That's a wonderful "fault".
[Alert! Long, boring story follows with homespun advice at the end!]
Have you ever wondered why RCDs are set to trip at 20 or 30mA? Probably not but I'll tell you anyway:
it's generally reckoned (or it was) that 20mA through the heart muscle is enough to be fatal. I understand this is because it causes nerve damage, and afterwards it's extremely difficult to get the heart to beat normally - those paddles beloved of Casualty and other medical dramas on TV are high voltage, but LOW current for this reason. And anyway they are to shock the heart into a normal rhythm if it can still beat, not to actually restart it if it fails. Don't believe what you see on TV!
[Here it comes; don't say nobody warned you!]
Many years ago, in the early 1980s when I worked in broadcasting, RCDs were becoming common (previously they were only used in applications where earths were hard to guarantee). We took delivery in Bristol of an outside broadcast truck ( a "scanner"). It had originally been made as one of a pair for the MOD, but the contract was cancelled and the BBC picked them up instead -- presumably on the cheap. For us it was a small truck, but all the same jammed full of up-to-date TV electronics, and thankfully not a valve in sight anywhere - this, for us, was real progress!
The engineering managers are (were) the people in charge of each OB, responsible for commissioning all the technical arrangements, from microwave links, cameras and recorders right down to the availability of Portaloos (if needed). EMs are also where the buck stops for all aspects of site safety. Although in those days things were a lot more laid back than now, the corporate culture was that anyone, even a junior technical person, could ask for something to be stopped or not done if it was considered unsafe. There was an effective 'safety culture' and the BBC had a very good record as a consequence.
So the Bristol EMs arrived in a gaggle to check over the shiny new vehicle in the garage. One (who shall be nameless) noted that the power distribution board was fitted with 100mA RCDs - not the common 30mA ones.
100mA RCDs? Surely not! Our engineering manager was cross - how could someone have allowed this to get through quality control? He refused to accept it, and talked all the other EMs into agreement: they wouldn't allow the vehicle out on a job until this mistake was fixed.
More junior engineers involved in the acceptance testing tried to explain, but this was a SAFETY MATTER and couldn't be argued with. So new, 30mA RCDs were procured and fitted, and the EMs called back down to the garage for another inspection.
There was a bit of sniggering, as the junior guys knew what was coming: The EM responsible for the fuss was invited to ceremonially switch the vehicle on. "Click-CLICK!" Every time he tried, and he tried for a while, turning on everything necessary in the vehicle caused the RCD to trip out. The vehicle simply wouldn't power-up properly any more.
The junior engineers knew exactly what was going on. The truck had a large number of bits of kit with switched mode power supplies - back then a new approach, but now common in PCs, mobile device and laptop charges, and almost all domestic electronics that are mains powered, simply because they're more efficient than the older designs.
I won't bore you with how they work, but switched mode supplies have a then-unusual characteristic: they often leak very small currents to earth. You can feel this on modern kit (especially Apple stuff in my experience) - touch the metal case of a MacBook or iPad when it's charging and you may get a tingle - not a fault, but a slightly unpleasant 'feature'.
The truck had so many switched mode supplies in it that a 30mA trip was simply too small to work - they pushed the leakage current up to around 75mA when it was fully powered up. Links at Andover made it, and they had been quite right: fitting 100mA RCDs allowed the power supply leakage, but would trip at less than 30mA if there was a real, dangerous fault.
So the original 100mA RCDs were put back, and the vehicle went on to give many years of perfectly safe service, including jobs like the first Antiques Roadshows, the London Marathon, Wimbledon, umpteen golf tournaments, and even the Sidmouth Folk Festival (that was a fun job!).
Morals of the story: (1.) electrical safety requires understanding, not just rote-learned knowledge, so if in doubt don't, but also don't be afraid to question things if you think they're not safe. (2.) 20mA through the heart IS enough to kill you, and it's a tiny current.
Tips to stay safe:
0. DON'T touch anything you suspect. Disconnect it and test it (if you have the knowledge and equipment), or get someone competent to do it for you. As a rule, fuses, breakers and RCDs serve as warnings of fault conditions. If one trips, find out why (as the OP has been doing).
The rest of this list applies in extreme conditions only, such as being in a flood, when you have to make decisions in life-threatening circumstances. In all other circumstances, see (0.) above.
1. Put your LEFT hand in a pocket and only use your RIGHT hand to touch anything you suspect. If everywhere is sopping wet, stand on your right leg only. Why? Your heart is on the left, and any shock you get goes to ground (i.e. your feet). You are less likely to get electrocuted between your right hand and the ground than your left. Many fatalities happen because the victim has one hand well earthed (for example holding an earthed metal case) when they touch the live device, so the current flows along one arm, across the chest and down to the other hand. Guess what's in the middle of that route...
2. If you think something may be live (mains) yet you have to touch it, brush your right index finger's knuckle over it first. If you get a shock your muscles will tense up and pull your hand away. don't use an outstretched fingertip - when the muscles tense, it will cause your hand to make better contact, rather than jerk your hand away.
3. Don't ever grab things with an open hand (see 2). If you get a shock you will be physically unable to let go.
4. When working on electrics don't wear jewellery of any sort. Rings, bracelets and even pendants have killed people who were careless. I have never worn a wedding ring for that reason.
5. If you have ever had a mains electric shock and lived, remember: you didn't survive because it couldn't kill you, or because "they often don't", but merely because your heart wasn't in the right place in the circuit. How much are you prepared to gamble next time?
And finally...
6. Leaving aside the mains wiring itself, some equipment, when faulty, is much more dangerous than you might think. For example, microwave ovens, old-fashioned cathode ray tube TV sets, battery powered flashguns (even the ones inside cameras), and those switched-mode power supplies (of PCs etc.) all have MUCH higher voltages than mains inside them, and store charge at those voltages. These can easily kill, even when they've been switched off and unplugged for some time. And they have done so! Unless you know, don't disassemble things or poke around in broken equipment, just in case.
Hope that's useful -- you never know...
E.
