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lethality of HV
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Robert Morein
A friend of mine is afraid to work on a three gun projector, which coincidentally has the same anode voltage as the 7704, 7904, etc, 24 KV.
It is my understanding that this voltage is not lethal, primarily because the CRT is too small to store a lethal charge. Elsewhere, I have read that the lethal threshold, at least as far as TV tubes are concerned, is around 32 inches. Comments? Any reassuring words for my friend? |
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Craig Sawyers
A friend of mine is afraid to work on a three gun projector,It all depends on the current. For an anode accerating system, there is very little (or no) current, so the power supply only has small capacitors in the voltage multiplier. There is maybe 10-20 milliJoules stored. Remember that the human body has a capacitance of 160pF typically, and you get a static charge walking across a rug or stroking a cat of about 10kV in dry conditions. You can certainly *feel* that - it causes me to yelp every time I get out my car in dry weather - closing the door is a real adventure. As to the lethality, I don't know the formal details, but I gave myself a fright with my 7704A once. In a moment of brain fade I unplugged the HT from the CRT (with the scope off), and made the mistake of touching the exposed pin on the connector where it exits from the supply. Youch! Of course the capacitors hold the charge (duur), so you get a nice 21kV belt. It is enough to make you not want to repeat the exercise, but I'm still here to tell the tale. The anode supply in older scopes is far more dangerous, because you have maybe 500V after the rectifiers across around 150uF of smoothing capacitor. That is 9 Joules, and more than enough to send you off to the pearly gates if you grab it with a vengeance. Once the skin is punched through, the body's resistance is only around 100 ohms - so you get 5A flowing from the 500V for 15ms (RC time constant). A rather scary site at t.html says that 5A (but for 1 second) causes "Ventricular fibrillation (the rhythmic pumping action of the heart ceases.) Muscular contraction and nerve damage occur. Death is most likely." I guess that the energy comes into this - so 5A for 15ms is the same as 75mA for 1 second. They say about 75ms "Extreme pain, respiratory arrest, severe muscular contractions. Individual cannot let go. Death is possible." So watch that old tubed gear! Craig |
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When I was in technical school I spent many evenings and weekends repairing
50s TV sets and never had a problem keeping away from the HV. Then I spent 20 years working on solid state electronics, mostly audio. I got quite used to using my fingers as signal tracers. Finally I decided to work on one of the old TV sets. I was amazed at how hard it was to keep my fingers out of the set. I kept wanting to touch a tube socket to see if that would effect the signal. Perhaps the more experience one has with modern electronics, the more dangerous it might be for them to work on an old 500 series scope! Mike Csontos |
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More about static electricity - it takes 5kV to even have a sensation from
it, due to skin resistance, etc. That is why static damage is so easy. Some components like JFETS can be damaged at potentials under 100V. I use an antistatic workstation and wriststrap all the time. If you live in a dry environment there is even more risk of static damage (or have nylon carpets in your shop area). I even have a static locator to identify and remove static generating materials from my "protected area". Some of the worst offenders are packing materials. Very few people use antistatic packing materials. Bubble wrap and styrofoam peanuts (& coffee cups) are the worst. Even your clothing can generate dangerous levels of ESD. Stick with natural fibers. Static damage is rarely immediate - it creates a "walking wounded" component that can fail a year or more out. The carbon generated from the ESD strike can cause a transistor junction to "grow" over time, temperature and other conditions in IC's. Antistatic straps have a 10Mohm resistor built in for safety reasons. In my car I use an insulated key as a lightning rod which lessens the shock immensely. I used to set up and train employees in medical manufacturing companies about ESD control. |
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In the Tektronix book "Biophysical Measurements" book, it states that it
only takes 10mA of current to stop the human heart if a circuit path through it is created (like hand to hand or hand to foot). Most old timers put one hand in their pocket so that there is no path. You will still get an alarming little shock but it won't be lethal. The threshold of pain is about 1 mA. Conversely most pacemakers only generate 10 microamps to pace the heart. Your friend is wise to be afraid. I have worked in more than one shop where a person was electrocuted due to carelessness. One must respect high voltage and electricity 24/7. I also belong to a Hammond organ technical group and it never ceases to amaze me the risks some people take. I am constantly telling people to hire a professional technician if they are not absolutely certain of what they are doing. Vintage tube equipment has many different lethal voltages. You will notice that in almost every manual, even for the TM series, warnings about lethal voltages inside. I know that most of you are seasoned technicians - this is more directed to newbies and persons more used to CMOS and TTL voltages in digital equipment. |
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Craig Sawyers
Most oldOh dear - maybe I now class as an old timer ;-) I learnt respect for hight voltage in the 1970's when I built an experimental pulsed carbon dioxide laser. This thing had an array of high voltage ceramics totalling 20nF charged up to 20kV (4 Joules). In the dark you could see any sharp corner glowing blue with corona discharge. Boxed the thing up in a copper foil lined perspex box with a hefty strap to ground to make sure that any unpleasant event went somewhere safe. And yes - while measuring anything on this lethal beast (using a P6015) one hand was very firmly in the pocket. I nearly gave a colleague a heart attack with one episode - I managed to get the probe tip half way between the top and bottom plate that sandwiched the capacitors - and it arced across top plate-probe-bottom plate with a huge BANG. Gave me such a fright that I yelped, and my colleague ran in with a white face expecting to find me on the floor. It really stuffed the screw-on tip of the probe too.... Although that was bad enough, this thing spat 0.5 Joule pulses of light out (in the infrared) with a pulse length of about 100ns - a peak power of 5MW. Of course over the years I have had the odd lapse of concentration - like the belt from the 7704A HT supply - but fortunately I've got away with it so far. Might have to revisit the care I take now the years are passing. Craig |
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Greyhawk
I Was Working On A High Energy Strobe System On The Bench, Late At Night 26 Years Ago Using A 500v Regulated Supply, I Was Tired, And Had The Supply Set To Max Current, And Voltage. Being Tired, I Had The Single Meter Set To Current (Thought 0 Volts), Clipped The Ground Lead To The PCB, And Held Onto The Ground Lead To Steady The PCB, And Grabbed The Positive Lead. At Full Voltage, And 500 Mils For A Minute (A Lead In Each Hand), After Getting My Thoughts To What Was Happening (Very Difficult Under The Present Circumstances)Stood Up And Walked Backwards, And Pulled The Leads Out. The Doctor Said I Should Have Been Dead On The Spot. I Guess The Great Spirit Was On My Side That Night. I Hope No One Here Has To Learn The Hard Way As I Did, Not Only Should Safety Be A Concern, But Mental Awareness, And Another Person In The Vicinity (In Case). Also. As You Get Tired, Dangerous Mistakes Are Made. I Still Have The Scars On Both Hands 26 Years Later As A Reminder Of My Stupidity.
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Greyhawk ----- Original Message -----
From: Fred Olsen To: TekScopes@... Sent: Saturday, March 06, 2004 14:15 Subject: Re: [TekScopes] lethality of HV Robert Morein wrote: > A friend of mine is afraid to work on ... 24 KV. > Comments? War stories aside for a moment, I think the essence of the question is "afraid". It's my non-humble opinion that his fear is born of inexperience at that level, perhaps also a lack of proper training, and is therefore a 'good thing'. In such situations it's quite common for someone to make a mistake due to the distraction caused by their fear. I would submit that if your friend has no means (or need) to acquire the training, then the experience, which would change the fear to a rational respect - then he has no business messing about where he shouldn't and needs to have it done for him. He's not silly, he's not a coward; he's being quite sensible. Certain situations don't allow for mistakes - not even one. I also have practiced 'one hand only' around high voltage, for something like .......... years. (Hmmm. Where in the world did those number keys go?) Always careful, and I still have the marks to show the effects of an unexpected breakdown, or of waiting six months for an RF burn to heal, or simply of a lapse in concentration. Yes, Craig, we're old. ;<)) Fred -- <>< -- Outgoing checked by Norton AV Yahoo! Groups Sponsor ADVERTISEMENT ------------------------------------------------------------------------------ Yahoo! Groups Links a.. To visit your group on the web, go to: b.. To unsubscribe from this group, send an email to: TekScopes-unsubscribe@... c.. Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service. |
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One of the better (and readily available) devices to painlessly dissipate
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static charge is a quarter coin. I hold on the most of the surface of the coin and touch grounded surface with exposed part of coin. That way, discharge current gets distributed over larger skin area and I do not feel it as a shock, as a matter of fact, do not feel it at all. Regards Miroslav Pokorni ----- Original Message -----
From: "Gary Allsebrook" <regman10@...> To: "Craig Sawyers" <c.sawyers@...>; <TekScopes@...> Sent: Saturday, March 06, 2004 8:44 AM Subject: RE: [TekScopes] lethality of HV Some components like JFETS can be damaged at potentials under 100V.MOSFET is even more sensitive, junction can take more abuse than oxide layer at gate of transistor. Regards Miroslav Pokorni |
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Brian Goldsmith
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----- Original Message -----
From: "Greyhawk" I Was Working On A High Energy Strobe System On The Bench, Late At Night 26 Years Ago Using A 500v Regulated Supply, -- Also. As You Get Tired, Dangerous Mistakes Are Made. I Still Have The Scars On Both Hands 26 Years Later As A Reminder Of My Stupidity. **** Is that why the Shift key gets depressed for each word that is typed? Brian Goldsmith. |
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Twenty years ago, when I worked for Siemens in Munich, after hours, if there
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was no one in the lab we were not allowed to work with anything electrical, that included 5 V lab supply, soldering iron etc. We used to joke that in such circumstances we should put a seat belt while seating in desk chair, in case that someone falls asleep, falls out of chair and gets hurt, but we were only our thirties, still feeling immortal. Regards Miroslav Pokorni ----- Original Message -----
From: "Greyhawk" <greyhawkeng@...> To: <TekScopes@...>; "Fred Olsen" <fwolsen@...> Sent: Saturday, March 06, 2004 11:40 AM Subject: Re: [TekScopes] lethality of HV I Was Working On A High Energy Strobe System On The Bench, Late At Night26 Years Ago Using A 500v Regulated Supply, I Was Tired, And Had The Supply Set To Max Current, And Voltage. Being Tired, I Had The Single Meter Set To Current (Thought 0 Volts), Clipped The Ground Lead To The PCB, And Held Onto The Ground Lead To Steady The PCB, And Grabbed The Positive Lead. At Full Voltage, And 500 Mils For A Minute (A Lead In Each Hand), After Getting My Thoughts To What Was Happening (Very Difficult Under The Present Circumstances)Stood Up And Walked Backwards, And Pulled The Leads Out. The Doctor Said I Should Have Been Dead On The Spot. I Guess The Great Spirit Was On My Side That Night. I Hope No One Here Has To Learn The Hard Way As I Did, Not Only Should Safety Be A Concern, But Mental Awareness, And Another Person In The Vicinity (In Case). Also. As You Get Tired, Dangerous Mistakes Are Made. I Still Have The Scars On Both Hands 26 Years Later As A Reminder Of My Stupidity. ---- Yahoo! Groups LinksService. |
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Ironically it was at Siemens where I was working for when one fellow got
electrocuted. It seems he grabbed on to a 480VAC 3 phase buss while pulling himself up from underneath a linear accelerator. Instant death. Good point about being tired. It's best to come back another day. I have worked for many companies that had the "2 man rule". Of course nothing could save you from 480V 3 phase. A little fear, like fear of God is healthy - respect a requirement. |
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Craig Sawyers
Bet that would blow the gate on a MOSFET ;-)
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Craig -----Original Message-----
From: Rex W. Athey [mailto:mister_twister@...] Sent: 07 March 2004 02:26 To: TekScopes@...; Craig Sawyers Subject: Re: [TekScopes] lethality of HV Speaking of High Voltage - check this out.... and the explanation..... DEFINITELY LETHAL!!!!!!!!! |
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In message <051a01c4034e$adc5cb90$0300a8c0@LianLi>, Robert Morein <morepub@...> writes
A friend of mine is afraid to work on a three gun projector, which coincidentally has the same anode voltage as the 7704, 7904, etc, 24 KV.Well he should be cautious. Isn't the point here that it isn't just the static charge on the scope we are talking about but the capability of the PSU to deliver as no matter how small the energy charge that the tube will hold I'll bet you a pound to a penny that the PSU has more than enough oomph to kill you. I don't blame him. It scares me too. To the point that I have a number of separate meters to dangle round stuff before I power it up (when it is very high volts) so I don't have to dab round any more than necessary. Paranoia I know but there's only one of me and components are easily replaced. Robin It is my understanding that this voltage is not lethal, primarily because the CRT is too small to store a lethal charge.-- Robin Birch |
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