Safely modify a medical isolation transformer

[jcc]

I’m not an electrician. Not 100% sure if you can leave the isolation transformer floating an be compliant with NEC. But a least it should have several labels or stickers that says: “No equipment ground”

Also, not 100% sure, but if you have a GFCI on the primary side. And you touch two points on the secondary side, or even one point if the secondary is grounded, it will not trip the GFCI.

[james_s]

The NEC regulates the wiring built into your house, it doesn't apply to devices you plug into a receptacle.

[jcc]

If there is no path to ground, the water in the bucket will be at nearly equal potential throughout, making electrocution an impossibility if you do not grasp the live conductors in your hands.

I’m no expert. But electricity takes all paths. Not only the shortest path. Electricity goes to the source. Not ground. On a grounded system the electricity goes to ground because the source is connected to ground. The electrical device will create a voltage gradient in the water. In this case, because there is no ground connection (mains are a grounded system), the voltage gradient will follow the pattern of an electric dipole. It is more complex. Because there are multiple contacts points. But for simplification assume that what is thrown into the water is a cable with ends exposed.

In the end, it depends on several factors if you get electrocuted. Skin resistance, solids in the water that increase conductance (pure distilled water does not conduct), voltage, current, etc.

[Zero999]

Yes, electricity takes all paths, it's just that most of it flows through the one of least resistance.

A complete circuit is required for electricity to flow. In the case of the mains, the neutral side of it is connected to ground, so a person can receive a shock by touching the phase, if another part of their body is touching ground. If an isolation transformer, with a floating secondary is connected to the main, the person won't be shocked if the only touch one side of the secondary, because there isn't a complete circuit.

Now you might ask, why is the mains earthed, if it increase the risk of a shock? The answer is it's done to prevent the mains from floating at a much higher voltage, with respect to earth, in case of a lightning strike, or a higher voltage conductor comming into contact with the mains.

[Doctorandus_P]

A very simple test I often do on my oscilloscope, just to see if it's still working and I've got the probe for the right channel is to touch the probe tip( 1:10, so 10M input) with a finger.

This often measures 70Vac or higher, and significantly changes when I wave my other hand around, or go near some 230Vac wiring such as a desk light.
This is all perfectly normal.

70/1e7 = 7uA which is not much current.

And yes, my oscilloscope is properly grounded.

Just did a little test with my DMM (Wavetek Meterman 35XP)
I connected one of the probes to the Earth lead of a plugged in extension cord, and held the tip of the other probe in my hand. By moving a bit I could measure upto 25Vac.

Same with an Aneng AN8009.