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Thermal (T- and TA-Line)

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Overload protection by thermally operated CBEs

Fig. 1 Thermal only CBE

Fig. 2 Contact force versus deflection

  1. Latch-type mechanism

  2. Spring-type mechanism

Thermal circuit breakers for equipment, CBEs, (figure 1), simulate the electrothermal behaviour of the protected components (conductors in wiring, motors, transformers, etc.) by a simple, but very clever device: The thermo-bimetal.

This mechanical element can simulate the heating effect of the current, can transform electric energy into a motion (deflection) and trigger a mechanism to cause automatic interruption of the current which produces these effects.

To use the heat created by the current instead of the magnitude of the current itself offers a great advantage, because heat determines the admissible stress of the insulation and the admissible duration of the various overload conditions encountered in practical applications.

Thermally operated CBEs, therefore, take good care of the surplus energy required for start-up or high-torque operation of motors. They cope well with high inrush spikes which occur in switching power supplies, transformers, tungsten filament lamps, etc. and avoid nuisance tripping due to such transients.

The CBEs of the T-Line use a «latch-type» mechanism. High contact force can be maintained until the unit trips. This prevents electrical «noise» due to contact bounce and reduces the risk of contact welding which may occur with spring type mechanisms (figure 2).


The strong points of thermal CBEs are:

  • Good simulation of the thermal behaviour of the protected component

  • Capability of coping with start-up and inrush currents

  • Suitability for a wide range of frequencies

  • Simplicity / reliability

  • Favourable price

Thermally operated CBEs are temperature sensitive. This, in most applications, is an advantage because the withstand capacity of the component to be protected is almost always temperature sensitive, too. The variation of the operating characteristics of thermal breakers with ambient temperature is closely matched to the admissible thermal stress of PVC insulations. For other insulations, the matching is not as close but the tendency exists, in principle, in any application where the protective device and the component to be protected are operating in an environment of practically identical ambient air temperature.

Thermal CBEs can, to a certain degree, be adjusted to special requirements concerning the withstand capacity of the protected item.

Their delay time can be influenced in several ways. The task may be achieved by using a different method of heating the bimetal. Figure 3 illustrates two methods.

The most widely used method is the direct heating of a bimetal strip by the internal losses produced by the current passing through the bimetal (example A). Where such losses are insufficient to produce enough heat and to cause sufficient deflection, a heater winding is wrapped around the bimetal strip to obtain the required heat. Since the heat has to pass through an insulation before it reaches the bimetal, a time lag will occur and a delayed action will result (example B).

The typical tripping zone of thermal CBEs is shown by figure 4. It changes with ambient temperature in a similar way as the withstand characteristic of a PVC insulated wire does (figure 5). The possibilities can be extended by using a shunt terminal as shown in figure 6.

The shunt terminal provides a parallel switched circuit to the main current sensing circuit.

Fig. 3a Simulation by bimetals (directly heated)

Fig. 3b Simulation by bimetals (indirectly heated)

Fig. 4 Typical tripping zone

Fig. 5 Range of protection

Fig. 6a Circuit diagrams - standard version

Fig. 6b Circuit diagrams - shunt terminal

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