We know that a circuit breaker has two operational positions-open and close. During the closing operation, the circuit-breaker contacts close against few opposing forces. We shall discuss those forces in details here. It is very important to know these forces. Again, during the opening operation, the closed contacts travel back as fast as possible.
The operating mechanism a circuit breaker provides optimize force to achieve the opening and closing operations against these mechanical resistances.
The mechanism must operate the circuit breaker very fast, in order to reduce breaking and making times. It must maintain the interval between the instant of receiving trip signal and final contact separation within preferably 30 milliseconds, i.e., 1.5 cycles. Although, for medium and low voltage circuit breakers used in distribution system it may go up to about 3 cycles.
During closing operations, the contact must close fast without hesitation, with adequate contact pressure at the end of contact travel. Otherwise, there may be a chance of contact welding can result. Also, there may be such situation just after closing operating the breaker needs to trip. So, the mechanism must also be capable of giving CO (close-open) operations.
The circuit breaker must close rapidly without hesitation at all currents from zero to rated making current. At the end of the closing operation the mechanism latches till the next tripping signal comes. Obviously, due to high speed of the moving contact there must be a tendency of bounce back by the contact after the closing impact. The CB operating mechanism must provide this feature too. The mechanism must open the contacts without any delay immediately just after receiving the tripping pulse. The mechanism also supports the auto reclose cycle.
Closing Operation
In normal currents, closing of CB contacts does not cause any problem. Here, the operating mechanism only overcomes friction of different parts of the levers and gears. It also provides acceleration the moving masses attached with the moving contact. However, the mechanism has also to close the contacts against a short circuit current during faulty conditions. Here, the moving contact has to overcome the additional thermal and electromagnetic stresses.
During closing operation, the arc initiates well before the contacts touch each other. We call it as pre-arcing. This pre-arcing causes heat thereby causing high pressure in the contact gap. The operating mechanism closes the contacts with sufficient speed to reduce this pre – arcing time. Also, the contacts must close against the thermally generated pressure in the contact gap.
Again, when the contacts close during short-circuit, a large electromagnetic force sets up by the making current. This force pushes back the contacts too. We must design the circuit breaker for a rated making capacity. The highest peak current against which the circuit breaker can be closed at a given voltage. The making capacity depends on the force and speed with which the mechanism closes the circuit breaker.
Opposing Forces during Closing Operation
Electromagnetic Forces between Contacts
When the moving contact touches the fixed contact during short circuit condition the electromagnetic forces appear at the instant of contact touch. The magnitude of this force is proportional to square of the current. This force acts against the closing of the contacts. The mechanism must efficiently close the CB on short circuit condition.
Action of Opening Spring
During closing operation the mechanism charges the opening spring. The trip spring pressure due to charging opposes the closing operation. The moving contacts of circuit-breakers are opened by spring pressure. While closing this spring opposes the closure.
Inertia of Movable Subassemblies
There are numbers of gears, levers arrangement including tie rods in operating mechanisms. The mass of these sub-assemblies possesses its own inertia. This inertia also opposes sudden acceleration of the movable sub assembly parts. The mechanism must be quite cable of overcoming these inertias.
Opposing Forces due to Medium such as Oil, SF6 Gas
The movable sub-assembly including moving contacts moves in the dielectric medium like compressed air/gas/oil. These are kept in high pressure. Also, due to pre – arcing the pressure of these dielectric increases further in the contact gap.
Static and Dynamic Friction
Along with all the opposing force, there will be static and dynamic friction acting on the moving parts of the circuit breaker. This friction may slow down the traveling speed of the assembly.
The operating mechanism has to act against all the above-mentioned forces during a closing operation specially during a fault.
Opening Operation
The opening operation has to accelerate the moving contacts and linkages rapidly to achieve early arc quenching. It must also maintain the desired speed of contact separation. The contact separation speed ranges from 2 to 7 m/s. High speed opening may cause bounce forward of the moving contact after completing of opening. Therefore, the mechanism has to provide sufficient damping at the end of the contact travel.
The charged tripping spring has to supply the entire energy for tripping or opening operation. The operating mechanism has to supply sufficient opening forces by releasing the energy of the tripping spring.
Opposing Forces during Opening Operation
Electromagnetic Forces due to Contact Grip
The fingers of fixed of female contacts tightly hold the moving or male contact. Their grip opposes the sudden movement of moving contact. During the short-circuit condition, the electromagnetic forces tend to increase the grip of the finger-contact assembly. The forces of contact grip increase in proportion to square of current. Hence it is significant during higher short-circuit currents.
Static and Dynamic Friction
The gear-lever assembly with operating tie links, bearing, etc. offer static friction. This friction forces oppose the movement too during opening operation.
Inertia of Movable Parts
The inertia of the moving contact assembly opposes the sudden acceleration during opening. Operating mechanism supplies energy for overcoming the opposition due tom these inertias.
Opposing Forces due to Quenching Medium
It is needless to say, the dielectric medium like compressed air, dielectric oil or SF6 gas also oppose the movement.
The opening operating mechanisms should be capable of overcoming these opposing forces during normal as well as short circuit opening conditions.