PROTECTION AGAINST SHOCK TO EARTH.

Instantaneous medium-sensitivity residual-current earth-leakage (r. c. e. l. ) circuit breakers provide a simple and effective means of protection for inadequate earth-return paths, but their use should be limited to situations in which their operation can cause the minimum of inconvenience, e. g. for domestic installations, for final subcircuits, and for restricted earth-fault protection. Definite minimum-inverse-time-delay earth-leakage devices are the only means by which discrimination can be assured between protection stages when earth faults are restricted by unwanted or intentional impedances. They make possible the limitation of fault power in earth-return paths by restrictive-impedances earthing. High-sensitivity r. c. e. l. protection, preferably at operating currents below no-let-go levels of shock current, or at 20 mA, at which level the restriction of respiration is unlikely, ensures almost complete safety from death from shock to earth in hazardous situations, where any failure of the earthing system or accidental exposure of live conductors might be lethal.