Unfortunately the safety provided by an isolation transformer vanishes the moment you connect a non-isolated scope ground clip to the primary side of the SMPSU under test, and worse, it prevents any upstream RCD/GFCI tripping.
In fairness, the hazard of doing this is hardly different to testing the high voltage secondary circuit of a tube type device using a conventional transformer type supply.
Many thousands of such circuits were probed with normal Tektronix or other probes over tens of years, with vanishingly small numbers of injuries.
One thing,though, is the possibilty of the SMPS having a "common" side which takes up a larger part of the board.
It would obviously be wise to place the earth clip on that side,as the "hot bits" would be smaller & harder to touch. ( Of course, this when using an isolation transformer---just in case some " newbies" missed that bit)
To retain the safety advantage, either use a fully differential HV probe, or an isolated input scope or a CAT II or better battery powered scopemeter, and beware of sneak grounds introduced by other testgear, etc.
Differential probes were, until comparatively recently, too expensive, so Techs had to rely upon a "functioning brain".
Even now, such probes are far from cheap.
Otherwise its live working rules:
- Start with a clear and tidy workbench and make sure there is nothing grounded where you may accidentally come into contact with it.
Really good idea,& hobbyists should embrace it,but realistically, this was pretty much "honoured in the breach" much of the time in workshops.
.[/li]
[li]Remove all rings, bracelets, metal watches, phone or music player headsets etc.
.[/li][/list]
This becomes more important in the current day,where people are wont to festoon themselves with all sorts of crap.
- Keep one hand in your pocket to avoid shocks across your chest.
. - Test before touching!
. - Don't hold the probe in place with your hand.
. - When ever possible switch off before moving the probe.
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All good!
- Use a large heavy insulating mat, checked for damage, to isolate yourself from ground.
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I've seen this done a few times, but quite rarely----it sounds more like what you would do if testing heavy industrial switchboards.
Very few TVs would have ever been fixed.
This is something we would have adhered to if getting into the power supply side of TV or Broadcast Transmitters, but small stuff like Monitors,TVs, etc were regarded as significantly less hazardous.
- Do not work if tired, distracted or intoxicated.
.
Intoxicated is certainly a "no-no",but most Techs are tired most of the time---it's the nature of the job!
- Do not allow pets or young children in the room.
. - Have a clearly marked isolating switch to cut power to the bench.
. - In case of electrocution, make sure that all personnel are trained to cut the power before touching the casualty.
Yes!,Yes!,& Yes!
That's how techs in the TV repair trade worked without killing themselves before GFCIs became common.
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