Editing Residual-current device (section)

===Differences in disconnection actions===
Major differences exist regarding the manner in which an RCD unit will act to disconnect the power to a circuit or appliance.

There are four situations in which different types of RCD units are used:

# At the consumer power distribution level, usually in conjunction with an RCBO resettable circuit breaker;
# Built into a wall socket;
# Plugged into a wall socket, which may be part of a power-extension cable; and
# Built into the cord of a portable appliance, such as those intended to be used in outdoor or wet areas.

The first three of those situations relate largely to usage as part of a power-distribution system and are almost always of the ''passive'' or ''latched'' variety, whereas the fourth relates solely to specific appliances and are always of the ''active'' or ''non-latching'' variety.  ''Active'' means prevention of any ''re-activation'' of the power supply after any inadvertent form of power outage, as soon as the mains supply becomes re-established; ''latch'' relates to a ''switch'' inside the unit housing the RCD that remains as set following any form of power outage, but has to be reset manually after the detection of an error condition.

In the fourth situation, it would be deemed to be highly undesirable, and probably very unsafe, for a connected appliance to automatically resume operation after a power disconnection, without having the operator in attendance{{dash}}as such, manual reactivation of the RCD is necessary.

The difference between the modes of operation of the essentially two different types of RCD functionality is that the operation for power distribution purposes requires the internal latch to remain set within the RCD after any form of power disconnection caused by either the user turning the power off, or after any power outage; such arrangements are particularly applicable for connections to refrigerators and freezers.

Situation two is mostly installed just as described above, but some wall socket RCDs are available to fit the fourth situation, often by operating a switch on the fascia panel.

RCDs for the first and third situation are most commonly rated at 30{{nbsp}}mA and 40{{nbsp}}ms. For the fourth situation, there is generally a greater choice of ratings available{{dash}}generally all lower than the other forms, but lower values often result in more nuisance tripping.  Sometimes users apply protection in addition to one of the other forms, when they wish to override those with a lower rating. It may be wise to have a selection of type four RCDs available, because connections made under damp conditions or using lengthy power cables are more prone to trip-out when any of the lower ratings of RCD are used; ratings as low as 10{{nbsp}}mA are available.