The PAT Test Test done properly requires you to plug the mains cable of the item under test into the PAT test machine, and then you manually take a ground test cable attached to the machine and attach this via its crocodile clip or whatever to the metal chasssis of the item under test.
The machine then sends often several 10's of amps from the ground pin of the items mains cable, through the chassis, and back to the PAT test machine, whilst measuring the resistance of this loop.
For safety reasons, you would want this loop to be a very low resistance one, meaning the earth from the items mains plug is securely bonded to the chassis, and can take a large current and still maintain the bond, and people won't die.
This current can't really flow through anything else other than this loop.
This is the procedure for a Class 1 device, i.e.one that has a mains earth on the mains plug, and a metal chassis that could, under fault conditions, become live and dangerous.
A Class 2 device that doesn't have a mains earth connection or a metal chassis that could become live and kill you, is not tested with this method, it is tested with a CLass 2 method that doesn't involve sending 10's of amps through the unit under test.
If you are a Numpty, you might attempt a Class 1 test on a Class 2 device and send 10's of amps down the earth connection of an RCA socket or XLR, and blow stuff up.
Don't know if you do PAT tests in your world, but it basically involves a machine that might put several 10's of amps between the mains earth and the chassis of the unit under test.
This is to test that the mains earth has a good connection with the chassis, so should a fault occur that put mains on the chassis, you won't die.
Wouldn't it take out all the things which are common to mains earth, like the micro controller etc...?
Now that you mention its a very likely scenario that there could've been a fault at the mains ground connection. That's pretty common, but also in those cases I see more damage done on different parts of the circuits.
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