When an alternate path to ground is established, a residual current operated device (RCD) or residual current circuit breaker (RCCB) detects the leakage current. The RCD isolates the power supply from the leakage path by breaking the circuit. Unlike fuses, these types of circuit breakers can be reset and reused, and they play an important role in protecting people and equipment.
In this article, the requirements for RCD leakage current detection and tripping will be reviewed, and how an ultra-low power voltage supervisor or Reset IC can be used as a leakage current detector. It will also explain how voltage supervisors can benefit medium voltage circuit breakers such as air circuit breakers (ACB) and molded case circuit breakers (MCCB) using microcontrollers.
Using Voltage Supervisors as Leakage Current Threshold Detectors
When a faulty device or an electric shock occurs, leakage current occurs, and the purpose of the RCD is to respond to a current of 5 mA to 500 mA. As stated in the International Electrotechnical Commission TS60479-1 standard, muscle contractions occur within 200ms of 50 mA, and exposure for more than 3s increases the likelihood of ventricular fibrillation.
RCDs detect leakage current by sensing the difference between the active and neutral currents. If the line and neutral currents are unbalanced and the leakage current exceeds a predetermined threshold, the circuit breaker trips, interrupting and isolating the power supply. RCD devices can be divided into three main stages. The first is the sensing phase, during which the leakage sensor senses the leakage current. In the second stage, the detection circuit sets the leakage current threshold. In the third stage, the relay is tripped to isolate the source of leakage.
In the past, relays have been used as detection circuits to set leakage current thresholds. Modern RCDs use integrated circuits such as voltage monitors to improve the accuracy and response time of detecting leakage currents and driving solenoid relays.
Figure 1 shows how TI’s TPS3840 voltage supervisor detects leakage current.
In this example, leakage sensors, such as differential current transformers or zero-phase current transformers, are represented by current sources. For the current sense circuit, a resistive divider is used to convert the input current to a voltage, which is then sensed by the TPS3840. The TPS3840 integrates an accurate reference bandgap and voltage comparator. Trigger points are factory programmed through one-time non-volatile memory (OTP), and the voltage thresholds are programmable from 1.6 V to 4.9 V with a typical accuracy of 1%.
When the voltage on the VDD pin rises above the threshold, the RESET pin will be pulled high to interrupt the microcontroller or drive an electromagnetic relay. Additionally, a single external capacitor can be used to extend the RESET response time to accommodate varying RCD response times depending on the magnitude of the leakage current.
This reference design highlights a common use case for a low-power reset IC and watchdog timer. The TPS3840 device has a current consumption of less than 1μA and can be configured as a comparator (through a simple 3-pin configuration) or as a general-purpose power supply monitor that can be daisy-chained as a sequencer. Additionally, the response and delay times are configurable with external capacitors. For applications that require a running watchdog, the TPS3431 device can be used, which is an independently programmable watchdog timer with an accuracy of ±2.5% (typ.).
Threshold Detection (TPS3840): Self-powered threshold detector using a voltage supervisor as a comparator (3-pin configuration, no external power supply).
Very low quiescent current (<700nA) architecture with 1.6VC4.9V (0.1V step) set threshold range; user-controlled response time (80μs to 600ms via external capacitor) fast power-up ( Startup delay < 250 μs).
Power Supply Monitoring and Sequencing Reset (TPS3840): Can be used for multi-rail reset monitoring and startup sequencing.
Provide active low and active high configuration, the threshold range is 1.6VC4.9V, the TD range is 80μsC600ms.
Watchdog Timer (TPS3431): User adjustable timer with enable pin and wide input range.
Low current consumption (up to 20A) for watchdog function and user configurable timeout using external resistors and capacitors.
This reference design highlights a common use case for a low-power reset IC and watchdog timer. The TPS3840 device consumes less than 1A, can be configured as a comparator (with a simple 3-pin configuration), can be used as a general-purpose power supply monitor, and can be daisy-chained into a sequencer. Additionally, the response and delay times can be configured using external capacitors. For applications requiring watchdog operation, the TPS3431 device can be used, which is an independently programmable watchdog timer with ±2.5% accuracy (typ).
The voltage supervisor in this reference design meets three key specifications:
Fast power-up and programmable response time. When the voltage monitor rises from zero input voltage to above the trip point or threshold voltage, it takes a certain amount of time to start and react. Circuit breakers need to quickly detect leakage current levels and have the flexibility to configure trip response times based on leakage levels and durations to avoid false trips caused by transients. The startup time of TPS3840 is 200s, which can respond quickly.
Ultra-low input current. In Figure 1, the power supply pin (VDD) of the supervisor IC is aligned with the input signal monitoring pin (SENSE). Since it is powered by the input signal, the voltage monitor should have high input impedance to minimize the error (IIN) on the voltage divider (IDIV). The TPS3840 voltage supervisor consumes ultra-low current with a typical IQ of 350 nA,
Low VPOR enables low VOL. VPOR is the minimum input voltage required for the controlled output state. When VIN < vpor, the output tracks the input and may trigger a relay. vpor should be as low as possible to provide a margin between the relay's enable voltage level and the voltage supervisor low output voltage level (vol). ="" tps3840 has active low configuration with low vpor and vol of 300="" mv to avoid false reset.
If the voltage supervisor is integrated in the microcontroller, it is recommended to use an external watchdog timer. An external watchdog ensures that the microcontroller does not latch up by periodically detecting pulses sent from the microcontroller’s general-purpose input/output pins. If the software malfunctions and misses a pulse, an external watchdog timer can reset the microcontroller.
The TPS3430 Programmable Watchdog Timer is a good choice because it provides a programmable watchdog that monitors timeouts and reset delays to meet the timing requirements of any microcontroller. If higher reliability is required, both a voltage supervisor and a watchdog should be used. The TPS3823’s integrated watchdog and voltage supervisor are a good choice, offering fixed thresholds and watchdog timeout options.
Not only can voltage supervisors monitor the microcontroller’s voltage supply to ensure proper operation, but they can also be used as leakage current detectors, helping to enhance your circuit breaker designs.
The reference design uses a signal processing front end and a self-powered module for the Electronic trip unit (ETU) used in circuit breakers. This design uses a FRAM-based microcontroller to process the current input from the signal conditioning amplifier to obtain the three-phase neutral-to-ground current. Two gains are used to extend the phase current measurement range.
This reference design can also be self-powered using a rectified current input. The TIDA-00498 is designed for fast repetitive trips (within 30mS) over a wide current and temperature range.
Measures three currents (phases) with an input range (0.2 to 12 INOMINAL) and two currents (neutral, ground) with an input range (0.05 to 2 INOMINAL) with an accuracy of ±3%.
Fast start-up using TI’s MSP430 FRAM MCU with start-up time of less than 30ms including single-cycle RMS compensation.
Brown-out detection provides about 200s post-power-on reset time.
Self-powered power supply (based on rectified current input) with MOSFET-based shunt regulation.