I always use a Rcd plug, it is easy to forget that most solution hose's have metal strands going all the way through them.I had a problem with one of my machines a few years back on a job, i kept getting pains in my hand ,thouht nothing of it tbh, thought it was a touch of rsi.When i went to fill up with more water i put my hand in the tank to take out a bit of debris and felt this tingling in my hand.The next day i took my machine apart and found there was a leak through the heating element, i got it fixed it and pat tested.I have no idea why it was only a small shock and i don't think a pat test would of picked it up anyway, but i was very lucky.Rcd plugs are certainly not full proof in my experience, but they are a must, buy the best one you can afford and don't take any risks.