lethal voltages. - Page 1
Forum section: electronics / repair

author: m3vuv
author_rank: Super Contributor
timestamp: October 30, 2019, 01:18:28 am
content: 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?.

author: tggzzz
author_rank: Super Contributor
timestamp: October 30, 2019, 01:27:30 am
content: Quote from: m3vuv on October 30, 2019, 01:18:28 amTryaddingsomeconventionalpunctuationsoitisreadable.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.

author: andy3055
author_rank: Super Contributor
timestamp: October 30, 2019, 01:34:43 am
content: 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!

author: tggzzz
author_rank: Super Contributor
timestamp: October 30, 2019, 01:48:34 am
content: Quote from: andy3055 on October 30, 2019, 01:34:43 amI 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.

author: TheMG
author_rank: Frequent Contributor
timestamp: October 30, 2019, 01:53:10 am
content: 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.

author: m3vuv
author_rank: Super Contributor
timestamp: October 30, 2019, 02:25:32 am
content: 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!!

author: vk6zgo
author_rank: Super Contributor
timestamp: October 30, 2019, 02:51:50 am
content: Quote from: TheMG on October 30, 2019, 01:53:10 amSorry, but this is a myth, the capacitance of a CRT is quite small.QuoteI'm not sure what circumstances they may be--- maybe if you are standing on a ladder holding the tube?QuoteI 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.

author: Psi
author_rank: Super Contributor
timestamp: October 30, 2019, 03:12:33 am
content: 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.

author: andy3055
author_rank: Super Contributor
timestamp: October 30, 2019, 03:27:28 am
content: 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

author: george.b
author_rank: Frequent Contributor
timestamp: October 30, 2019, 03:27:59 am
content: 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), 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.

author: m3vuv
author_rank: Super Contributor
timestamp: October 30, 2019, 04:02:27 am
content: that was an interesting read!,cheers.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 30, 2019, 08:39:37 am
content: Quote from: TheMG on October 30, 2019, 01:53:10 amIs the case of the CRT capacitor is the dielectric the vacuum or the glass?Quote from: vk6zgo on October 30, 2019, 02:51:50 amVery 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.

author: Gyro
author_rank: Super Contributor
timestamp: October 30, 2019, 12:37:11 pm
content: Quote from: shakalnokturn on October 30, 2019, 08:39:37 amNo, 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.

author: dom0
author_rank: Super Contributor
timestamp: October 30, 2019, 12:58:34 pm
content: 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.

author: shakalnokturn
author_rank: Super Contributor
timestamp: October 30, 2019, 03:01:28 pm
content: 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...

author: m3vuv
author_rank: Super Contributor
timestamp: October 30, 2019, 07:13:13 pm
content: I normaly don my brown trousers when working on the pda/eht side of things!!

author: janoc
author_rank: Super Contributor
timestamp: October 30, 2019, 08:17:36 pm
content: Quote from: m3vuv on October 30, 2019, 07:13:13 pmThere 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 arenasty. 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).

author: jogri
author_rank: Frequent Contributor
timestamp: October 30, 2019, 09:35:16 pm
content: 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.

author: jdragoset
author_rank: Frequent Contributor
timestamp: October 30, 2019, 10:47:05 pm
content: 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)

author: vk6zgo
author_rank: Super Contributor
timestamp: October 31, 2019, 12:04:56 am
content: Quote from: jogri on October 30, 2019, 09:35:16 pmAnd 40+ years later, you are still waiting for the effects to kick in!

author: mcovington
author_rank: Regular Contributor
timestamp: October 31, 2019, 04:57:30 am
content: 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?

author: andy3055
author_rank: Super Contributor
timestamp: October 31, 2019, 06:37:45 am
content: 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!

author: james_s
author_rank: Super Contributor
timestamp: October 31, 2019, 07:09:56 am
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: October 31, 2019, 09:40:44 am
content: Quote from: mcovington on October 31, 2019, 04:57:30 amFor one anecdote, seehttps://www.eevblog.com/forum/testgear/ground-of-oscilloscope-always-connected-to-earth/msg1967150/#msg1967150

author: vk6zgo
author_rank: Super Contributor
timestamp: October 31, 2019, 10:36:52 am
content: Quote from: mcovington on October 31, 2019, 04:57:30 amI'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.

author: tggzzz
author_rank: Super Contributor
timestamp: October 31, 2019, 11:46:50 am
content: Quote from: vk6zgo on October 31, 2019, 10:36:52 amCounter-conclusion: if they had been as careless/ignorant as many posts on this forum indicate, there would have been deaths.Analogy: running out into the road without looking isn't necessarily lethal, but it does significantly increase the probability of becoming a KSI statistic.Similarly, lack of KSI amongst those that look firstdoesn'timply there are no problems associated with not looking.We teach our kids how toavoiddangers they are too young to recognise. We should teach newbies how toavoiddangers they don't realise exist.

author: mcovington
author_rank: Regular Contributor
timestamp: November 01, 2019, 01:11:16 am
content: Quote from: andy3055 on October 31, 2019, 06:37:45 amPoor fellow. I wonder in what condition he survived. He might have had brain damage. Do you know whether he recovered to lead a normal life?

author: mcovington
author_rank: Regular Contributor
timestamp: November 01, 2019, 01:16:38 am
content: So we have accounts of fatalities from the AC line and probably from power-supply outputs in the over-100-volt range that can deliver tens of mA or more.None, so far, from CRTs; despite the higher voltage; the current is lower.Of course, I do not want anyone to experience an electric shock. One thing my doctor told me a while back is that after any non-trivial electric shock, there can be aftereffects on the heart due to the body's chemical response, and you should go to the emergency room even if you seem to be OK.

author: 0culus
author_rank: Super Contributor
timestamp: November 01, 2019, 01:20:03 am
content: The account says he was gone, so I assume he did not survive? Odd choice of language though, as above it says he nearly died?I think another factor is that truly high amperage hazards (utility side of your electric, industrial electric service, electric railroads, etc) are usually also arc flash hazards...and arc flash will definitely mess you up, and often is fatal blow in those situations, as it were.

author: james_s
author_rank: Super Contributor
timestamp: November 01, 2019, 01:38:01 am
content: A friend of mine worked in an old building about 10 years ago and an electrician who came out to service something was electrocuted and died on the spot. Again not CRT related but line voltage, either 208 or 480V 3 phase I'm not sure.As I said earlier, I think it's highly unlikely for the EHT in a CRT device to kill a person, but there are "low" voltages that are still sufficiently high to be dangerous and can deliver a lot of current if one does not know what they're doing.

author: 0culus
author_rank: Super Contributor
timestamp: November 01, 2019, 01:51:25 am
content: Yeah. A service like that will easily deal more than enough fatal current before the circuit breaker or fuse is even breaking a sweat. Hence, GFCI and arc fault breakers standard in household wiring. On the industrial or utility side of things...there's no fuse and the system will happily deliver massive current to ground through you with no problems and create arc flash situations.

author: mengfei
author_rank: Regular Contributor
timestamp: November 01, 2019, 02:16:56 am
content: I had an ARC jump on me from an Old CRT TV I was repairing.I was reaching for a wire at the back of the Flyback X-former close to the anode cap, as I was reaching in & about an inch or two from the anode cap an ARC jumping out of the wire, could have had a crack, close to the cap hit my wrist which I slammed to the side of the tv case. There was a slight burn but the pain was from the slammingSo I "guess" although that pumps out around 25KV+ the current is very very small to make your heart stop.

author: andy3055
author_rank: Super Contributor
timestamp: November 01, 2019, 03:44:53 am
content: Quote from: mcovington on November 01, 2019, 01:11:16 amNever heard from him after that. He went back to Australia and the other guy came to complete the work. Just before he left, he came on site to pick up his stuff. I bet he recovered well. If I remember he was in his early 20s. Skinny fellow!

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 02, 2019, 12:58:53 am
content: Beware of bulk capacitors in SMPS charged to 300-400V they store enough energy to shake you for seconds.The capacitor known to be a "frequent" killer is the one in microwave ovens when the bleeder resistor goes open.Although not as tricky as ground referenced AC, that most people beware of when they actually understand the risk, capacitors or secondary side DC have no RCD/GFCI.As mentioned before the risk after a severe enough DC shock is aftermath "poisoning" from electrolysis.

author: dom0
author_rank: Super Contributor
timestamp: November 02, 2019, 05:21:53 pm
content: Quote from: mcovington on November 01, 2019, 01:16:38 amEnergy is much lower, too. The output caps of a HV multiplier may hold something like 20-30 mJ, meanwhile a good sized SMPS input capacitor will hold several Joules. Discharging the anode lead is supposed to produce a "zap", meanwhile, if you short the input caps on a higher power (100+ W) SMPS, which is not recommended, it will produce a real BANG.

author: james_s
author_rank: Super Contributor
timestamp: November 03, 2019, 05:33:14 pm
content: One of the Electrohome monitors that was very common in arcade games of the early 80s has a second fuse after the bulk filter capacitor. When the flyback or HOT fails it will normally blow this fuse leaving the capacitor fully charged. I have been bit a couple times by one of those, even though it's "only" 170V it packs an enormous punch.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 04, 2019, 12:08:42 am
content: Quote from: james_s on November 03, 2019, 05:33:14 pmElectrohome design is to say the least, idiosyncratic!Back in the day, I discovered another way to get "zapped", when digging round in the guts of a washing machine.This machine had quite a large oil filled capacitor connected across the Mains during use.----I am vague about its purpose, it was many years ago.I turned the washer off, unplugged it from the ac Mains, & delved confidently inside.Anti-intuitively, I "copped" a nice shock.It turns out that mechanical switches are quite fast compared to the 20ms occupied by the 50Hz cycle, so the power can easily be cut off part way through a cycle, charging the cap to whatever polarity & amplitude the cycle is in at that instant."Sod's Law" decrees that it won't be at a zero crossing!I knew special relays could be very fast--- changing the RF drive on the old Marconi TV transmitters only caused the loss of a few lines on the transmitted picture, but I always thought ordinary switches were fairly leisurely in action!

author: james_s
author_rank: Super Contributor
timestamp: November 04, 2019, 02:10:05 am
content: Despite a few oddities in the design, the Electrohome G07 is a dependable monitor that is easy to work on and makes a good picture when they are dialed in. I have a few of them in games and while I've had to replace most of the original flybacks (LOPT) I have not had any trouble again with any that I've fixed.The only purpose I can think of for a large capacitor across the mains is power factor correction. Odd that it wouldn't have been right across the motor though in which case the motor windings would quickly discharge it.

author: mengfei
author_rank: Regular Contributor
timestamp: November 04, 2019, 02:18:20 am
content: ...Oh & one of the small lethal device that could give you a BIG shock are those E-caps around camera flash! those have a rating of 400 Volts!

author: EHT
author_rank: Frequent Contributor
timestamp: November 04, 2019, 09:26:31 pm
content: QuoteSo I have a related question - what is the lowest voltage which we should consider dangerous? Has anyone had a surprise shock from a circuit they though was low enough to be safe, but were wrong?I think 50V is usually referred to so I tend to go by that. We know 110V is high enough to travel through anyone careless enough to touch it; what about 40-80V range? Supplies at this voltage are often pretty high current ...

author: shakalnokturn
author_rank: Super Contributor
timestamp: November 04, 2019, 10:22:49 pm
content: I remember getting tingled, not a nasty shock, from VCR VFD supply.The books I learnt my basics from stated the "safe" voltages at 50V DC and 24V AC if I remember correctly.The "high current" source always makes me frown, it usually reveals a misunderstanding of the danger. The example I have heard the most is the car battery being dangerous due to the amount of current it can deliver, true, it is dangerous in some respect due to that. Yet I'm still waiting for someone to get electrocuted on a car battery...The availability of current brings other risks, if 10mA is considered the safe limit before becoming a life hazard high current sources are much lower current than expected...For most work I won't put my fingers in until I understand the equipment well enough, then when I do know the risks I'll go to the trouble of discharging capacitors that will add-up to more than 30V. That means most audio amplifiers are considered dangerous.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 05, 2019, 12:33:41 am
content: Quote from: shakalnokturn on November 04, 2019, 10:22:49 pmYes it seems that Ohm's Law is a new & startling concept to some people, including many who would be expected to understand it!Quote

author: james_s
author_rank: Super Contributor
timestamp: November 07, 2019, 08:34:58 pm
content: I've never heard of anyone being electrocuted by a car battery but they can certainly be dangerous. My dad has a scar on his finger from a ring that got very hot almost instantly from a short between a wrench and car body.