Automatic power off is a protection measure, wherein;
— primary protection is provided by primary insulation of live parts or by bulkheads or enclosures, and
- fault protection is provided by equipotential bonding and fault tripping.
In electrical installations, automatic power cut-off is a commonly used protection measure. Where this protection measure is used, Class II devices may also be used.
Protection by automatic power cut-off in electrical installations should be designed and implemented in such a way, that both basic protection is provided in these installations, and damage protection.
In the event of a short circuit between the live part and the accessible conductive part (e.g. the conductive housing of the electrical appliance), the principle of protection in case of damage is, that the expected contact voltage exceeds 50 V alternating current or 120 V DC was turned off so quickly, that there are no dangerous pathophysiological effects. To meet this requirement, any short circuit in an electrical device, as a result of which dangerous touch voltage will appear, it must cause a current flow of a value ensuring the automatic disconnection of the power supply in a sufficiently short time to guarantee the required level of human safety. It follows from here, that auto power off protection consists of:
— stworzeniu odpowiedniej drogi dla prądu zwarciowego, known as a short circuit loop, and
— wyłączeniu prądu zwarciowego przez odpowiednie urządzenie ochronne w dostatecznie krótkim czasie.
The creation of an appropriate short-circuit loop requires the installation of protective conductors connecting all accessible conductive parts of electrical devices:
— z punkiem neutralnym sieci w przypadku układu sieciowego TN,
— z ziemią w przypadku układu sieciowego TT i IT.
Protective devices that automatically switch off the power supply are:
— urządzenia ochronne nadmiarowoprądowe (overcurrent),
— urządzenia ochronne różnicowoprądowe RCD.