Editing Residual-current device (section)

=== RCD with integral overcurrent protection (RCBO) {{anchor|RCBO2}} ===
<!-- Courtesy note per [[WP:RSECT]]: [[RCBO]] and others link here -->
[[Image:FI-offen.jpg|thumb|upright=1.4|right|Opened three-phase residual-current device]]

Residual-current and [[overcurrent_protection|over-current]] protection may be combined in one device. Such a device is termed an RCBO (residual-current circuit breaker with over-current protection). In the US and Canada such devices (in the form of circuit breakers) are known by the term ''GFCI circuit breaker'' (lacking an American-specific acronym for this combination and seemingly taking for granted the presence of over-current protection in any circuit breaker type device). They are effectively a combination of a RCD and a [[Circuit_breaker#Arc interruption|MCB]].<ref>{{cite web |title=RCBOs {{!}} RS Components |url=https://sg.rs-online.com/web/c/automation-control-gear/circuit-protection-circuit-breakers/rcbos/ |website=sg.rs-online.com |accessdate=15 June 2020}}</ref>

As well as requiring both {{Not a typo|line}} and neutral inputs and outputs (or, full three-phase), some RCDs/GFCIs require a functional earth (FE) connection. This serves to provide both EMC immunity and to reliably operate the device if the input-side neutral connection is lost but {{Not a typo|line}} and earth remain.

For reasons of space, many devices, especially in DIN rail format, use flying leads rather than screw terminals, especially for the neutral input and FE connections. Additionally, because of the small form factor, the output cables of some models (Eaton/MEM) are used to form the primary winding of the RCD part, and the outgoing circuit cables must be led through a specially dimensioned terminal tunnel with the current transformer part around it. This can lead to incorrect failed trip results when testing with meter probes from the screw heads of the terminals, rather than from the final circuit wiring.

Having one RCD feeding another is generally unnecessary, provided they have been wired properly. One exception is the case of a [[Earthing system#TT_system|TT earthing system]], where the [[earth loop impedance]] may be high, meaning that a ground fault might not cause sufficient current to trip an ordinary circuit breaker or fuse. In this case a special 100{{nbsp}}mA (or greater) trip current time-delayed RCD is installed, covering the whole installation, and then more sensitive RCDs should be installed downstream of it for sockets and other circuits that are considered high-risk.