There are (at least) three possibilities:
- Static electricity.
- Current flow to something grounded
- Capacitive flow.
Most everyone is familiar with static electricity I think. Sparking to a properly grounded electrical box is to be expected. Static charge can be built up and stored on your body in many way, like rolling acrylic wheels of a desk chair across the floor. Or walking on a carpet. After being discharged, if you don't build up another charge, then there is nothing to discharge, so you don't feel anything.
For there to be current flow, there would have to be a potential. It is possible (though unlikely) that the light switch is mis-wired and presents a potential to the screw. If you touch it while also touching something grounded like the desk, a land-line phone, etc. then an electrical path exists through you. If you are not grounded, like when you repeated touching the screw, there is nothing to feel.
A capacitive path is even less likely than the second possibility. I am only aware of it under high voltage situations, such as this video of a flying helicopter being bonded to high voltage. It is current flowing each a.c. cycle back and forth and stored in a conductive object. It stands to reason that if huge sparks occur at, say, 200,000 volts, that a small version could occur at 120 volts using your body as a capacitor. If you are sensitive enough, you might be able to feel it. However, if you use a less sensitive part of your body, then maybe you wouldn't. Electrical sensitivity could well vary millimeter by millimeter across a fingertip. This explanation also requires that there be an (erroneous) potential on the light switch screw.
If the second or third situations exist, there is a wiring fault, so it is good an electrician will be checking it.
Just thought of another one:
A fourth possibility is that there is a strong radio transmitter near by. The capacitance of your body is more of a factor at higher frequency. Maybe you would feel it sometimes but not other times because the transmitter is not continuously on. There isn't really much to do about it.
There needs to be multiple compounding faults for you to get a jolt like that from the tub. The tub should be grounded to earth, but to give you a jolt, it would need to be instead bonded to one of the mains lines. Since your detector is no longer complaining, it must be bonded to the neutral line. While a bad situation, in normal use there are likely no adverse effects from this.
The other thing that needs to happen is a polarity reversal from either the power monitor or one of the plug adapters. For example, if you accidentally got an UK to Argentine adapter, it would appear outwardly to be for Oz, but in fact the neutral and active pins would be reversed. This too is bad, but also usually no adverse effects are apparent.
Now you have active mains power running down the neutral washer wiring, which inappropriately includes the tub. If the tub were properly grounded and active power somehow came in contact with it, the breaker would trip or the fuse would blow. Having been instead bonded to neutral, and now hot due to polarity reversal, two somewhat innocuous errors combine to become a potentially deadly combination!
First identify which device is reversing polarity. Looking at a wall outlet, with the ground pin on the bottom, both UK and Oz outlets have neutral on the right. Anything that switches sides should be destroyed. Not just thrown out, destroyed, as in render unusable.
The washer should be inspected and the connection to neutral identified and removed. It may be simply a wire with worn insulation that needs to be replaced, or something more intentional. It's impossible to say without inspection. The situation has not changed from before. While the tub bonded to neutral cannot ever be called "safe", as long as polarity is observed, nothing really bad should happen. You can still get shocked from this because the neutral and true ground are likely at different potentials, but nothing like the full 220v jolt you got.
Best Answer
This is why GFCI sockets are used in kitchens.
It sounds like you have a bad ground circuit or possibly switched hot and neutral wires on the sockets.
Time to call in an electrician before someone dies.
My congratulations on your survival, we once lived in a house that developed some sort of issue that caused the cold water tap outside the front door to be hot. I found out about the bite and dangers of electricity when I was 5 years old.