Editing Residual-current device (section)

==Limitations==
A residual-current circuit breaker cannot remove all risk of electric shock or fire. In particular, an RCD alone will not detect overload conditions, phase-to-neutral short circuits or phase-to-phase short circuits (see [[three-phase electric power]]). Over-current protection ([[fuse (electrical)|fuse]]s or [[circuit breaker]]s) must be provided. Circuit breakers that combine the functions of an RCD with overcurrent protection respond to both types of fault. These are known as RCBOs and are available in 2-, 3- and 4-pole configurations. RCBOs will typically have separate circuits for detecting current imbalance and for overload current but use a common interrupting mechanism. Some RCBOs have separate levers for residual-current and over-current protection or use a separate indicator for ground faults.

An RCD helps to protect against electric shock when current flows through a person from a phase ({{Not a typo|line}} / hot) to earth. It cannot protect against electric shock when current flows through a person from phase to neutral or from phase to phase, for example where a finger touches both {{Not a typo|line}} and neutral contacts in a light fitting; a device cannot differentiate between current flow through an intended load from flow through a person, though the RCD may still trip if the person is in contact with the ground (earth), as some current may still pass through the persons finger and body to earth.

Whole installations on a single RCD, common in older installations in the UK, are prone to "nuisance" trips that can cause secondary safety problems with loss of lighting and defrosting of food. Frequently the trips are caused by deteriorating insulation on heater elements, such as water heaters and cooker elements or rings. Although regarded as a nuisance, the fault is with the deteriorated element and not the RCD: replacement of the offending element will resolve the problem, but replacing the RCD will not.

RCDs are not [[Selectivity (circuit breakers)|selective]], for example when a ground fault occurs on a circuit protected by a 30 mA I<sub>Δn</sub> RCD in series with a 300 mA I<sub>Δn</sub> RCD either or both may trip. [[#Break_time_(response_speed)|Special time-delayed types]] are available to provide selectivity in such installations.

In the case of RCDs that need a power supply, a dangerous condition can arise if the neutral wire is broken or switched off on the supply side of the RCD, while the corresponding {{Not a typo|line}} conductor remains uninterrupted. The tripping circuit needs power to work and does not trip when the power supply fails. Connected equipment will not work without a neutral, but the RCD cannot protect people from contact with the energized wire. For this reason circuit breakers must be installed in a way that ensures that the neutral wire cannot be switched off unless the {{Not a typo|line}} conductor is also switched off at the same time. Where there is a requirement for switching off the neutral wire, two-pole breakers (or four-pole for 3-phase) must be used. To provide some protection with an interrupted neutral, some RCDs and RCBOs are equipped with an auxiliary connection wire that must be connected to the earth busbar of the distribution board. This either enables the device to detect the missing neutral of the supply, causing the device to trip, or provides an alternative supply path for the tripping circuitry, enabling it to continue to function normally in the absence of the supply neutral.

Related to this, a single-pole RCD/RCBO interrupts the energized conductor only, while a double-pole device interrupts both the energized and return conductors. Usually this is a standard and safe practice, since the return conductor is held at ground potential anyway. However, because of its design, a single-pole RCD will not isolate or disconnect all relevant wires in certain uncommon situations, for example where the return conductor is not being held, as expected, at ground potential, or where current leakage occurs between the return and earth conductors. In these cases, a double-pole RCD will offer protection, since the return conductor would also be disconnected.