Circuit breaker monitors leakage current

Sharath Kumar, Gemplus R&D Centre, Dubai, United Arab Emirates -- EDN, 6/21/2001

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 (http://www.crmagnetics.com), 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 10V and is directly proportional to the sensed ac currents.

A high-speed comparator, IC_1B, 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 IC_1B as a precision, fast-acting voltage comparator. IC_1A provides a stable 6V reference to IC_1B. When the voltage on the noninverting input of IC_1B 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.

A 12V, 0.5A 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.

Free Magazine Subscription

Printer-friendly version