This is a topic to warn you about failing differential circuit breakers in your homely electric installation.
We all (must) know that the test button in those switches is there to perform a monthly test to see if the circuit breaker is working. My word of advice is to distrust this procedure and go a bit further based upon my recent experience. I, like most of us, refrain to perform the monthly test and only do it once in a blue moon or simply when it comes to mind. The other day I was lurking youtube and found one talking about a simple differential switch tester (a Triac, a Diac, 4 resistors, 3 capacitors, a potentiometer, a SDPT switch and a 50mA AC analog panel) with which you can test not only if it works but "when" it works, I mean, how many mA are needed to trip the typical 30mA differential switch we all have at home for our security.
The fact was that I had a couple of "Very Expensive Super Immunized 30mA Differential Switches With Automatic Reconnection" at home from a "Very Reputable Brand", both installed on independent branches of the electrical grid at home and with no more than 4 years of use. Both of them pass the test button test OK but one didn't work at all as a differential switch
The one in my house tripped the switch at 25mA (very common for a 30mA breaker) and auto reconnected like a charm but the other in my "den"
didn't trip at all, not even at 50mA... although, as I said before, it passed the button test perfectly . This is the one responsible of protecting my work bench where I do all things electric and electronic, my garage and my mechanical shop
My word of warning is to be aware that the "monthly test button test" may not be definitive and your safety may be in danger: Place a load of between 15mA to 30mA between the live and earth and see if the differential switch go down.
Sorry if this is not the correct place to put this thread. Feel free to move it to a better place if necessary.
[EDIT] I'm talking about a GFCI Breaker or Ground Fault Circuit Interrupter.