I read on the other Electrical forum an old discussion about RCD's and American GFI's. In this discussion it was stated that RCD's do not give Overload protection, however I would like to point out that this is wrong, RCD's DO give overload protection, only basic, but it is there. For example, a 100A 30mA RCD will trip if the difference between the load on the Phase conductors (Live and neutral) exceeds 30mA or if a 30mA or higher current is detected on the CPC (Earth). Further, if the total load being drawn through the RCD exceeds it's maximum rating, then the RCD will trip and disconnect the circuit. However, and this is where many people get confused by this, a standard RCD does this by electronic determination of the load current via the detection circuitry unlike the MCB's and RCBO's. The torus forming the heart of the RCD has a small detection circuit attached to it via an op amp. The magnetic field detected by this circuit is carefully measured and if it exceeds a certain thresh-hold, as determined by the maximum load rating, then it will disconnect the circuit To clarify this, RCBO's combine the operation of the MCB and the RCD into one unit. The RCD part operates in exactly the same way as the standard RCD, however for overload protection the RCBO contains magnetic or thermal trips, in good quality units, such as MK, Merlin-Gerin and Hager, the RCBO contains BOTH thermal and magnetic trips, just as an MCB does. It is these differences that dictate the use of such devices. RCBO's are used to protect individual circuits that need that extra level of safety, but overload protection is still a vitally important aspect, whereas RCD's are useful, in certain circumstances, for replacing Isolators in Consumer units, Distribution Boards etc where effective disconnection may be required in an emergency or for maintenance etc, but overload protection is not required. I hope this clarifies the difference between these devices, a proper understanding of the devices will certainly help to determine their proper use.