What is Resistance Switching?
Resistance switching, also known as shunt resistance, is a method used to interrupt an arc in a circuit breaker by diverting a portion of the current through a fixed resistance (R) connected in parallel with the contact space or arc. This helps to reduce the restriking voltage and transient voltage surge that occurs when the circuit breaker interrupts the current.
When a circuit breaker interrupts the current flow, an arc is formed between the contacts as they are separated. This arc is caused by the ionization of the medium surrounding the contacts, such as air or a gas. The ionization occurs because of the high-energy electrons that are present in the medium, which are generated by the high current flow in the circuit.
As the arc is extinguished, the voltage across the contacts of the circuit breaker increases suddenly and becomes very high, this is called restriking voltage. This high voltage can cause the arc to restrike, resulting in an unreliable interruption of the current. It can also cause severe voltage in the system, which can endanger the operation of the system.
To avoid this problem, resistance switching is employed in circuit breakers with high post-zero contact space resistance. By connecting a fixed resistance in parallel with the contact space or arc, the resistance switching method helps to reduce the restriking voltage and the transient voltage surge.
When a fault occurs, and the contacts of the circuit breaker open, an arc is formed between the contacts. With the arc shunted by the resistance R, a portion of the arc current is diverted through the resistance. This results in a decrease in the arc current and an increase in the rate of deionization of the arc path. This means the arc resistance is increased, leading to a further increase in the current through the shunt resistance R. This process continues until the current becomes so small that it fails to maintain the arc, at this point the arc is extinguished, and the circuit breaker interrupts.
Alternatively, the resistance may be automatically switched in by transference of the arc from the main contacts to the probe contact as in the case of an axial blast circuit breaker. In this case, the time required for this action is very small. Having the arc path substituted by a metallic path, the current flowing through the resistance is limited and then easily broken.
The shunt resistor also helps in limiting the oscillatory growth of restriking voltage transients. The shunt resistance R is added in the circuit to control the restriking voltage and the transient voltage surge caused by the interruption of current. It helps limit the arc current, which limits the restriking voltage and the transient voltage surge.
Advantages of Resistance Switching:
- Reducing the RRRV (Rate of Rising of Restriking Voltage): The shunt resistance helps in reducing the rate of rising of restriking voltage, which is the rate at which the voltage across the contacts increases after the arc is extinguished. This helps to prevent the arc from restriking and ensures a reliable interruption of the current.
- Reducing high-frequency restriking voltage transients: The resistance switching also helps in reducing the high-frequency restriking voltage transients that occur during switching out inductive or capacitive loads. These transients can cause damage to the equipment, and by reducing them, the resistance switching helps to protect the system.
- In a multi-break circuit breaker, it helps in distributing the transient recovery voltage more uniformly across the contact gaps: In a multi-break circuit breaker, the shunt resistance helps to distribute the transient recovery voltage more uniformly across the contact gaps. This helps to prevent damage to the equipment and ensures reliable interruption of the current.
- The resistance switching is not required in the plain circuit breaker because their contact space is low: The plain circuit breaker has a low post-zero resistance of contact space, which means that the resistance switching is not required. Instead, other methods such as vacuum interruption, or SF6 gas interruption are used to interrupt the current.