When people are talking about voltage potentials between well-functioning electronic equipment, such as oscilloscopes and PC computers, for instance, they are frightened because they can measure a high voltage with a sensitive multimeter. There can be a considerable voltage between their respective chassis. However, it is only a very small AC current, too small to cause any injury, it can be somewhere in the high micro-amps, but yes, it sometimes it can cause a mild electrical shock that should be considered more unpleasant rather than dangerous. If there were any dangerous AC current, the GFI in my house would instantly kick open the AC circuit in the room.
When using my digital and analog oscilloscopes, voltmeters and the likes under these situations, (which I have been doing for over 25 years doing electronic design and repair work.), I always connect the ground connection of the oscilloscope or other test instrument first to the D.U.T. This means that the probe tip of the oscilloscope or signal generator, etc. is at the same ground potential before I connect a signal or a meter or an oscilloscope probe and can do no harm to the D.U.T.
I am not a lone maniac who chooses to work in this way, hundreds of other technicians in service shops in this tiny country operate in this very same way.
In fact, almost every house in this country has 2-wire AC plugs and people connect things together all the time without any injuries to themselves or their equipment, thanks to GFI. Once in a while people bring in equipment that they have burned out an USB port because of connecting equipment where there are power ground loops. I always remind people to unplug at least one of the devices when connecting USB. But even different AC grounded circuits in the same room can have a nasty low-voltage AC but with a high-current that can also damage equipment and cause a tickle.
I haven't had any damage to any equipment that I've serviced, nor to myself, nor to the oscilloscopes and other test equipment in my work area. I have been occasionally been tickled a bit by the bite of this inter-chassis AC potential.
Before I plug anything in that I might be asked to repair or modify, I always make visual check of the equipment for rust or corrosion or other water damage or just check if the equipment shows any dampness, water drops, etc. If I am unsure of the equipment, I use a DVM to check the AC power cord for resistance to the chassis ground before plugging it in. That is just common sense. Electronics is about working with electricity and can be dangerous, so caution must be a thing to learn and practice. I am not against grounding equipment if it is something like an electric washing machine, a refrigerator or a high-powered circular saw that is not double-insulated.
On the other hand, having a grounded oscilloscope on my workbench could put me in great danger. It could allow me to get a serious shock from some other piece of equipment that has a AC mains short to the chassis if I am touching the metal ground of the scope and the chassis or some live AC mains wire in the circuit of some piece of correctly working or even defective or unsafe equipment. I wouldn't be in mortal danger if the oscilloscope was ungrounded.[/b]
For safety's sake I would not allow any three wired grounded outlets around my work bench test area.
In servicing thousands of devices, I can recall only two or three that ever had a AC mains short to ground. These few items could have been really dangerous, but a visual inspection and a DVM resistance check revealed the danger of the problem before I even turned them on. One was a miswired 3-wire AC EU-style line cord the other was due to someone trying to repair the item and connecting one side of the AC line to the chassis.
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