General information circuit breakers
Fig. 1 Thermical CBE
Fig. 2 Contact force versus deflection
Circuit breakers for equipment, CBEs, provide protection against hazards of electricity in equipment. For the TA45-line «Protection» includes the safeguarding against harmful thermal effects of overcurrents and the prevention of accidents caused by electricity.
Overcurrent protection is achieved by the automatic interruption of sustained overcurrents with the help of a thermal release tripping the CBE when the duration of an overcurrent exceeds a predetermined limit. The essential part of such a release is a thermo bimetal (figure 1, figure 6a). 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.
Bimetals can also handle frequencies in a fairly wide range, e.g. from DC to 400 Hz, without necessitating any change in ratings or characteristics. The CBEs of the TA45-line use a «latch-type» thermal release. 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).
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.
Thermal magnetic CBEs have two releases to achieve automatic interruption of an overcurrent (figure 7):
A thermo-bimetal for overload current
An electro magnet for short circuit current
Consequently, the operating characteristic is essentially composed of two zones, linked by a zone (3) where either one or the other mode of tripping will be effective (figure 8).
The electro magnet should be dimensioned so that it will not trip during transients likely to occur in the intended application. This determines the level of the current below which instantaneous tripping should not occur.
The upper level, indicating the current above which instantaneous tripping must occur, is of interest in considerations concerning the selective action of two protective devices.
In the short circuit range of overcurrents (above 8....12 times the rated current), the faster interruption obtainable with the magnetic release is an advantage. It can help to save the heater windings of indirectly heated bimetals from overheating and it can improve the breaking capacity of the CBE. The CBEs primarily intended for overload protection are usually capable of interrupting, without back-up assistance, currents up to 100 to 300 amps and be fit for further use after such an interruption. The performance at higher fault levels usually relies on back-up assistance by fuses or breakers.
Fig. 7 Thermal-magnetic CBE
Fig. 8 Tripping zones of thermal magnetic CBEs
Thermal mode of tripping
Magnetic mode of tripping
Either thermal or magnetic mode
The prevention of accidents can be achieved in several ways. To safeguard persons from the possible risks of injuries arising from an unexpected restarting of an electric motor when the voltage recovers after a power failure, undervoltage releases can be fitted to the basic CBE. This release will trip the CBE when the voltage drops below a certain level. The restarting requires a manual ON operation.
Fig. 3 Undervoltage release
Fig. 4 Mechanical lock-out latch
Undervoltage releases can be combined with overcurrent releases in one integral unit. The TA45-line utilizes a special version of an undervoltage release as illustrated by figure 3. It differs from the conventional version by using an additional latch, reducing the anlatching force significantly. The release can thus be operated with far less power and utilize rectified AC to avoid any humm while the CBE is in the ON position. The wiring diagram is shown by figure 6b. Typical examples for the use of undervoltage releases are floor cleaning machines, high pressure cleaning equipment etc.
To prevent injuries caused by dangerously exposed moving parts of a machine, a mechanical lock-out latch can be fitted to the basic CBE. A spring loaded pin will cause the CBE to trip when a protective cover is removed from dangerous parts, like the cutting knifes of a shredder. The CBE can not be switched ON as long as the protective cover is not in its place. Figure 4 shows the operating principle. Figure 6c shows the wiring diagram.
Protection may also be necessary when at a remote location a dangerous situation occurs which could escalate if the CBE did not interrupt the current. To avoid such a risk, a remote trip release ca be fitted to the basic CBE to achieve tripping on sensor command. The operating principle is shown by figure 5, the working diagram by figure 6d.
Fig. 5 Remote trip release
The various possibilities of combining different protective functions is also reflected by the wiring diagrams as shown in figure 6.
Fig. 6a shows the wiring diagram fo the basic CBE, with one protected pole. The TA45 can be outfitted with two protected poles for additional safety against faults to earth.
Fig. 6e shows the more complex diagram, utilizing a shunt connection (P1-5) and a change over auxiliary contact.
The wiring diagrams for CBEs with an undervoltage release, a mechanical lock-out latch and a remote trip release are shown by 6b, d, and c.
CBEs of the TA45-line are available with rocker or push button actuators and protective covers to obtain the desired degree of protection.
The strong points of the TA45-line are:
Thermal overload protection
Remote trip release
Mechanical lock-out latch
3 pole version
Push button actuation
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