"Fitted one of these this week and it failed the 5 times test at 180. Went out and got another one which failed in the same way. Scratched my head and began to wonder if I was doing something wrong or if my tester had a fault. Tested an existing RCd and that was fine so called MK technical helpline to be told that there is a known fault with their RCBO's at the moment failing on the 5 times test. Have been advised that those affected have a serial number in the range F1534 to F1552 and F1601 to F1602. Very frustrating and although wholesalers should know about it, they are still selling them!"