Circuit breaker monitors leakage current

Texas Instruments LM319A

The residual-current circuit breaker in Figure 1 continuously monitors the supply lines for any leakage current and immediately disconnects the supply if necessary. Load-supply wires, both live and neutral, pass through the magnetic core of the CR4311-5 transducer, which monitors the supply current. Under normal circumstances, because the current flowing in both conductors is equal and opposite, no flux is generated in the transducer core. However, under faulty conditions, the current in the live wire exceeds the current in the neutral wire, which catalyzes the production of flux in the core. This transducer core has a secondary winding that generates a voltage based on the produced flux. The generated voltage ranges from 0 to 10 V and is directly proportional to the sensed ac currents.

IC1B is a precision, fast-acting comparator and controls whether the relay is active based on a preset reference-voltage level.
Figure 1. IC1B is a precision, fast-acting comparator and controls whether the relay is active based on a preset
reference-voltage level.

A high-speed comparator, IC1B, detects this generated voltage and compares it with a set reference. If the detected voltage is within a tolerable range, the relay remains active, and the load remains connected to the mains supply. However, if this voltage exceeds tolerable limits, the circuit immediately deactivates the relay, thereby disconnecting the faulty load. Any further or repeated attempts to restart the device with the faulty load result in repeated tripping of the relay. You have to manually disconnect the faulty load and restart the device.

The circuit configures IC1B as a precision, fast-acting voltage comparator. IC1A provides a stable 6 V reference to IC1B. When the voltage on the noninverting input of IC1B rises above the preset reference voltage on its inverting input, the output goes high. This output controls Q1. When Q1 turns on, Q2 turns off, which deactivates the relay.

Two trim potentiometers facilitate tripping at user-preset levels. R2 controls the coarse setting, and R1 provides for finer adjustments. Typically, the muscles in the human body can tolerate current up to 20 mA. Hence, R1 and R2 must have settings that cause the relay to trip at leakage currents of greater than 15 mA that the transducer senses from the load-mains supply wires. R3 allows control over the hysteresis. D1 to D3 provide protection. C1 and C2 are decoupling and charge-pump capacitors, respectively.

The residual-current circuit breaker uses a transducer to monitor the supply current and a relay to disconnect the mains from the load.
Figure 2. The residual-current circuit breaker uses a transducer to monitor the
supply current and a relay to disconnect the mains from the load.

A 12 V, 0.5 A mains power-supply unit is sufficient to effectively run the circuit. The relay contacts must have a rating suitable for the load. Figure 2 shows the wiring layout for attaching the circuit to an ac-mains circuit. All components are standard industrial grades and are commonly available.

Materials on the topic

  1. Datasheet CR Magnetics CR4310/11
  2. Datasheet Texas Instruments LM319A
  3. Datasheet CDIL BC107
  4. Datasheet Unitra BC148

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