As mentioned previously, circuit breakers can detect a fault either thermally or magnetically. The magnetic hydraulic circuit breaker is an all round device and can operate in many different areas. The magnetic hydraulic circuit breaker can be divided into two different categories; sealed and unsealed breakers. Sealed breakers, made for currents below 20 A, are less dependent of the surrounding environment.
The trip of a magnetic hydraulic circuit breaker is achieved by a solenoid. As the current flows through the breaker, it also passes through a coil which creates a magnetic force. When a fault current occurs, the magnetic force of the solenoid increases until it reaches a certain level where a sear is trigged and opens the breaker. This level is called “the ultimate trip current” and it is the lowest current that still guarantees a break.
The most usual at Ericsson Power Solutions is to use a time delayed magnetic hydraulic circuit breaker that make it susceptible to some current transients. To time delay a magnetic hydraulic circuit breaker the coil is wound on a sealed, non ferrous tube which is filled with a silicone fluid. Inside the tube lies a movable core, held under the tension of a spring. At normal current the magnetic field is not strong enough to move the core. During an overload or a transient, the strength of the increased magnetic field attracts the core towards an end piece. If the over current lasts long enough or the transient is big enough, the core has moved all the way to the end piece and thereby magnetizing it. The trigger then is seared by the magnetic force and the circuit breaker trips. see Figure 2.5 and 2.6.
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| Figure 2.5: Cross section of a magnetic hydraulic circuit breaker |
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| Figure 2.6: Delay function of a magnetic hydraulic circuit breaker |
If a short circuit occurs the current rushes and the magnetic leak flux from the coil will magnetize the end piece instantaneously (before the core has been pressed against it) making the sear trip the circuit breaker immediately. This current is referred to as the “instantaneous trip current”. The trip current is varied by the number of turns in the coil and not affected by variations in temperature but since the silicon liquid gets more viscid as the temperature decreases, the trip time will vary with varying temperature. Instantaneous tripping is independent of ambient temperature.
To influence the time-delay, liquids with different viscosity and springs with varying stiffness are used. Because of gravity, the mounting of a magnetic hydraulic circuit breaker effects the trip time. If the coil is placed correctly, with the core horizontal, the gravity is of a very small significance. But if a magnetic hydraulic circuit breaker is placed with the core vertical or at any angle, gravity will affect the trip time.
At high over currents, the magnetic hydraulic circuit breaker only depends on the current which often is considered to be an advantage. In complement to a thermal circuit breaker the magnetic hydraulic circuit breaker does not have to cool down between two ruptures. Another advantage is that a magnetic hydraulic circuit breaker has a lower voltage drop compared to a thermal circuit breaker.
As for a melting fuse, selecting a magnetic hydraulic circuit breaker also means that important parameters must be considered. Even though it is in general the same parameters, there are differences in how to deal with them; a basic understanding of parameters for magnetic hydraulic circuit breakers follows.