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I've lost an old machine or two to legacy CRTs that "snapped". Are there any devices that can be hooked between a computer and a monitor to keep a computer safe in the event that a legacy monitor blows? Something with a fuse in it or something like that?

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What do you mean by "snapped" or "blows"? I've never heard of a CRT monitor damaging a computer aside from some kind of impact damage (e.g. if the monitor is thrown at or dropped on the PC). –  Lèse majesté May 18 '12 at 6:42
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I've never heard of a CRT failure damaging a PC. A CRT flyback transformer can generate 20000 Volts though. It might be cheaper to buy a replacement LCD monitor than to source a device to protect against these sort of voltages. I expect fuses are too slow. –  RedGrittyBrick May 18 '12 at 9:38

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Hmm... over the decades I've had many monitor failures between personal systems and a multi-location company network I manage.

Having one take out a computer??? Never. It'd be a pretty rasty, grotty cheap monitor that'd pass that kind of voltage down the video cable. For the results you describe, your problem's likely a failed ground circuit, which is a safety issue. I'd be more worried about my bodily safety than any fiddling old computer. Make sure the ground wire in the computer & monitor power cables have continuity and that the wall socket ground is functional before worrying about anything else.

No fuse is going to take care of the problem, you're talking about a circuit failure that's somehow, due to bad ground most likely, dumped high voltage into the video circuitry and caused the monitor to use the video cable as its ground via the video card. The VGA signal voltage is 1V or less and it would take some sort of voltage clamping circuitry array that I've never heard of being manufactured to protect your HD-15 connector's pins from passing that voltage into the video card.

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hmm well, as I recall...now that I think about it, the monitor in question that caused this to happen came from :-p Walmart, and was running the basement of my grandmother's house at the time. She had 120V sockets in some parts of the house and 60V in others, it was a pretty old house. Could that have factored into it perhaps? –  leeand00 May 18 '12 at 13:29
    
Were all the sockets two prong only? If there were sockets with the third prong (ground pin), did the electrician installing them actually connect them to ground (third green or bare wire in the romex). I've seen cowboy contractor installs in older homes where they connected the ground pin to the neutral (white wire-bad move) or just left it unconnected (really bad two wire romex installation). All three-wire grounded sockets are supposed to be connected with three wire romex or two wire in EMT conduit with metal boxes and there must be continuity between the pin and the service panel ground. –  Fiasco Labs May 19 '12 at 4:02
    
For a computer and monitor to have proper grounding, they both must be plugged into a three wire socket and the ground circuit must be intact. –  Fiasco Labs May 19 '12 at 4:05
    
And the grounding is what protects the one device from the other, yes? –  leeand00 May 19 '12 at 18:24
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As a final assessment on the situation, make sure the computer, monitor and any equipment that attaches to it are plugged into a quality surge protected power strip and that power strip is connected to a socket with a tested known good ground. The strip will tie all the grounds for the equipment together and the circuit ground will put all their chassis grounds at earth potential. –  Fiasco Labs May 20 '12 at 15:27

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