Electrical – Does voltage in the PC’s case indicate a circuit grounding problem

A few things that can cause this:

1. Faulty iMac supply.
2. "Hot fault" in the wiring... A bad junction somewhere, or a physically damaged wire, or wires that are too small for the current draw (and the fuse or circuit breaker!).
3. Exotic interference. Is there an electronic switch or timer, or a low-quality UPS, or one of those infernal "SpySmart Meters" somewhere?

Note that for items 1 and 2, you should see a voltage drop -- measured at the outlet that the iMac is plugged in to. A drop from 120V to 110V yields roughly a 16% drop in incandescent brightness. A drop from 120V to 100V, causes about a 31% drop in brightness.

What iMac model is it? What specs does it list for power? iMacs usually run at about 80W to 200W, but I read once that they can have short, peak draws of 1000W.

A surge protector will do nothing in this case, but a battery backup with brown-out protection will protect the computer and may lesson the flickering.

If it is a "hot fault" type problem, there is a very real danger of fire. A fuse or circuit breaker cannot guard against such faults either, but you should immediately verify that the circuit breakers are properly sized for the wire gauge(s) of each circuit (the smallest wire gauge, if more than one was used).

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.