There is a risk of electric shock in all electrical devices, whose operating voltages exceed the voltage values recognized as permissible. In such cases, it is necessary to apply appropriate technical means of protection against electric shock. The basic means of protection against electric shock are measures, which prevent direct contact between humans and the live parts of the electrical circuits, and thus prevent shock current from flowing through the human body. Technical means of protection against electric shock, whose task is to prevent shock current from flowing through the human body, they are called primary protection or protection against direct contact.
Basic protection, although it significantly reduces the risk of electrocution, this, however, does not completely eliminate it. During the operation of the devices, there are disturbances in their normal operation (usually consisting in damage to the basic insulation), as a result of which voltages appear on the accessible conductive parts. Therefore, it is necessary to apply - in addition to the basic protection - additional measures, whose task is to reduce its effects in the event of an electric shock hazard. Technical means of protection, whose task is to reduce the effects of electric shock when you touch the live ones (as a result of damage to the insulation) conductive parts available, they are called protection against damage or protection against indirect contact.
The effects of an electric shock depend primarily on the intensity of the shock current and the time of its flow. Therefore, action, the aim of which is to reduce the probability of harmful consequences of destruction, consists in the application of appropriate inducing measures:
— zmniejszenie wartości prądu rażeniowego,
— skrócenie czasu przepływu prądu rażeniowego.
The shock current is reduced by reducing the touch voltage or increasing the shock circuit resistance. When doing this nothing gives any positive results, measures are taken to ensure that the shock current is switched off sufficiently quickly.
According to the PN-HD standard 60364-4-41 in AC electrical appliances with a voltage up to 1 kV and direct current to 1,5 kV electric shock protection should be provided by the application:
— środka ochrony podstawowej i niezależnego środka ochrony przy uszkodzeniu lub
— wzmocnionego środka ochrony (e.g. reinforced insulation), which will provide both basic protection, and damage protection.
The protection measures admitted to use are:
- automatic power off,
— izolacja podwójna lub izolacja wzmocniona,
— separacja elektryczna do zasilania jednego odbiornika,
— bardzo niskie napięcie SELV lub PELV.
If certain conditions of the protection measure cannot be met, additional measures should be taken yes, that the combined protection applied reaches the required level of security. An example is FELV circuits, in which the voltage reduction alone is not sufficient to ensure the required degree of safety.