The Danish authorities have undertaken a large study of the realibility of RCDs in which hundreds of installations have been tested. From 2 to 10% of the units were found not to work as intended. Some passed the test button test, but failed to work when tested with proper equipment test. The failure rate was found to increase with age and brand name products were not immune to failure.
A comprehensive report in English is available at:
http://www.sik.dk/content/download/5099/71907/file/HPFI%20rapport%20-%20engelsk.pdf
