Understanding the Trip Device

A trip device is a mechanical and electrical assembly coupled to a circuit breaker’s operating mechanism. Its function is to release the latch mechanism, forcing the breaker to open automatically under specified conditions. This rapid disengagement of the electrical circuit protects both equipment and personnel from hazardous faults.

In residual current protection applications, when leakage or electric shock occurs, the zero-sequence current transformer detects an imbalance—the vector sum of the phase currents deviates from zero. The transformer’s secondary winding produces a voltage signal, which is amplified through an integrated circuit. Once the preset threshold is reached, the leakage trip device acts within 0.1 seconds, disconnecting the power supply to safeguard life and prevent equipment damage.

Types of Trip Devices

Thermal Trip Device

The thermal trip unit protects against overload conditions. Current passing through the trip device heats a bimetallic strip (direct-heating type). The strip deforms as it warms, and when deflection reaches a set point, it strikes the trip lever, releasing the mechanism and opening the circuit.

Thermal-Magnetic Trip Device

This hybrid device combines thermal protection for overloads with magnetic protection for short-circuits. The thermal element reacts to gradual overcurrent, while the magnetic coil responds almost instantaneously to severe fault currents.

Electromagnetic Trip Device

An electromagnetic trip unit is connected in series with the protected circuit. Under normal current, the magnetic force generated by the coil is weaker than the opposing spring force, and the armature remains stationary. During a short-circuit event, the fault current produces a magnetic pull strong enough to overcome the spring, moving the armature and triggering the trip mechanism.

Undervoltage Trip Device

The undervoltage release is wired in parallel on the supply side of the circuit breaker. It prevents closure or causes immediate opening if supply voltage falls below a safe threshold. When voltage drops below approximately 40% of nominal, the magnetic force cannot hold the armature, and the breaker trips. Conversely, when supply voltage is between 75% and 105% of nominal, the breaker can close normally.

Shunt Trip Device (MX)

The shunt trip release enables remote tripping of a low-voltage circuit breaker via an external control signal. Its coil is connected in parallel on the supply side. When energized, the magnetic field moves the armature, mechanically actuating the trip mechanism to open the breaker.

Electronic Trip Device

Electronic trip units integrate all the above protective functions with adjustable settings. Using electronic circuitry, they monitor current, amplify signals, and activate the trip coil when necessary.

What Is a Shunt Trip Device (MX)?

The term “shunt trip” describes a mechanism whereby an external power source energizes a coil to force the breaker into the open position.

• “Shunt” – Refers to tripping the breaker, interrupting the current path.

• “Trip”  – Denotes electromagnetic excitation. Energizing the internal coil produces the magnetic force needed to actuate the latch.

• “M” – Derived from Magnet, emphasizing electromagnetic operation.

• “X” – Represents excitation or execution, indicating “energize to execute trip.”

When an external control voltage—often from an emergency system such as a 24V DC fire alarm output—is applied to the coil, the magnetic field drives the mechanism, causing the breaker to open instantly.

Operating Principle

Fundamentally, a shunt trip unit is a trip coil with a mechanical linkage to the breaker latch. Applying the rated control voltage to the coil triggers tripping regardless of load current.

Fire Protection Example:
In a fire scenario, the fire control center issues an alarm signal, typically DC 24V. For non-essential circuits, the associated breaker is fitted with a shunt trip device. The fire signal energizes a relay whose contacts close in the trip coil circuit. This action cuts power to non-fire loads, preventing hazards and prioritizing supply to emergency systems.

Shunt trip devices are available in multiple voltage ratings—12V, 24V, 220V, in both AC and DC versions.

Common control arrangements:

1. Direct Control: Supplying the specified voltage (e.g., DC 24V) directly to the coil trips the breaker.

2. Relay-Driven Control: When the coil rating is higher (e.g., 220V) but the control signal is low-voltage (e.g., DC 24V), an intermediate relay switches the higher voltage to the coil.

Auxiliary Contact Integration

While the shunt trip device initiates breaker opening, operators may require confirmation of its status. This is where an auxiliary contact (OF) becomes essential. The OF contact provides a remote electrical indication of the breaker’s open or closed state, enabling integration into monitoring systems and ensuring that trip commands have been executed successfully.