I added a dead man footswitch to my bread boarding arrangement where I might have numerous bare gator clips, breadboard wires, open barrier strips, etc. flying around. A momentary normally-off footswitch you can get anywhere typically used with sewing machines, drill presses, etc. So when your foot leaves the switch, all power is cut off. The switch feeds a power strip on my bench into which I plug the (silicon rectified) power pack for whatever circuit I'm currently bread boarding. I also have an inline fuse in place closely under whatever current I happen to need for the power pack, so I'm not waiting to get all the way up to the 15 amp breaker on the power strip before I get cutoff.
I figure if I ever get caught in a shock situation I should still be able to get my foot off the switch breaking the power, or I'll make myself fall to the ground and cut the power "dead man" style. In a short circuit situation the inline .5, 1 or 2 amp fuse (depending on project) will blow well ahead of the mains and power strip for faster protection there. I've read of people making sure they have a master cutoff near their bench, but if that master switch still involves consciously moving your free hand to a wall switch somewhere your upper body muscles might already be locked or you might not have the frame of mind to get to that master off switch. And that's assuming you even have a free hand to shut it off with. Also I don't want a master switching off all my test gear, just the power pack I'm using. The footswitch also makes it easy to change out parts as I can just remove my foot, discharge the smoothing caps and resume the experiment. And I can power up the project with both hands off of anything at all.
The only problem with the arrangement is that if you are using a tube rectified supply you have to hold the footswitch until things warm up, so I'll bread board only with silicon diode power packs, no biggie.
My wife worries when I'm in the basement alone, she really liked this improvement, thought I'd share it. Cant be too careful.
I figure if I ever get caught in a shock situation I should still be able to get my foot off the switch breaking the power, or I'll make myself fall to the ground and cut the power "dead man" style. In a short circuit situation the inline .5, 1 or 2 amp fuse (depending on project) will blow well ahead of the mains and power strip for faster protection there. I've read of people making sure they have a master cutoff near their bench, but if that master switch still involves consciously moving your free hand to a wall switch somewhere your upper body muscles might already be locked or you might not have the frame of mind to get to that master off switch. And that's assuming you even have a free hand to shut it off with. Also I don't want a master switching off all my test gear, just the power pack I'm using. The footswitch also makes it easy to change out parts as I can just remove my foot, discharge the smoothing caps and resume the experiment. And I can power up the project with both hands off of anything at all.
The only problem with the arrangement is that if you are using a tube rectified supply you have to hold the footswitch until things warm up, so I'll bread board only with silicon diode power packs, no biggie.
My wife worries when I'm in the basement alone, she really liked this improvement, thought I'd share it. Cant be too careful.
Yes that too, I have a 2.5 amp Stancor (one of those plain gray ones uncased) always between the power strip and the power pack, that is bolted always to the bread board arrangement.
For filter caps I made my own tool by grinding an old phillips screwdriver to a point, then I heat shrinked the shaft except for the very tip so I can probe deep without hitting other stuff and solders a 470 ohm 5 watt resistor into the alligator clip and heat shrunk that whole clip. I leave that tool clipped to ground all the time. So its easy enough to just lift the tool and tap each capacitor for a second after I shut off the power.
For filter caps I made my own tool by grinding an old phillips screwdriver to a point, then I heat shrinked the shaft except for the very tip so I can probe deep without hitting other stuff and solders a 470 ohm 5 watt resistor into the alligator clip and heat shrunk that whole clip. I leave that tool clipped to ground all the time. So its easy enough to just lift the tool and tap each capacitor for a second after I shut off the power.
Last edited:
Having experienced a decent shock the risk of body locking up is significant. An effective dead man switch needs to have sufficient mechanical force to push your foot off in case of a significant shock. The result is significant muscle fatigue during prolonged work sessions. It is worth considering the location and set up of the switch to increase likelihood of foot coming off in case of shock.
Another issue with extensive safety kit is the sense of security this gives you. As you think the consequences of an incident are non serious there is more natural inclination to work in a less safe manner pushing things beyond what you would do without the safety kit. Unless you continue to expect things will kill you, you will end up doing something that, even if it doesn’t get you with a shock, could hurt you, exploding capacitors or wiring fires come to mind.
So if you do this make sure it will be effective and operate without your conscious physical action and still work as if it doesn’t exist!
Take Care!
Another issue with extensive safety kit is the sense of security this gives you. As you think the consequences of an incident are non serious there is more natural inclination to work in a less safe manner pushing things beyond what you would do without the safety kit. Unless you continue to expect things will kill you, you will end up doing something that, even if it doesn’t get you with a shock, could hurt you, exploding capacitors or wiring fires come to mind.
So if you do this make sure it will be effective and operate without your conscious physical action and still work as if it doesn’t exist!
Take Care!
Yes uncritical satisfaction in ones own safety measures can hurt you. This arrangement in no way, I hope, makes me complacent. My goal was to not have to consciously "get to" a master switch with a hand if the nightmare happens.
The one handed rule still applies always, especially when I'm working on my house wiring. For house wiring, I'm actually more cautious of the neutral wires in the box than the "hot" wire. I know the breaker de-energized the hot wire, but I could have neutrals passing through the electrical box I'm working on that are still carrying a return load from a different live branch breaker. Electricians will take any nearby neutral to complete a circuit. So opening up a neutral bundle in a box is always more dangerous than touching the hot you know you shut off. That neutral may be series carrying back 12 amps for your A/C compressor in a box you thought you de-energized.
Also I like to heat shrink open probes all but the point for probing around helps when "threading a needle" with the probe past other bare wires. Heat shrinking a screwdriver or probe shaft works best if you put some hot glue under the heat shrink because polyolefin will just slide around on chrome or a probe.
The one handed rule still applies always, especially when I'm working on my house wiring. For house wiring, I'm actually more cautious of the neutral wires in the box than the "hot" wire. I know the breaker de-energized the hot wire, but I could have neutrals passing through the electrical box I'm working on that are still carrying a return load from a different live branch breaker. Electricians will take any nearby neutral to complete a circuit. So opening up a neutral bundle in a box is always more dangerous than touching the hot you know you shut off. That neutral may be series carrying back 12 amps for your A/C compressor in a box you thought you de-energized.
Also I like to heat shrink open probes all but the point for probing around helps when "threading a needle" with the probe past other bare wires. Heat shrinking a screwdriver or probe shaft works best if you put some hot glue under the heat shrink because polyolefin will just slide around on chrome or a probe.
I am able to work and troubleshoot at low voltage (750v) and handle under high voltage (20000v). what I can say is that when I work on a tube amp, I never use my thumb and forefinger to manipulate a conductor or element with tension or danger. the biggest risk for me is the little finger, it is he who comes into contact most often when we pay attention and when the hand is folded. I have an isolation transformer on the power of my work table and I never work when there is someone next to me, even my cat. I want to take the opportunity to recall here that no one ever think to protect their eyes and yet, it is the eyes that are the most fragile and most exposed in case of short circuit or explosion. I happened to do a short circuit on a bar in 380v + neutral, I can say thank you to my mask protection because sense him, I walk in the street with a blind man. one last thing, the greatest danger is the habit.
I always wear a pair of safety glasses when powering up a circuit.
A reversed cap can take out your eye if it launches pieces and i have read about eyesight damage from arc flash, so the safety glasses offer an amount of UV protection to prevent that.
It isn't really an inconvenience and often i forget that i am wearing them and walk into the house with them still on (which the missus actually appreciates).
A reversed cap can take out your eye if it launches pieces and i have read about eyesight damage from arc flash, so the safety glasses offer an amount of UV protection to prevent that.
It isn't really an inconvenience and often i forget that i am wearing them and walk into the house with them still on (which the missus actually appreciates).
I worked on up to 10kv dc and a lot of 600Vac of over 100 amps.
Mandatory was the helmet, UV glasses, double earplugs (buds + eggs) with built in communication set, the lab certified (each month?) 10kv gloves + the over leather gloves + all arc flash suits, and some body armors if working over 600Vac, serious stuff, always WORK with at least ONE assistant.
That is in the industry/motor testing etc.
At home, you are mostly in the garage or basement, so the number one rule:
1. wear good insulated shoes! with socks, and if possible use a carpet under your work area to break ground path.
2. wear googles
3. wear insulated gloves, one or 2 layers
4. use a VARIAC
5. MANDATORY: use a GFCI industrial certified, I installed mine in the basement and wired it myself, tested it many times.
6. Discharge the amp before workign, measure any voltage on caps to double check, work with one hand.
7. Unplug the equipment, use a secondary power bar with a fuse and easy to turn off switch, so if you forget to unplug or switch off the amp or the bar at least one is off,
8. If taking measurements live: use proper distance, use ear protectors, glasses in case a capacitor or resistor blows
9. always have at least 1 fuse in the circuit
10. Again for live test, first do a sequence of the procedure without voltage , clips might slip, some clipping point might cause an arc, be unstable etc.
11. PRE-plug the test equipment before power on.
12. Do not look directly to the equipment in critical aspects, NEVER lean inside the equipment etc.
Bottom line: if you take a 120 V shock you will not conduct to earth, the GFCI will also protect you.
I built at home amps with 800Vdc final rectified power at 150Ma x2 So this is mandatory. Especially when working alone, be extra cautious.
Mandatory was the helmet, UV glasses, double earplugs (buds + eggs) with built in communication set, the lab certified (each month?) 10kv gloves + the over leather gloves + all arc flash suits, and some body armors if working over 600Vac, serious stuff, always WORK with at least ONE assistant.
That is in the industry/motor testing etc.
At home, you are mostly in the garage or basement, so the number one rule:
1. wear good insulated shoes! with socks, and if possible use a carpet under your work area to break ground path.
2. wear googles
3. wear insulated gloves, one or 2 layers
4. use a VARIAC
5. MANDATORY: use a GFCI industrial certified, I installed mine in the basement and wired it myself, tested it many times.
6. Discharge the amp before workign, measure any voltage on caps to double check, work with one hand.
7. Unplug the equipment, use a secondary power bar with a fuse and easy to turn off switch, so if you forget to unplug or switch off the amp or the bar at least one is off,
8. If taking measurements live: use proper distance, use ear protectors, glasses in case a capacitor or resistor blows
9. always have at least 1 fuse in the circuit
10. Again for live test, first do a sequence of the procedure without voltage , clips might slip, some clipping point might cause an arc, be unstable etc.
11. PRE-plug the test equipment before power on.
12. Do not look directly to the equipment in critical aspects, NEVER lean inside the equipment etc.
Bottom line: if you take a 120 V shock you will not conduct to earth, the GFCI will also protect you.
I built at home amps with 800Vdc final rectified power at 150Ma x2 So this is mandatory. Especially when working alone, be extra cautious.
I can verify from experience that on the AC side of the line, a GFCI will trip with only a minimal tingle in your wayward finger. Also, sobriety while working with dangerous voltages is a plus. Have a celebratory drink while listening after....
I remember the arc flash when the lineman tried a fuse after lightning hit the pole in front of the house. The transformer was shorted. It was like a bomb, I couldn't see right for hours. I found a new respect for both electricity and linemen that night...
I remember the arc flash when the lineman tried a fuse after lightning hit the pole in front of the house. The transformer was shorted. It was like a bomb, I couldn't see right for hours. I found a new respect for both electricity and linemen that night...
A GFI/RCB protects against line current, but NOT currents inside the amplifier.
Your little finger from +600V to chassis is insignificant to the GFI.
I did that, many decades back, little finger was numb for a very long time. It still isn't 70%. (Nerves "can" heal but it takes a very very long time.)
I must point out that on a wooden floor you are not very grounded; but if your "safety" process requires you to stand on a hard-wired switch, you ARE grounded for-sure. Rubber tennis-shoes reduce the risk, but I have tripped a GFI (didn't feel it) with rubber soles on a concrete floor. REAL shock-rubber is, as gabdx says, tested frequently by a professional laboratory.
Your little finger from +600V to chassis is insignificant to the GFI.
I did that, many decades back, little finger was numb for a very long time. It still isn't 70%. (Nerves "can" heal but it takes a very very long time.)
I must point out that on a wooden floor you are not very grounded; but if your "safety" process requires you to stand on a hard-wired switch, you ARE grounded for-sure. Rubber tennis-shoes reduce the risk, but I have tripped a GFI (didn't feel it) with rubber soles on a concrete floor. REAL shock-rubber is, as gabdx says, tested frequently by a professional laboratory.
Good point, I looked over my switch and the housing is all plastic, a little metal around the pedal trim and the middle plane of the pedal is also plastic with a rubber surface. I'd have to open it up to see how the switch plunger actually contacts the pedal part, but appears to be plastic on plastic I'd need to open it to see if the switch is plastic as well. I would hope such switch makers thought of this. It's not a 1930's sewing machine pedal which in those days was probably steel throughout. But I will open up the switch to check it out. I do wear shoes.
Last edited:
I think if you have a serious shock you will not be able to remove the foot of the pedal...
Your brain can be paralyzed, or it can take a few seconds to be able to do it, which can still cause the heart to go into fibrillation. A strong cap discharge which goes to your brain can cause damage in a fraction of a second, the switch is useless to protect your nervous system.
Your brain can be paralyzed, or it can take a few seconds to be able to do it, which can still cause the heart to go into fibrillation. A strong cap discharge which goes to your brain can cause damage in a fraction of a second, the switch is useless to protect your nervous system.
i agree with you . i think the best way is a real deam man foot switch ,it is a pedal where you have to stay pressed to make contact, but it is much less convenient and comfortable. I totally agree with the mounting of a protection "in mind" with a detector of ground fault "DTT" in French, it is much more effective and much safer and faster than your pedal.
I have to say, it's vastly important to consider safety, and to consider measures to insure said safety.
But in all my 45 years of professional servicing, etc, I've never had to impliment such an idea as this "dead man switch" thing you speak of.
Isolation transformers, current-limiting equipment (aka Dim Bulb testers) and proper respect for grounding and electricity derived from formal training all come into play when I do servicing.
Nevertheless, good luck, be safe.
3 people took a shock at the equipment before my time, no switch could have prevented the damages....
1. Static discharges, someone had his hand 10 cm from the part, he got zapped and hurt himself due to muscle contraction and hitting furniture... (note this was a 5 tons rotor, which wasn't discharged of static electricity after a test run) the person developed Parkinson soon after...
2. pure stupidity: someone took in each hand the + - contacts of a live tester without gloves to move them somewhere... short life on permanent disability, arc holes etc...
3. another time someone touched the DC prong , plus or minus with fingers by mistake, lost use of fingers, throughout holes, just a 0.5 second DC shock at 7kv 0.1A
In the 2nd case someone disconnected the power to save the persons life , there is no way if he had a normally open pedal that he could have lift his leg, no way... maybe after 2 minutes when all the muscles are cooked to the legs muscles?
, in cases 1 and 3 it happened very fast, no switches could have helped..
I had a few electric shocks, very minor, one I was fixing a junction box under a roller conveyor, I was replacing live a twist-on, I tried first with naked fingers and started the work... but I changed my mind and put on my work gloves because I was on my fours under the metal thing and lot of conduits around me, and I thought I was OK, well I got a good 2 seconds zap with the elbow and hand through the twist-on and the gloves!!!
It hurt a lot, lesson learned, electricity can find a way in industrial settings because of all the metal dust, the rust-preventing pastes, and all the sweat.
Usually I can touch life wires without issues if I touch only one polarity.
One thing to watch for when building amps is with your VARIAC, STILL use a ground to the amp case, take the time to wire the variac box , to the ground , to the amp. I had a nasty zap with a defective power switch, the built in light was powered with 120V and it energized the case after short circuiting!
with caps, never take any chance, it can damage your brain seriously.
1. Static discharges, someone had his hand 10 cm from the part, he got zapped and hurt himself due to muscle contraction and hitting furniture... (note this was a 5 tons rotor, which wasn't discharged of static electricity after a test run) the person developed Parkinson soon after...
2. pure stupidity: someone took in each hand the + - contacts of a live tester without gloves to move them somewhere... short life on permanent disability, arc holes etc...
3. another time someone touched the DC prong , plus or minus with fingers by mistake, lost use of fingers, throughout holes, just a 0.5 second DC shock at 7kv 0.1A
In the 2nd case someone disconnected the power to save the persons life , there is no way if he had a normally open pedal that he could have lift his leg, no way... maybe after 2 minutes when all the muscles are cooked to the legs muscles?
, in cases 1 and 3 it happened very fast, no switches could have helped..
I had a few electric shocks, very minor, one I was fixing a junction box under a roller conveyor, I was replacing live a twist-on, I tried first with naked fingers and started the work... but I changed my mind and put on my work gloves because I was on my fours under the metal thing and lot of conduits around me, and I thought I was OK, well I got a good 2 seconds zap with the elbow and hand through the twist-on and the gloves!!!
It hurt a lot, lesson learned, electricity can find a way in industrial settings because of all the metal dust, the rust-preventing pastes, and all the sweat.
Usually I can touch life wires without issues if I touch only one polarity.
One thing to watch for when building amps is with your VARIAC, STILL use a ground to the amp case, take the time to wire the variac box , to the ground , to the amp. I had a nasty zap with a defective power switch, the built in light was powered with 120V and it energized the case after short circuiting!
with caps, never take any chance, it can damage your brain seriously.
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.