While I can provide no numbers, I would say that the majority (probably all)  of houses I have inspected  or worked on have never had the RCD test button pressed except by an electrician when installed or working on the premises. I usually have to explain what it is to the resident and tell them to test it when they change the clocks (twice per year). I am not sure how many ever do. I do find the rare failure when testing, either of the test button or not tripping  with fault currents, rarely do the test times fail. Many properties old and new have the consumer units hidden in cupboards full of stuff or in inaccessible positions so they are difficult to access which increases the probability of them never being touched.


Due to the requirement to use metal consumer units in domestic premises there are issues about protecting supply tails from earth faults on TT installations especially if the consumer unit is some way from the supply. Many of us install upfront 100mA time delayed RCDs to protect the tails, don't tell anyone but we often put these in plastic enclosures as it can be argued it is not a consumer unit,  and provide a backup in the event of a downstream 30mA RCD failing to operate on a fault. There were some very long threads on the old forum about this. Some like to use a 100mA S type RCD as the main switch on dual RCD or RCBO based consumer units as a backup on TT systems. 


TT has been relatively rare here except in the countryside but the introduction of electric car charging points and the requirement for TT has identified the problems of retro fitting TT onto premises with PME supplies and the issues about trying to maintain separation from two earthing systems in domestic and some commercial situations.

While I can provide no numbers, I would say that the majority (probably all)  of houses I have inspected  or worked on have never had the RCD test button pressed except by an electrician when installed or working on the premises. I usually have to explain what it is to the resident and tell them to test it when they change the clocks (twice per year). I am not sure how many ever do. I do find the rare failure when testing, either of the test button or not tripping  with fault currents, rarely do the test times fail. Many properties old and new have the consumer units hidden in cupboards full of stuff or in inaccessible positions so they are difficult to access which increases the probability of them never being touched.


Due to the requirement to use metal consumer units in domestic premises there are issues about protecting supply tails from earth faults on TT installations especially if the consumer unit is some way from the supply. Many of us install upfront 100mA time delayed RCDs to protect the tails, don't tell anyone but we often put these in plastic enclosures as it can be argued it is not a consumer unit,  and provide a backup in the event of a downstream 30mA RCD failing to operate on a fault. There were some very long threads on the old forum about this. Some like to use a 100mA S type RCD as the main switch on dual RCD or RCBO based consumer units as a backup on TT systems. 


TT has been relatively rare here except in the countryside but the introduction of electric car charging points and the requirement for TT has identified the problems of retro fitting TT onto premises with PME supplies and the issues about trying to maintain separation from two earthing systems in domestic and some commercial situations.