Editing Residual-current device (section)

==Application==
[[Mains power plug|Electrical plugs]] with incorporated RCD are sometimes installed on appliances that might be considered to pose a particular safety hazard, for example long extension leads, which might be used outdoors, or garden equipment or hair dryers, which may be used near a bath or sink. Occasionally an in-line RCD may be used to serve a similar function to one in a plug. By putting the RCD in the extension lead, protection is provided at whatever outlet is used even if the building has old wiring, such as [[Knob-and-tube wiring|knob and tube]], or wiring that does not contain a grounding conductor.  The in-line RCD can also have a lower tripping threshold than the building to further improve safety for a specific electrical device.

In North America, GFI receptacles can be used in cases where there is no grounding conductor, but they must be labeled as "no equipment ground". This is referenced in the National Electric Code section 406 (D) 2, however codes change and someone should always consult a licensed professional and their local building and safety departments. An ungrounded GFI receptacle will trip using the built-in "test" button, but will not trip using a GFI test plug, because the plug tests by passing a small current from {{Not a typo|line}} to the non-existent ground. It is worth noting that despite this, only one GFCI receptacle at the beginning of each circuit is necessary to protect downstream receptacles. There does not appear to be a risk of using multiple GFI receptacles on the same circuit, though it is considered redundant.

In Europe, RCDs can fit on the same [[DIN rail]] as the [[miniature circuit breaker]]s; much like in miniature circuit breakers, the busbar arrangements in [[consumer unit]]s and [[distribution board]]s provides protection for anything downstream.