Protection against Electric Shock
Protection against Electric Shock
The purpose of protection against electric shock is to prevent devices under voltage from causing electric shock to their users. The manufacturer of the device is responsible for designing these protective measures. This must also take into account faults where parts of the device that are normally not live become live, for example a washing machine with a casing that gives an electric shock due to a fault to the body.
Protection against electric shock is divided into three classes:
Class I Protection
This is a protective method using an earthing conductor, which protects by automatically switching off the power supply in the event of a fault. The plugs of such devices also have two contacts for the protective earth network. These devices may only be used in electrical systems equipped with protective earthing. Otherwise, in case of a fault, they can cause a life-threatening electric shock!
The earthing conductor or protective earth network is now an essential safety element in electrical systems, but this was not always the case. In older houses and flats, Class I devices are still used without being earthed. The earthing conductor literally connects the casing of these devices to the ground via a wire and an earth rod driven deep into the soil. Where it is not possible to drive an earth rod, such as on the 6th floor of a block of flats, it is still possible to create a perfectly functioning earthing network.
WARNING! Modifying the network without proper qualifications can lead to life-threatening accidents!
If a Class I appliance fails and its casing becomes live — in other words, a fault to the body occurs — the earthing conductor directs the voltage to the ground. The residual-current device (RCD) detects the fault current and cuts off the circuit. In the Hungarian TN electrical system, such a fault to the body also counts as a phase-to-neutral short circuit, and due to the high amount of fault current, the circuit breaker will also trip, cutting off the circuit.
Examples of appliances with this type of protection: washing machines, hobs, ovens, refrigerators, larger power tools, metal floor lamps, desktop computers.
Class II Protection
This is a protective method without an earthing conductor. The device is made with double or reinforced insulation and has no protective earth terminal, so the protection is independent of the electrical network.
The casing of such equipment, made of electrically insulating material (plastic), will not become live even in the event of a fault, and will not cause electric shock when used according to the instructions. If their casing or power cable is damaged, they must not be used further!
Examples of appliances with this type of protection: hair dryers, smaller kitchen mixers, plastic night lamps, small radios, power supplies for larger devices, mobile phone chargers.
Class III Protection
This class includes devices operating on extra-low voltage, with a maximum of 50 V AC or a maximum of 120 V DC supply. Touching extra-low voltage does not cause electric shock.
Such devices must not be earthed, because their power supply provides complete separation from the electrical network. If they were earthed, a fault in another device could cause a voltage higher than the extra-low voltage to appear on them.
Examples of devices in this class: children’s toys, modern televisions and computer monitors, laptops, 12 V lamps, LED strips.
