RCDs (residual current devices), also commonly called safety switches, are designed to cut the power in a fraction of a second in the event of a fault condition, thereby preventing electrocution. The electrics in your house may be too old to have RCDs installed, or you might want to use a power tool outdoors on an outlet that doesn't have an RCD fitted - use this handy portable one instead. Buy one right now if you don't have RCDs already - it's a lot cheaper than a funeral.
Note: RCDs are not a substitute for common sense around mains electricity. While they will work in most situations, they may not in some cases and are not a substitute for current overload devices such as fuses and circuit breakers. Wiring, appliances, leads and power outlets must always be maintained in a safe condition.
