Electricity is life-threatening. Since it is neither visible nor audible, sensible protective measures and clearly defined standards are important. Therefore, in electrical engineering, the protection classes serve as a classification and marking of electrical equipment for their safety against electric shock.
The protection classes are standardized according to the "Basic safety standard for protection against electric shock" (DIN EN 61140 / VDE140-1).
The protection class should not be confused with the IP protection class. While protection classes describe the measures taken against voltages that are dangerous to touch, the IP protection class refers to the degree of protection of the housing against moisture and contact.
There are four protection classes, with only I to III being approved in the EU.
Protection class 0 has no protection against electric shock other than basic insulation. Therefore, protection must be ensured by the environment of the equipment.
Furthermore, there is neither a symbol nor any other marking for this protection class.
In Germany and Austria, equipment of protection class 0 is not approved, and it is not to be included in the future international standard. The reason for this is on the one hand the lack of further protective measures against electric shock and on the other hand the lack of possibility to connect the devices to a protective conductor system.
Protection class I devices have the most comprehensive safety measures of all protection classes, as they are of double design:
- By the basic insulation of the active parts
- By a metallic sheath, to which the PE (Protection Earth) conductor is connected.
This means that in addition to the basic insulation, grounding is established on the electrically conductive parts. In electrical engineering, this is also referred to as equipotential bonding.
Depending on the device, this can be done in two ways:
via a "Schuko plug" (appliance plug with protective contact)
via a permanently installed connection cable
Here, the PE protective conductor is able to connect all electrically conductive parts of the device to the ground and thus safely dissipate electrical currents.
Devices that are found in protection class I are:
- Electric stoves
- Washing machines
For devices of protection class II, the protection against dangerous body currents is also double:
- by the basic insulation of the active parts
- by an additional protective insulation or at least a reinforced basic insulation
In this way, indirect contact with the active parts is prevented in the event of damage to the basic insulation.
- Vacuum cleaner
- Power tools
- Hair dryer
- Hand lamps
Attention: Conductive parts of devices of protection class II must not be connected to the protective earth conductor (PE conductor).
Devices of protection class III are characterized by a protective extra-low voltage. This may be a maximum of 50V for AC voltages and 120V for DC voltages. For safety extra-low voltages, accumulators or motor generators are used as voltage sources. If the protective extra-low voltage is taken from the power supply network, a safety transformer must be used to ensure safe isolation between the input and output circuits. The cables of protective extra-low voltage circuits must be protected against direct contact by basic insulation and must be laid separately from other circuits. If this is not possible, they must have an additional non-conductive sheath. Protective extra-low voltages below 25 V AC or 50 V DC have a particularly high protective effect. They fulfill both "protection against direct contact" and "protection against indirect contact".
The nominal values are:
- 6 V for certain medical devices
- 12 V for devices used in bathtubs and showers
- 24 V for electrically operated toys
The protective extra-low voltage must not be confused with the functional extra-low voltage. In the case of functional extra-low voltage, as the name indicates, the low voltage is determined by the function of a system or device. Examples of this are metrological equipment, telecommunication and telephone systems, intercom systems, etc.
The main difference compared to the protective extra-low voltage is that with the functional extra-low voltage
- a safety transformer is not required, and
- the circuit may be grounded
Devices used in bathtubs or showers
Electrically operated toys
Caution: Circuits with protective extra-low voltages must not be grounded or connected to the protective earth (PE) conductor!