The specification of RCD devices is such that they are designed to trip when greater than 30mA of current flows to earth. This current could be flowing through you. If this is the case the RCD trips in a very short space of time, a fraction of a heartbeat, And you are saved from electrocution. There are many devices that leak current to earth, it might only be a few mA. heating elements in kettles, cookers, ovens are all known to leak current. As they get older they tend to leak more. A cooker we had regularly, but not always, used to trip the RCD when cooling down.