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| How dangerous or lethal is HV in CRT devices? |
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| EPAIII:
In my younger days one thing I always hated working on was TV sets. They all had CRTs back then. And I have been hit with the residual charge on the CRT a time or three. Not from a working flyback, but just the charge remaining on the tube after it was disconnected. It was wise to discharge it more than once. But those shocks were not too bad and I was OK in a few minutes if not immediately. The thing is, the consumer TVs were deliberately made with a limited current being available from the HV circuit. It only needed to energize the tube which was as much of an insulator for that HV as they could make it. Yes, they thought of consumer safety back in the 1950s. BUT, one time I was working on a professional, broadcast quality TV video monitor. A color one. And it had about 35 or 40 KV on the tube, not the usually lower HVs of the consumer TVs. And probably capable of somewhat more current. In this incident it was ON and I was probing around the flyback circuit and got on the wrong pin. That full Voltage went through the plastic of the probe I was holding, in my hand, out my foot, and through my sock and shoe. I found the burned in, pin holes in both places. My foot, with sock and shoe was resting on a metal foot rest of the work bench, which was grounded. It must have passed through at least a half inch (2 cm) of insulating materials in addition to my body. I felt the effects of that one for three or four hours that day. I probably should have been checked out by medical personnel, but I was young and indestructible. Today, with my 79 year old heart, it would probably kill me. So, dangerous? Yes, and no. For the most part no. But it could be dangerous or even fatal under the right circumstances. And definitely NO FUN! Edit: So ... Been there, Done that, And didn't get the tee shirt. I wonder if I could get one printed; "I survived 40,000 Volts!" Not many people can say that, can they? |
| Psi:
--- Quote from: AntiProtonBoy on January 28, 2022, 05:29:35 am ---The real danger comes from the tube itself --- End quote --- +1 for this. The tube is a large capacitor, HV + Capacitor = danger. |
| dazz1:
--- Quote from: bob91343 on January 28, 2022, 04:37:36 am ---As I remember it, those CRT anode potentials were indeed dangerous. For color TV they were up in the 25 kV range. It doesn't take much capacitance to store a lot of energy at those potentials. And energy is what can get you. Any voltage above around 100V will cause conduction of human tissue, enough to cause problems. As for electrocution, I believe the old electric chairs were around just a few kV. --- End quote --- Hi The upper "safe" limit is about 48VDC. You see this applied in battery packs etc. This reference https://burncenters.com/safety/the-short-term-and-long-term-effects-of-electric-shock-on-the-human-body/ states even 50V can be lethal. Lots of things affect a person's conductivity. One of them is fatigue, which chemically reduces body resistance and increases the risk of doing something stupid. I am super careful with HV. Your first mistake could be your last. I connect probes etc with power off. If there is something I need to do with power on (eg probe something), I do a power off rehearsal of the actions first. |
| Shock:
Reminds me of this video. But seriously... "dangerous to lethal" sums it up perfectly. |
| David Hess:
--- Quote from: Marco on January 28, 2022, 09:37:24 am ---A CRT had a lot less HV capacitance than a mid sized van de Graaff. For an operating device it was probably less dangerous than mains, more likely to knock you off. On the other hand, it might knock you into a mains connection. --- End quote --- I measured some Tektronix CRTs and the post deflection acceleration connection had 10s of picofarads, and discharged rapidly. I do not know about color television CRTs; they are much larger. |
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