An RCD or trip switch, is a device to protect people and animals from being electrocuted, by automatically switching off the electricity if there is a fault.
It can also provide some protection against electrical fires. RCDs offer a level of personal protection that ordinary fuses and circuit-breakers can’t provide. Many homes now have a fixed RCD installed on or near the main fuse box.
For added protection, for example, when using power tools outside, a portable RCD can be plugged into a standard socket which you can then plug your appliance into. It is important to remember that portable RCDs only provide protection for the equipment plugged into them.