blueport

I know this is an extremely old post & I only discovered it by accident doing a search. However, I just have to comment on this as stated above: 'a short to earth (ie a person touching active and neutral)' This is incorrect in a couple of ways & I hope it was just down to a perhaps rushed answer. First, an RCD's purpose is not to detect a 'short' to earth, although it certainly will. A circuit breaker or fuse is designed for that purpose. For a start, a person caught in an 'earthed' situation is not considered to be 'shorted', which has the connotation of being a direct short to earth .i.e minimal resistance. A person has much resistance & is just another load on a circuit to a fuse or circuit breaker, hence they will not trip & even if they did, never in time to save a life. Secondly, & most importantly, an RCD will NEVER trip when a person is connected between active & neutral if that person is NOT touching earth. i.e if you are working in a roof space & lying across timber trusses working on a 'live' circuit & manage to get hooked up between active & neutral & cannot disengage from the connection ( i.e. get belted off by the shock like I have on 2 major occasions), on an RCD protected circuit...you are dead. It is all over for you. An RCD will never trip in this instance as it monitors the current out v's current back in between the active & neutral. In this case it will be equal as you are across active & neutral. If you have some leakage to earth then the RCD will trip depending on its rating & the leakage. I will detect that not all current is coming back on the neutral therefore the difference must be going to earth & it will trip. In most cases an RCD will save you because most electrocutions are in earthed situations. They have saved many lives. I just thought that this needed clarifying. Cheers.