Pre-insertion Resistor (PIR) in Circuit Breaker

In electrical power systems, circuit breakers play a crucial role in safeguarding the network by interrupting fault currents and isolating faulty sections. However, the rapid interruption of high fault currents can lead to severe transient overvoltages, stressing the system and the circuit breaker itself. To mitigate these overvoltages and enhance the reliability of circuit breakers, Pre-insertion Resistors (PIRs) have been developed as an effective solution. In this article, we will explore the concept of Pre-insertion Resistors and understand how they contribute to the smooth operation and longevity of circuit breakers in power systems.

Pre-insertion Resistor (PIR)
Pre-insertion Resistor (PIR)

What is Pre-insertion Resistors (PIRs)?

A Pre-insertion Resistor (PIR) is a specialized resistor placed in parallel with the main contacts of a circuit breaker. It is designed to limit the rate of rise of transient recovery voltage (TRV) during the opening operation of the circuit breaker. The TRV is the voltage that appears across the breaker contacts after current interruption, and its rapid rise during fault clearance can lead to severe overvoltages, which can damage the breaker and other connected equipment.

The primary function of a PIR is to provide a controlled path for the fault current to discharge during the early stages of the opening operation, thereby slowing down the rate of rise of TRV. This controlled discharge helps prevent overvoltages and ensures a smoother interruption process.

Working Principle of Pre-insertion Resistors:

When a fault occurs and a circuit breaker is required to open, the main contacts start separating. At this moment, the Pre-insertion Resistor comes into play. Initially, the PIR carries a fraction of the fault current as the contacts begin to open. This controlled current flow through the PIR creates a voltage drop across the resistor. As a result, the TRV across the main contacts is limited, ensuring a gradual rise rather than an abrupt surge.

Once the main contacts are fully open, the fault current is entirely transferred to the arc-quenching medium (e.g., vacuum, SF6, etc.) within the circuit breaker. The Pre-insertion Resistor is bypassed, allowing the breaker to function normally after the fault has been cleared.

Advantages of Pre-insertion Resistors:

  1. Transient Overvoltage Mitigation: The most significant advantage of PIRs is their ability to suppress transient overvoltages during the opening operation of the circuit breaker. By limiting the rate of rise of TRV, PIRs protect the breaker and connected equipment from voltage stresses that could lead to insulation failures or equipment damage.
  2. Extended Circuit Breaker Life: By reducing the severity of overvoltages, PIRs contribute to prolonging the operational life of the circuit breaker. Lower voltage stresses result in less wear and tear on the breaker contacts and insulation, increasing its reliability and reducing maintenance requirements.
  3. Improved System Reliability: PIRs enhance the overall reliability of power systems by ensuring smoother and controlled breaker operations. This helps prevent unnecessary outages and reduces the risk of collateral damage during fault clearance.
  4. Cost-Effective Solution: Installing Pre-insertion Resistors is a cost-effective approach compared to other methods of mitigating transient overvoltages, such as using complex circuit configurations or specialized voltage suppression devices.


Pre-insertion Resistors (PIRs) are invaluable additions to modern circuit breakers, addressing the challenges posed by transient overvoltages during fault clearance. By controlling the rate of rise of transient recovery voltage (TRV), PIRs protect the circuit breaker and connected equipment from damaging voltage stresses, leading to improved reliability and longevity. As power systems continue to evolve, PIRs will remain a key component in enhancing the stability and efficiency of circuit breaker operations, making them an indispensable tool for power system engineers and operators alike.

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