The breaking or cut-off is that electric current , for a switch, a relay - or a contactor contact , a load break switch or circuit breaker specified in the current cutoff.
In the case of fuses ( fuses and circuit breakers ) and circuit breakers, the breaking current (here this is the short-circuit current !) Is also referred to as the switching capacity . The safety device must be able to safely switch off a short-circuit current within its switching capacity without damage to the fuse holder or surrounding components caused by an uncontrolled arc or excessive heat generation.
In the case of inductive loads in particular, operational shutdown represents the more critical change in switching status compared to activation: Inductors store energy proportional to the square of the current in their magnetic field , which when switched off means that the current continues to flow under all circumstances ( self-induction ). If there is no protective circuit ( free-wheeling diode , RC element or spark extinguishing combination, varistor ), it will continue to flow through the opening switch contacts by igniting an arc (see also switching arc ). When switching off with semiconductor components , this overvoltage leads to a breakdown and, without protection, to the destruction of the semiconductor switch .
Switching off leads, especially with inductive loads, to an interference pulse that spreads on the power lines and disturbs other consumers. This can also be reduced with a spark extinguishing combination.
The breaking capacity of switches and contactors is specified for various types of load (resistive load, inductive load). With high currents and voltages, a switching arc can occur even with a resistance load. If protective circuits cannot be used, it must be ensured (especially with direct current or inductive loads) that the switching arc is extinguished (spark extinguishing chambers, hard gas switch, compressed air).
With capacitors, there are no problems with breaking currents or breaking overvoltages.
Shutdown in the event of a short circuit
In the event of a short circuit, fuses and circuit breakers must interrupt the circuit without being destroyed or triggering fires. In the mains short-circuit currents occur by some 100 to some 10,000 amps. The breaking capacity of fine fuses in devices is often too low for this, so that in the event of a short circuit the higher-level miniature circuit breaker is often activated.
Circuit breakers in medium-voltage switchgear usually have to switch off the current if an arc fault occurs in the network, for example due to a lightning strike. The high power that occurs there requires a quick shutdown in order to avoid destruction. In this way, the circuit breaker protects transformers and generators; it must not only be able to switch off the short-circuit current, but also withstand the associated electromagnetic forces. In the case of lower cut-off currents, short-circuiters are sometimes used in conjunction with high-performance high-voltage fuses ( HV HRC fuses ): if an arc fault is detected, the short-circuiter closes and ensures that all three three-phase external conductors are switched off more quickly and completely.
In motor vehicles, on the other hand, the voltage is often only 12 volts, so that the high short-circuit currents here (up to around 1,000 amperes) can also be managed with very simple fuses.
- Peter Pössnicker: "Introduction to fuses." (PDF; 1.5 MB), eska-fuses.de, Eska Friedrich Schweizer, 2007, accessed on January 22, 2016.