that girl really waffles on.an rcd monitors current flow between active and neutral.If an imbalance is detected between active and neutral it will mean that this imbalance is travelling to earth.The rcd's sensing equipment detects this leakage current and operates the release mechanism in turn switching off the power supply provided the leakage is equal or greater than the rcd rating ie 30mA
*Many electrical installations have a relatively high earth impedance. This may be due to the use of a local earth rod (TT systems), or to dry local ground conditions. These installations are dangerous and a safety risk if a live to earth fault current flows. Because earth impedance is high, 1. not enough current exists to trip a fuse or circuit breaker, so the condition persists uncleared indefinitely 2. the high impedance earth cannot keep the voltage of all exposed metal to a safe voltage, all such metalwork may rise to close to live conductor voltage. These dangers can be drastically reduced by the use of an ELCB or Residual-current device (RCD). The ELCB makes such installations much safer by cutting the power if these dangerous conditions occur. This approach to electrical safety is called EEBAD. In Britain EEBAD domestic installations became standard in the 1950s. In non-technical terms if a person touches something, typically a metal part on faulty electrical equipment, which is at a significant voltage relative to the earth, electrical current will flow through him/her to the earth. The current that flows is too small to trip an electrical fuse which could disconnect the electricity supply, but can be enough to kill. An ELCB detects even a small current to earth (Earth Leakage) and disconnects the equipment (Circuit Breaker). Go through the source links. Also read and seach for residual current device.
