Thermomagnetic circuit breakers are equipped with two tripping mechanisms: the temperature-dependent part of the mechanism consists of a bimetal with a heating coil. Currents which exceed the nominal current of the protective device generate heat in the heating wire. The bimetal bends, until it causes the switching mechanism to switch off. The response to overload currents is time-delayed.
The magnetic tripping mechanism consists of a solenoid and a plunging or hinged armature. Currents, which exceed the nominal current of the protective device, generate a magnetic field in the coil. The current is amplified by the magnetic field and therefore attracts the anchor. Once the pre-defined limit value is reached, the anchor activates the trip mechanism and switches the protective device off. The circuit breaker responds to short-circuit currents and overload currents which are too high within three to five milliseconds.
|Nominal current (A)||Internal resistance (Ω)||Nominal current (A)||Internal resistance (Ω)|
1. Power in
2. Power out
12. Normally closed (NC)
14. Normally open (NO)
Thermomagnetic device circuit breakers are generally available with three different tripping characteristics. They are therefore designed to meet all requirements. In the typical tripping characteristic, the individual ranges and functions are displayed as an example.
a = operating range of thermal tripping
b = operating range of magnetic tripping
t = switching time (in seconds)
xl = multiple of the nominal current/tripping factor
1 = current range, for which the characteristic applies
2 = DC tripping range (gray)
3 = AC tripping (blue)
4 = tripping maximum
5 = tripping minimum
The characteristic curve indicates that the thermal tripping [a] responded considerably later than the magnetic one [b]. The reason for this is the time required by the tripping mechanism to warm up, which is dependent on the temperature. However, even lower currents, which flow for longer periods, are detected as overload currents and switched off. The magnetic tripping responds in no time at all to rapidly increasing currents, which exceed the nominal current.
Alternating currents trip faster than DC currents at the same nominal value. This is depicted by the blue area in the curve. This is generally regarded as the behavior for all characteristic curves. Nevertheless, it is only used for circuit breakers with the M1 characteristic curve. Circuit breakers with the SFB or F1 characteristic curve trip just as fast with direct current, so that they would respond sensitively during operation with alternating current. With this in mind, the tripping ranges for alternating currents are not displayed in the SFB and F1 characteristic curves.
Circuit breakers with the SFB trigger characteristic offer maximum overcurrent protection – even in large systems with long cable paths.
Protective devices with this characteristic prevent unnecessary prior switch-off in the event of brief current increases during operation. They simultaneously prevent long, maintained overload currents, which may lead to hazardous generation of heat in operating equipment.
Circuit breakers with the M1 characteristic curve trip later than those with SFB or F1 characteristic curves. They withstand starting currents for longer periods, but respond less swiftly to error situations. Drives blocked in error may be subjected to considerable damage caused by the connected overload current.
In comparison to the direct current characteristic curve, the alternating current characteristic curve is dragged forward to the axis of the nominal current multiple. Even at a lower multiple of the nominal current, alternating currents cause the circuit breaker to trip.
Gray characteristic curve: tripping range for direct currents
Blue characteristic curve: tripping range for alternating currents
These circuit breakers are not intended for protecting drives, which cause temporary starting currents higher than the nominal current. Circuit breakers with the F1 characteristic curve trip rapidly in the case of overloads. During operation, this may lead to frequent, unnecessary switch-offs.
Termination devices, which may become damaged during temporary overloads and slight increases to the operating current, are also protected by these circuit breakers.