Author Topic: lethal voltages.  (Read 2457 times)

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Offline m3vuv

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lethal voltages.
« on: October 30, 2019, 01:18:28 am »
ive seen lots of folks mentioning lethal voltages inside cro's,ive always been led to  belive the eht wont kill you as its lots of volts but no amps,sure i can see mains voltages can harm you but is the pda and eht so lethal as made out?i had a belt off of a colour tv eht pda  wire when i was about 11yrs old,threw me across the room but didnt kill me,one of the worst clouts ive had working was from a vauxhall cdi ignition system,had hold of the coil lead as my idiot bro in law turned the ignition on and off,actualy made me arm hurt for about half an hour!,so whats everyones views on eht etc?.
 

Offline tggzzz

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Re: lethal voltages.
« Reply #1 on: October 30, 2019, 01:27:30 am »
ive seen lots of folks mentioning lethal voltages inside cro's,ive always been led to  belive the eht wont kill you as its lots of volts but no amps,sure i can see mains voltages can harm you but is the pda and eht so lethal as made out?i had a belt off of a colour tv eht pda  wire when i was about 11yrs old,threw me across the room but didnt kill me,one of the worst clouts ive had working was from a vauxhall cdi ignition system,had hold of the coil lead as my idiot bro in law turned the ignition on and off,actualy made me arm hurt for about half an hour!,so whats everyones views on eht etc?.

Tryaddingsomeconventionalpunctuationsoitisreadable.

Then try touching the various voltages again. And again when you are older, your heart is weaker, and humidity etc is different. Then get someone you know to touch it.

Some risks are worth taking, because that's the only way to get the benefits. Touching HT/EHT isn't one of those.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline andy3055

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Re: lethal voltages.
« Reply #2 on: October 30, 2019, 01:34:43 am »
It all depends on the individual. I know an electrician friend of mine who can work on live 120 volts with no problem. He just wears rubber flip flops and makes sure no one touches him and is not electrically grounded. But then the other day he touched the live and the neutral with his bare hands and I was ready with a wooden pole to knock him off!

 

Offline tggzzz

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Re: lethal voltages.
« Reply #3 on: October 30, 2019, 01:48:34 am »
It all depends on the individual. I know an electrician friend of mine who can work on live 120 volts with no problem. He just wears rubber flip flops and makes sure no one touches him and is not electrically grounded. But then the other day he touched the live and the neutral with his bare hands and I was ready with a wooden pole to knock him off!

I did that once, but there was nobody with a wooden pole. Luckily it was a light touch, and the involuntary biceps contraction rapidly broke the circuit.

In topics like this there are too many uncontrolled variables to make predictions.
There are lies, damned lies, statistics - and ADC/DAC specs.
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Offline TheMG

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Re: lethal voltages.
« Reply #4 on: October 30, 2019, 01:53:10 am »
While the current generated by the high voltage flyback transformer itself is very low, the CRT tube itself acts like quite a significant high voltage vacuum capacitor. The output of the flyback transformer probably isn't going to kill you, but will certainly get your attention. However, the charge stored within the CRT can most definitely be fatal under the right circumstances, that is what you need to be particularly careful about, and always ground out the HT connector on the CRT tube to discharge before touching.

Also, in many CRT display designs, the circuitry driving the flyback as well as the deflection is not isolated from mains (not sure about oscilloscopes, but this is definitely true of many CRT TVs and PC monitors), so that presents an additional potential hazard if you were to get your fingers in the wrong place.

So always better to be extra careful when working with these things.
 

Offline m3vuv

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Re: lethal voltages.
« Reply #5 on: October 30, 2019, 02:25:32 am »
i suppose it comes down to the old saying "its volts that jolts but mills that kills",the clout i had from the tv tube was disconecting the eht cap with bare hands,i once picked up a 240v 2.5mm cable in an old silica mine in n wales,it had been tapped up but the conductor had pierced the insulation tape that i got hold of,i was standing on wet ground with wet feet,must of jumped 6ft in the air and landed on my friend ,it broke his collar bone!,i was ok,still was a tickle tho against the tv tube pda cap!!
 

Offline vk6zgo

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Re: lethal voltages.
« Reply #6 on: October 30, 2019, 02:51:50 am »
While the current generated by the high voltage flyback transformer itself is very low, the CRT tube itself acts like quite a significant high voltage vacuum capacitor.
Sorry, but this is a myth, the capacitance of a CRT is quite small.
Quote

The output of the flyback transformer probably isn't going to kill you, but will certainly get your attention. However, the charge stored within the CRT can most definitely be fatal under the right circumstances, that is what you need to be particularly careful about, and always ground out the HT connector on the CRT tube to discharge before touching.

I'm not sure what circumstances they may be--- maybe if you are standing on a ladder holding the tube?
Quote

Also, in many CRT display designs, the circuitry driving the flyback as well as the deflection is not isolated from mains (not sure about oscilloscopes, but this is definitely true of many CRT TVs and PC monitors), so that presents an additional potential hazard if you were to get your fingers in the wrong place.
So always better to be extra careful when working with these things.

I believe there were a few older TVs sold in Europe that did something like this, but they are a very small minority.
The vast majority of CRT based  TVs & monitors have isolated  deflection & EHT supples.
 

Online Psi

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Re: lethal voltages.
« Reply #7 on: October 30, 2019, 03:12:33 am »
There is no hard limit for what is lethal.
It's like asking what height you can fall from and not die.

The general rule of thumb is that above 30mA can kill but that does not mean 10mA is safe for everyone, or that no one survives 100mA.
However you would have to be the most unlikely person in history to die from like 5mA.

The voltage is only relevant in that more volts allows more current to flow and less voltage means less current will flow.
Either the current is limited by the power source not being able to supply more than a set amount (current limited source)
or buy the voltage being too low to allow much current to flow into a human body.

The resistance of the human body also varies quite a bit.
The lower the resistance the less voltage is needed to get dangerous current to flow.

Also the duration of the shock is important too. The peak current from like, a static shock, can be an amp but it's so short it's safe.
« Last Edit: October 30, 2019, 03:24:49 am by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 

Offline andy3055

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Re: lethal voltages.
« Reply #8 on: October 30, 2019, 03:27:28 am »
Here is some info. But by no means should one take chances at any level. I take no responsibility!

https://www.asc.ohio-state.edu/physics/p616/safety/fatal_current.html
 
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Offline george.b

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Re: lethal voltages.
« Reply #9 on: October 30, 2019, 03:27:59 am »
I think you're more likely to break something (like the CRT neck) when you jerk your arm from the shock than anything else. I got zapped by a live TV flyback transformer when I was a kid (playing around with the cool sparks :palm:), and my arm hurt for the better part of an hour, just from the muscle contraction. Still, there's no reason why you shouldn't take proper precautions and treat the HV section of anything CRT with respect.
Just because, say, you and I got away with it with only a sore arm doesn't mean everyone else will. The risk is there.
« Last Edit: October 30, 2019, 04:21:51 am by george.b »
 
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Offline m3vuv

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Re: lethal voltages.
« Reply #10 on: October 30, 2019, 04:02:27 am »
that was an interesting read!,cheers.
 

Online shakalnokturn

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Re: lethal voltages.
« Reply #11 on: October 30, 2019, 08:39:37 am »
While the current generated by the high voltage flyback transformer itself is very low, the CRT tube itself acts like quite a significant high voltage vacuum capacitor. The output of the flyback transformer probably isn't going to kill you, but will certainly get your attention. However, the charge stored within the CRT can most definitely be fatal under the right circumstances, that is what you need to be particularly careful about, and always ground out the HT connector on the CRT tube to discharge before touching.

Also, in many CRT display designs, the circuitry driving the flyback as well as the deflection is not isolated from mains (not sure about oscilloscopes, but this is definitely true of many CRT TVs and PC monitors), so that presents an additional potential hazard if you were to get your fingers in the wrong place.

So always better to be extra careful when working with these things.

Is the case of the CRT capacitor is the dielectric the vacuum or the glass?




I believe there were a few older TVs sold in Europe that did something like this, but they are a very small minority.
The vast majority of CRT based  TVs & monitors have isolated  deflection & EHT supples.

Very few designs I know of had non-isolated horizontal deflection, indeed far from a majority.

The Philips EM5.3E TV chassis is one of them. I seem to recall Sharp did that on at least one design too.
 

Offline Gyro

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Re: lethal voltages.
« Reply #12 on: October 30, 2019, 12:37:11 pm »
While the current generated by the high voltage flyback transformer itself is very low, the CRT tube itself acts like quite a significant high voltage vacuum capacitor. The output of the flyback transformer probably isn't going to kill you, but will certainly get your attention. However, the charge stored within the CRT can most definitely be fatal under the right circumstances, that is what you need to be particularly careful about, and always ground out the HT connector on the CRT tube to discharge before touching.

Also, in many CRT display designs, the circuitry driving the flyback as well as the deflection is not isolated from mains (not sure about oscilloscopes, but this is definitely true of many CRT TVs and PC monitors), so that presents an additional potential hazard if you were to get your fingers in the wrong place.

So always better to be extra careful when working with these things.

Is the case of the CRT capacitor is the dielectric the vacuum or the glass?

No, it's not a vacuum capacitor, the dielectric is the glass wall of the CRT cone between the inner and outer conductive Aquadag coatings. Note that this only applies to TV / monitor tubes - it's very rare to find a scope CRT with an external coating.
Chris

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Offline dom0

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Re: lethal voltages.
« Reply #13 on: October 30, 2019, 12:58:34 pm »
Some scope tubes will have a 10-40 MΩ in the anode lead, which would limit the discharge current to a few mA. The Tek 7603 for example has 36 MΩ in the lead which should limit the discharge (from ~12 kV) to 1/3 mA. Obviously it still needs discharging, but it doesn't make a noticeable spark. I never had any intention on finding out whether this would be dangerous or merely painful to touch. I always kept the anode lead grounded permanently while working on it.

The output of the HV multiplier on the other hand has no series resistance, so you are looking at something like 1 nF charged to 12 kV.
« Last Edit: October 30, 2019, 01:03:11 pm by dom0 »
,
 

Online shakalnokturn

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Re: lethal voltages.
« Reply #14 on: October 30, 2019, 03:01:28 pm »
Speaking of that... The last time I got a bite from a CRT must have been trying to disconnect the PDA plug on a Tek 2212.
I like the idea of their H.V. interconnect on the wire.
Only problem being the conductors are not far back enough to prevent it snapping-back at you once you have won the struggle to release the connection.

So, on the whole it's crap, I prefer having the option to slide a screwdriver on the side of the CRT to discharge rather than doing it with my hand...
 

Offline m3vuv

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Re: lethal voltages.
« Reply #15 on: October 30, 2019, 07:13:13 pm »
I normaly don my brown trousers when working on the pda/eht side of things!!
 

Offline janoc

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Re: lethal voltages.
« Reply #16 on: October 30, 2019, 08:17:36 pm »
I normaly don my brown trousers when working on the pda/eht side of things!!

There is another thing to consider - while the flyback output may not kill you outright, the high frequency high voltage burns that you can get from it are nasty. It carbonizes the skin (and muscle) where the current passes through. Such injuries can take very long time to heal, with a possibility for a serious infection (even losing fingers and/or sepsis is possible).
 

Offline jogri

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Re: lethal voltages.
« Reply #17 on: October 30, 2019, 09:35:16 pm »
Well, you forgot to mention one crucial factor when discussing the lethality of electrical injuries: time.

Sure, if the current is high enough you will die nearly instantaneous, but if it isn't there is a chance that you have injured (be it temporary or permanent) your heart, and no one can say for certain how long your pacemaker(s) will function: You could just as well drop dead hours after you touched a voltage source if your SA node (the natural pacemaker of your heart) was weakened by the voltage/current through it and couldn't withstand its normal operation.

And that's the reason why nearly every medical textbook tells you to go see a doctor if you got an electric shock. They will monitor your EKG for a full 24h to look for any anomalies that might develop.
But lets be honest, (nearly) no one does that. You get a shock, it hurts and you tell yourself that you are still fine, no harm done.
 

Offline jdragoset

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Re: lethal voltages.
« Reply #18 on: October 30, 2019, 10:47:05 pm »
Back in the vacuum tube B&W TV days, one quickly learned to respect the fly back HF, HV, as it would burn, rather than shock (skin effect).
With only a 1-B3 rectifier tube, it produced a typical 15 kV, 15 kHz output with probably a +35 watt drive.
(pre-historic microwave).
The wise folks at George Marshal Space Flight Center in Huntsville, AL said to always have your unused hand in your back pocket while taking HV measurements (I prefer clip leads and no-hands for power-up)
 

Offline vk6zgo

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Re: lethal voltages.
« Reply #19 on: October 31, 2019, 12:04:56 am »
Well, you forgot to mention one crucial factor when discussing the lethality of electrical injuries: time.

Sure, if the current is high enough you will die nearly instantaneous, but if it isn't there is a chance that you have injured (be it temporary or permanent) your heart, and no one can say for certain how long your pacemaker(s) will function: You could just as well drop dead hours after you touched a voltage source if your SA node (the natural pacemaker of your heart) was weakened by the voltage/current through it and couldn't withstand its normal operation.

And that's the reason why nearly every medical textbook tells you to go see a doctor if you got an electric shock. They will monitor your EKG for a full 24h to look for any anomalies that might develop.
But lets be honest, (nearly) no one does that. You get a shock, it hurts and you tell yourself that you are still fine, no harm done.

And 40+ years later, you are still waiting for the effects to kick in! ;D
 

Offline mcovington

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Re: lethal voltages.
« Reply #20 on: October 31, 2019, 04:57:30 am »
I don't take risks.  I don't want to feel *any* electric shocks (and haven't, for decades).

But there is another interesting question to ask:  What kind of accidents actually have caused serious injury or death?  I'm aware of deaths that are *power*-related -- power lines and relatively beefy high-voltage power supplies, e.g., in radio transmitters.  I don't think I've heard of a death resulting from someone working on a CRT or its power supply, but that doesn't mean there haven't been any, of course.

My point is, without denying that it's dangerous, we can still stratify different levels of danger and harm.  So... what actual incidents do people know about?
 

Offline andy3055

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Re: lethal voltages.
« Reply #21 on: October 31, 2019, 06:37:45 am »
Here is one:

I was once working as the electrical engineer at a plywood plant in Fiji. When they ordered a new DC drive for a veneer lathe, a young Aussie electrician from ABB came to install it. This guy nearly died while working on the panel as he forgot to turn off the 3 phase/440volt breaker on the other side of the room before trying to loosen one of the feeder cables coming in to the panel. When I got there, he was crouched on the floor unconscious and his palm around the cable where it was insulated. By some miracle his hand slipped down enough for him to be alive. I nearly touched him unknowingly but something told me he did not look good. My immediate reaction was to throw the isolator off before calling the mill workers to carry him down from the room to a car. He was off to the hospital rigid like a pole, his legs protruding from the open windows. He had burn marks on his hand and on his calf where it had made contact with the frame of the panel. After a few days in the hospital he was gone and another guy had to come and finish the project. (A-hole never thanked me!)
As I write this, I am shivering thinking how close it was for me!
« Last Edit: October 31, 2019, 06:39:52 am by andy3055 »
 
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Offline james_s

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Re: lethal voltages.
« Reply #22 on: October 31, 2019, 07:09:56 am »
The EHT probably won't hurt you, I mean it hurts, but it would be very unusual if you were directly injured by it.

CRT devices contain many lower but still quite high voltages that pack a lot more grunt. The 180V B+ or 170-340V DC bus in the power supply is certainly capable of being lethal.

Then there are the secondary effects, if you get zapped from the HV you could easily slice your hand wide open on sharp metal shielding when you jerk it back.

So yeah, probably not actually lethal but still not a good place for a total noob to go poking around.
 

Offline tggzzz

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Re: lethal voltages.
« Reply #23 on: October 31, 2019, 09:40:44 am »
I don't take risks.  I don't want to feel *any* electric shocks (and haven't, for decades).

But there is another interesting question to ask:  What kind of accidents actually have caused serious injury or death?  I'm aware of deaths that are *power*-related -- power lines and relatively beefy high-voltage power supplies, e.g., in radio transmitters.  I don't think I've heard of a death resulting from someone working on a CRT or its power supply, but that doesn't mean there haven't been any, of course.

My point is, without denying that it's dangerous, we can still stratify different levels of danger and harm.  So... what actual incidents do people know about?

For one anecdote, see https://www.eevblog.com/forum/testgear/ground-of-oscilloscope-always-connected-to-earth/msg1967150/#msg1967150
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
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Offline vk6zgo

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Re: lethal voltages.
« Reply #24 on: October 31, 2019, 10:36:52 am »
I don't take risks.  I don't want to feel *any* electric shocks (and haven't, for decades).

But there is another interesting question to ask:  What kind of accidents actually have caused serious injury or death?  I'm aware of deaths that are *power*-related -- power lines and relatively beefy high-voltage power supplies, e.g., in radio transmitters.  I don't think I've heard of a death resulting from someone working on a CRT or its power supply, but that doesn't mean there haven't been any, of course.

My point is, without denying that it's dangerous, we can still stratify different levels of danger and harm.  So... what actual incidents do people know about?

I've worked most of my life in  Radio  & TV Broadcasting, getting my first job in that field in 1965.
In all that time, some people I had worked with died in accidents.
Three were in aircraft crashes, & two in falls from high places (only one at work).
From Electric shock?------zero!

By and large, people that work with high voltage stuff for a living are very cautious, but that is different from the rather overwrought approach somewhat prevalent on this forum.
 
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