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In Reply to: RE: That's very dangerous posted by Triode_Kingdom on October 25, 2016 at 08:46:35:
Actually, I was being a little flippant when I spoke of the back-to-back rectifiers saving the day if the amplifier were plugged into a reverse-wired socket. Yes, they almost certainly would, but of course actually I would never depend on that as the way of checking that the socket was not mis-wired. In fact, I checked, double checked and triple checked that the socket was wired correctly before I plugged the amplifier in. I have full control of the amplifier, where it is plugged in, and into which socket, in my own house, and there would be nobody else touching it or messing with it.
So for the chassis or audio ground to become live it would require two simultaneous major independent failures (neutral becoming disconnected and also ground becoming disconnected).
One could compare this with some conceivable failure modes in a more typical amplifier configuration. A common set-up involves keeping signal ground separate from true chassis ground to avoid hum loops, with, say, a 100 ohm resistor between signal and true ground. Suppose now that an insulation breakdown occurred in the main HT transformer in the amplifier. This could lead to direct mains voltage being connected through to the signal ground. The puny 100 ohm resistor would be of no use at all in preventing the signal ground from being brought up to full mains AC voltage.
Or suppose a CD player with a cheapo imported all-voltage wall-wart power supply is plugged into the amplifier. If the insulation broke down in the HF transformer in the wall-wart, again the signal ground could be brought up to full mains voltage. Does one have so much faith in the quality of construction of cheap imports?
(Personally, in my transformered amplifiers I always use big back-to-back rectifiers to ensure that signal ground cannot deviate from true ground by more than 1.5 volts or so, for precisely such reasons. But not everyone does.)
All of the above scenarios are highly improbable failure modes, and any one of them could lead to mains voltage appearing where it could be a potential hazard.
It does not seem to me that the level of hazard in a sensibly operated direct mains device is totally off scale in comparison to the other examples.
Chris
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Follow Ups
- RE: That's very dangerous - cpotl 13:42:29 10/25/16 (0)
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