I've used the test button on a dedicated RCD to power down our barn every
night for years. Never failed yet (famous last words :-)

It could be :-)

I wonder who designed the RCBO test-buttons, they seem designed to
frustrate their usage - lots of travel, near impossible to press (if like me
you do it at arms length, eyes shut, other finger in ear, awaiting blast).
--
Dorothy Bradbury