MFAC34N1AA0001A | ALSTOM | Low-load trip relay | MFAC34N1AA0001A

The ALSTOM low-load trip relay is an electromagnetic relay with two internal coils:

One is a current-starting coil (TBJ/I), connected in series in the circuit breaker trip circuit. It starts when the protection output relay operates and trip current flows.

The other is a voltage-holding coil (TBJ/U), connected in parallel to the closing circuit via its normally open contact, used for self-holding after tripping.

The relay contact configuration typically consists of four sets: the normally open contact TBJ1 is connected in parallel with the trip output to hold the trip command until the circuit breaker auxiliary contacts open, preventing the output relay contacts from being burned by arcing; the normally open contact TBJ4 closes to prepare for the voltage coil self-holding circuit; two normally closed contacts TBJ2 and TBJ3 are connected in series in the closing coil circuit. When the relay operates, it immediately disconnects the closing circuit, preventing the circuit breaker from closing again, thus achieving anti-pumping.

“Low load” refers to the extremely low power consumption of the relay coil, minimizing voltage drop and power consumption in the control circuit. According to power industry standards, its current coil voltage drop is less than 5% of the rated voltage of the operating circuit, and the current initiation value is 30% to 50% of the circuit breaker tripping current, ensuring neither false tripping nor failure to trip. The voltage coil operating value is between 50% and 70% of the rated DC voltage, ensuring reliable operation during power fluctuations and preventing false tripping during grounding.

The core technical parameters are as follows:

Rated frequency: 50Hz or 60Hz.

Current initiation value range: 0.07 to 0.12 amperes, coil resistance: approximately 10 ohms, current initiation value not exceeding 50% of the circuit breaker tripping current, ensuring a reliability factor greater than 2.

The current coil voltage drop is less than 5% of the rated DC voltage of the operating circuit; this is the meaning of “low load”—minimal impact on the shunt current of the protection circuit.

The voltage operating value is between 50% and 70% of the rated DC voltage, ensuring reliable operation during DC power supply voltage fluctuations and preventing malfunctions during grounding.

The output contacts are rated at 220V/5A DC. With the normally closed contacts open, they can withstand 1000V DC for 1 minute, and between unconnected conductive parts, they can withstand 2000V DC for 1 minute.

The reset time is no more than 50 milliseconds, and the overtravel time is no more than 30 milliseconds.

The operating temperature range is -10°C to +60°C, the storage and transportation temperature is -25°C to +80°C, the relative humidity is less than 95% and there is no condensation, and the weight is approximately 2.5 kg.

This relay is widely used in ALSTOM MiCOM series protection devices and PACiS substation automation systems, working in conjunction with ALSTOM circuit breaker operating mechanisms. In practical engineering, circuit breakers with voltage levels of 35 kV and above generally employ electrical anti-pumping mechanisms, using this type of relay as the core to form an anti-pumping circuit, creating a double layer of protection together with the circuit breaker’s mechanical anti-pumping mechanism.
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