What's in the switches that fails??
They do NOT have to be replaced every 10 years. If they were made by Leviton Manufacturing Co., and are older than 23 years, they should be tested at least every month, AND IF THEY TRIP, SHOULD BE TESTED AFTER THEY WERE RESET. To test the GFCI, push the "Test" button. The GFCI should click, and they should NOT supply power to the outlet. Next push the "Reset" button and power should now be present at the outlet.
IF THERE IS POWER AT THE OUTLET AFTER THE “TEST BUTTON” IS PUSHED, THEN THE GFCI SHOULD BE REPLACED BECAUSE IT IS NOT PERFORMING ITS SAFETY FEATURE.
If they were not made by Leviton, I do not know if the other companies are using the newer electronic circuit that Leviton invented (around 1996). This new circuit will NOT ALLOW THE GFCI TO SUPPLY POWER IF THE SAFETY FEATURE IS NOT WORKING. However it is a good idea to test the GFCI regularly anyway.
As to what fails, the electromagnet that caused the device to trip is designed to trip in less than 25 thousands (0.025) of a second. In order to accomplish this, and still have the product fit into a wall outlet box, we overload the electromagnetic coil. The power to the coil turns off after the device ice trips which usually happens in about .016 seconds. If there is something that would keep the coil from burning out after it trips, then the GFCI will fail to perform as required.
This type of acceptance of a possible failure is similar to the electric starter motor in a car. If you would build a starter motor that could run continually, it would have to be built larger than the gasoline engine. A GM engineer whose last name is Kettering invented this. He is one half of the pair who supplied lots of money to Sloan Kettering.
One time I was sent two GFCI's that came in from a trailer campsite. I took them apart and found they were full of dead ants. I think that the ants were attracted to the GFCI either because the internal powder supply emits a small amount of heat, or we used a varnish to keep the printed circuit board dry and the varnish could have been an ant delicacy.
I think the ants were electrocuted when a group of them made a conductive path between the two power wires. The others might have also been electrocuted,or the heat was enough to wipe them out too