It only requires a very small continuous electric current – 40mA (a twenty-fifth of an amp) or more – flowing through the human body to cause irreversible damage to the normal cardiac cycle (‘ventricular fibrillation’) or death (‘electrocution’). When somebody comes into direct contact with mains voltage and earth, the current flowing through the body, is of the order of 230mA (just under a quarter of an amp).
Appropriate protection against serious injury or death calls for disconnection in a fraction of a second (40ms or one twenty-fifth of a second) at 230mA. For lower values of shock current, longer disconnection times may be acceptable but if disconnection takes place within 40ms fibrillation is unlikely to occur.
‘High sensitivity’ RCDs, rated 30mA or even 10mA, are designed to disconnect the supplywithin 40ms at 150mA and within 300ms at rated tripping current to protect the user. ‘Medium sensitivity’ devices, rated 100mA or more will provide protection against fire risks but will not provide full personal protection.