Overcurrent protection device
The overcurrent protection device , also referred to as an electrical fuse or OCP (= Over Current Protection), interrupts an electrical circuit if the electrical current exceeds a specified current strength for a specified time. There are different versions of overcurrent protection devices such as the fuse or the circuit breaker . All electrical fuses in a house or apartment are usually housed together with other circuit breakers in a distribution box or fuse box.
Overcurrent devices protect lines or other equipment from damage due to excessive heating that would result from the overcurrent flowing over a longer period of time. An overcurrent can be caused by an overload or a short circuit . Overcurrent protection is mandatory according to the standard.
The response time of an overcurrent protection device with different currents is called characteristic and is represented in time-current characteristics . There are overcurrent protection devices which, for safety reasons, remain in the interrupted state indefinitely after tripping and are only put into a functional state by manual actuation or by replacing the protective device, as is the case with the fuse by replacing the fuse cartridge. In addition, there is also the automatic reclosing function , which is used in industrial systems, in energy networks and other more automated areas. This is divided according to certain criteria such as switch-off time or number of repetitions.
The different types of overcurrent protection devices result from the intended use. This includes:
- Safety fuse , with tripping currents from a few tens of mA to a few kA, for example as a device protection fuse and in low-voltage networks .
- Electronic fuse , mostly part of electronic circuits, for example integrated in power supplies such as switched-mode power supplies . In some cases an automatic reclosing function is available.
- Self-resetting fuse , typically a so-called PTC thermistor , a form of load-dependent resistance
- Circuit breaker , an electromechanical overcurrent device, especially in the low voltage range.
There are also special applications:
- Motor protection switch for motor controls.
- Motor protection relay here only thermal tripping is used
- Circuit breakers in combination with network protection in high-voltage networks .
- Automatic traction device as the main fuse for the motors in trams.
Dimensioning and protection of lines and cables
The current carrying capacity I r of a cable is mainly determined by the conductor cross-section and the type of installation . However, it changes in the event of accumulations and if the ambient temperature of 30 ° C specified in DIN VDE 0298 is exceeded or not reached. An ambient temperature of e.g. B. 25 ° C results in a current carrying capacity for PVC - insulated cables
Selection of the overcurrent protection device
To prevent the overcurrent protection device from tripping when the system is fault-free, the nominal current of the protection device must be at least equal to or greater than the operating current of the consumer.