1. Circuit Analysis of the Trip Lockout Circuit The electrical trip lockout circuit is usually implemented by a trip lockout relay. Figure 1 is a wiring diagram of a trip lockout circuit applicable to a circuit breaker with one trip coil. The trip lockout relay TBJ has a current-starting coil TBJ/I, a voltage-holding coil TBJ/U, two pairs of normally open contacts TBJ1, TBJ4, and two pairs of normally closed contacts TBJ2, TBJ3. TBJ/I is connected to the trip coil circuit of the circuit breaker, and TBJ/U is connected to the closing circuit of the circuit breaker. TBJ1 is used for current self-holding. TBJ2 and TBJ3 are connected in parallel and then in series with the closing circuit. Figure 1 Electrical Trip Lockout Wiring Diagram When the trip relay TJ operates to initiate the trip, TBJ/I is energized, TBJ operates, TBJ1 closes to hold the trip command until the circuit breaker opens, and at the same time TBJ2 and TBJ3 open the closing circuit, TBJ4 closes, and the voltage self-holding circuit of TBJ is prepared. If the manual closing relay contact SHJ or the automatic reclosing contact ZHJ closes before the circuit breaker trips, i.e., before TBJ returns, then TBJ will remain open via the already closed TBJ 4 and SHJ or ZHJ, ensuring that TBJ2 and TBJ 3 will not close, thus achieving the purpose of tripping and locking. 2. Technical Requirements for Trip-Lockout Relays 2.1 Current Starting Value According to the 1984 regulations of the Ministry of Electric Power Industry on anti-accident measures and the design code for secondary circuits of power systems, the current starting value of the trip-lockout relay should be coordinated with the tripping current of the circuit breaker. Its current starting value should not exceed 50% of the circuit breaker tripping current, meaning that the current in the tripping circuit should be greater than twice the starting current of TBJ, ensuring that the reliability coefficient of the TBJ current is greater than 2. 2.2 Voltage Drop of the Current Coil According to the above regulations, the voltage drop of the current coil of the trip-lockout relay should be less than 5% of the rated voltage of the operating circuit. 2.3 Voltage Operating Value According to the regulations, the voltage operating value of the trip interlocking relay should not exceed 70% of the rated DC voltage of the operating circuit, ensuring reliable operation of the TBJ when the operating DC power supply voltage fluctuates within the specified range. Simultaneously, the TBJ voltage operating value should not be less than 50% of the rated DC voltage of the operating circuit to ensure that the TBJ does not malfunction when the operating DC power supply circuit is grounded. 2.4 Contact Performance The contact performance of the TBJ should be the same as that of the intermediate relay at the output in the relay protection device. Power industry standards stipulate that the contact performance of the intermediate relay at the output in the relay protection device should meet the following requirements: Return characteristic: return value ≥ 10% of the rated value (for reed relays, the return value should be ≥ 70% of the rated value); Closing capacity: DC circuit 220V, 5A; Mechanical life: 105 operations without load; Contact resistance: ≤0.1Ω when measured with a milliohm meter; ≤0.5Ω when measured with a digital multimeter; ≤0.1Ω when measured using the current-voltage method. 2.5 Insulation Performance a. When the same group of contacts is open, it can withstand a power frequency voltage of 1000V for 1 minute; b. Between conductive parts with no electrical connection, it can withstand a power frequency voltage of 2000V for 1 minute; c. Between all conductive parts and the mounting frame, it can withstand a power frequency voltage of 2000V for 1 minute. 3. Composition of the Trip Lockout Relay Starting Circuit 3.1 Changing the Parameters of the Relay Current Coil A current-type relay with a current-type operating coil is usually selected as the trip lockout relay TBJ. Its current coil operating value is selected according to the circuit breaker trip current to ensure the relay's operating sensitivity. The relay current operating value designed for this requirement is specified as 30% to 50% of the nominal rated value. As long as the relay current is consistent with the circuit breaker current, the relay sensitivity requirement can be met. The advantage of using a current-type relay as the trip lockout relay TBJ is that the trip lockout circuit wiring is simple. The current coil of TBJ can be monitored by the closing position relay HWJ. During operation, if TBJ is disconnected, HWJ will issue an abnormal alarm signal for timely handling. Its disadvantage is that when the circuit breaker trip current changes, a relay with the corresponding current specification must be replaced, which is relatively troublesome. 3.2 Relay Coil and Parallel Branch To reduce the workload of changing the parameters of the trip lockout relay TBJ due to changes in circuit breaker parameters, relevant technicians and manufacturers have been seeking a method applicable to various specifications of circuit breakers. Among them, the method of using a voltage-type relay with a parallel current branch in the voltage coil of the relay is gradually being recognized. The parallel branch can be composed of resistors, diodes, or Zener diode circuits. The following describes the current starting circuit of the trip lockout relay composed of a voltage-type relay and a parallel branch of resistors, diodes, and Zener diodes (as shown in Figure 2(a), (b), and (c)). Figure 2 Wiring diagram of the current starting circuit of TBJ (1) It is composed of a relay voltage coil and a resistor in parallel as shown in Figure 2 (a). The current starting value of relay J/I is 0.07~0.12 A and the coil resistance is 10Ω. The parallel resistors are generally divided into 4 groups. One or more resistors can be connected by connecting pieces to adjust the current shunt to change the relay action value and achieve matching with the tripping current of the circuit breaker. When using this circuit, attention should be paid to prevent the resistor from breaking. It is advisable to choose a metal oxide film resistor with a power of 8~10W. This type of resistor has high reliability. (2) It is composed of a relay voltage coil and a diode in parallel as shown in Figure 2 (b). The current starting value of relay J/I is 0.07~0.12 A and the coil resistance is 10Ω. The four diodes in parallel are connected in series in pairs and then in parallel across the two ends of the relay coil. The midpoints of the two series are connected. This wiring can ensure that the circuit works normally when one of the diodes is open, thus improving the reliability of the circuit. At the same time, the two branches can share a large current, preventing the diode from being damaged due to excessive current. After testing, with this wiring, when the current is 0.25~10A, the voltage across the terminals is 1.2~2.3V, and the current in the relay starting coil is 0.12~0.23A. This meets the requirement that the voltage drop is less than 5% of the rated voltage and ensures the sensitivity requirement of the relay. (3) The circuit is formed by the relay voltage coil and the parallel branch of the Zener diode as shown in Figure 2(c). The current starting value of the relay J/I is 0.07~0.12A, and the coil resistance is 10Ω. The Zener diodes VS1 and VS2 have a rated voltage of 1.5V and a rated current of 5A. The parallel connection of the two Zener diodes can improve the reliability of the circuit. The test shows that the circuit can meet the requirement that the voltage drop is less than 5% of the rated voltage and ensure the sensitivity requirement of the relay. The shortcoming of the current starting circuit of the jump-locking relay formed by the relay coil and the parallel branch is that the parallel branch is added at the 2 end of the relay coil, and the relay coil cannot be monitored. If the relay coil is disconnected, incorrect operation will occur. 4. Conclusion Circuit breaker tripping interlocking devices composed of electrical circuits are still widely used. Changing the relay current coil parameters is the primary method currently used due to its simple wiring and ease of monitoring. The method of constructing a tripping interlocking relay current starting circuit with the relay coil and parallel branches is also popular with manufacturers and operators because of its ease of parameter replacement. However, due to the complexity of the circuit and the inability to monitor the relay coil, further experience is needed, and its use should be approached with caution.