of electric shock that people in the hospital may exhibit.

Before you dismiss voltage's role in all of this, remember that the current your body will draw during an electric shock is directly proportional to the voltage present. You don't want a person's body to be the current limiting resistor.

So ladies and gentlemen, ground those chassis and be very careful when working on amps.

[Structo]

Yes, well I just threw that 500ma figure out there as that was what I remember a highschool teacher saying years ago.
But it would depend heavily on the voltage pushing that current.
It may be at 25ma but if you have 250v pushing it, it's goodnight Irene.

Seems like there was an voltage threshold that the skin on your hands could protect you from, like 30v or something.
Also depends on how sweaty and salty your hands are.

My dad worked for the phone company for years and years and he told me more than once that he could work on the DC powered system without getting shocked but if he happened to be touching the wires when a ring voltage came through it jumped to 90 volts and it would shock him.

[Phil_S]

... I remember thinking how glad I was that it was AC and not DC.

Actually, 120 V 60 Hz is a HEART STOPPER if it crosses the heart

I'm fairly zealous about the hand in pocket rule on live amps. That is a good defense for the heart, but probably not perfect.

The AC DC thing is that it is much easier to release your hand from an AC source as you get the opportunity every time it changes phase. With DC, it is possible that it won't let go of you.

Anyway, it was a graze. Enough to hurt, but not a situation where I was closing my fingers on it, the danger there being a loss of motor control so as to prevent the ability to release. This is a more serious problem with DC.

This whole thread makes me wonder why anyone would work on a live circuit.

[Doug H]

Have You Ever Been Shocked Working On An Amp?

No, but I've been shocked by line voltage in the house. Also been shocked by a car ignition system and a lawn mower ignition system. Not fun...

I was also shocked off of a tube phonograph once when I was a kid. I was using it as a preamp into my Silvertone amp-in-a-case. It quit working so I reached in there with it live and started wiggling stuff... Grabbed a hold of the selenium rectifier- I won't do that again.

I'm pretty strict about safety when working on my amps these days, given my record...

[skyboltone]

Way, way less than 500ma. More like 25ma.

Way, way less than 25mA.

ITE product safety standards require a leakage (touch) current limit of 0.25mA for conductive parts not connected to earth, and 3.5mA for conductive parts that are connected to earth.

There's no real hard limit,

Agreed, the 25ma figure was the GFCI standard for equipment ground fault protection. 5ma is the limit on GFCI protection for outlets. Like you said though, depends more or less on Grace. When I was teaching electricity a fellow teacher came into work one evening after having taken 4160VAC from hand to elbow that afternoon. The shock "surface tracked" down his forearm and left him with a little dead skin. He had opened a whole row of disconnects for cleaning but was turning them off one at a time as needed. He turned around to get a tool off his cart and returned to the wrong disconnect. I finished up my career writing safe work procedures for a huge industrial company to help avoid such mistakes.

[jerrydyer]

I got nailed so bad I though my cousin who was right next to me grabbed my hair and yanked me backwards. I shouted "stop" he just looked at me and laughed. I had to take a nap. Scary ..Ive never been the same.

[krash]

the insulation value of your skin varies according to humidity, moustire on your skin, and of course body chemistry etc.

The DC thing vs. ringing voltage on telecom lines is -48VDC that is present at all times on live telephone lines to power telephones and also to prevent corrosion by drawing a small amount of "sealing current". This voltage is usually not sufficient to give you an electric shock as long as you have dry skin etc., but it is very near the limit of a safe DC voltage. Plus this is current-limited to a very low amount of current from the telephone company, not to mention miles and miles of copper wire providing series resistance. Perhaps a bigger concern may be electrostatic charge that can build up on aerial phone wiring in the tens of kV. However ringing voltage in the USA is nominally 120VAC RMS and you will definitely get shocked by it if you are holding onto the wire while the phone rings. In other countries the nominal ringing voltage is higher. Because of the complex impedance and in particular the inductance of the phone wiring and high terminating impedance of a phone that's on the hook, ringing voltage can be as high as 300V peak. Telecom safety was my profession for a number of years and I also have two patents on telecom safety technologies (one is for a certain type of fuse, the other is lightning protection scheme that is in use in all Cisco DSL equipment).

The first time I traveled to Mexico I was working as a product safety engineer and I remember being stunned at how ridiculously unsafe the wiring installations were in that country. We have very good safety standards and certifications in the USA. Unfortunately for part-time boutique guitar amp builders like myself, obtaining a safety compliance certification is extremely costly in view of the quantity of amps we sell, and largely unnecessary for selling our products. The safety certification game in the USA is pretty much customer driven and not a legal requirement for most things that plug into a wall outlet.

Way OT I know...