Shunt trip (MX) versus undervoltage release (MN) on circuit breakers — why one is fail-safe and the other is not

Circuit breakers (ACBs, MCCBs) are often fitted with an auxiliary coil that can force an opening remotely or via a safety circuit, independent of the breaker's own thermal and magnetic protection. IEC 60947-2 defines two mechanically similar but functionally opposite options for this: the shunt release (MX) and the undervoltage release (MN). In practice they are regularly confused, while the difference in fail-safe behaviour determines precisely whether an emergency-stop or safety circuit does what is expected of it.

Shunt release (MX release) — tripping via an active signal

The shunt release (MX release) opens the breaker when voltage is applied to the coil — via an impulse command (typically within a few tens of milliseconds, on the order of ≤20 ms) or via a maintained command. Opening is guaranteed when the coil is energised within the specified operating voltage range (typically up to roughly 0.7× rated control voltage and above); below the lower limit of that range, or with no voltage on the coil at all, nothing happens.

This means the shunt release is not fail-safe: if control voltage is lost — due to a broken supply cable, a failed auxiliary transformer or a dead control-voltage battery — the shunt release can no longer open the breaker, even if an emergency-stop signal is issued. The shunt release is therefore suitable for commands that are only relevant while control voltage is present (for example, a remotely commanded normal shutdown), but unsuitable as the sole means of opening for a safety function that must specifically work when control voltage is lost.

Undervoltage release (MN release) — tripping when voltage disappears

The undervoltage release (MN release) works the other way round: the coil must remain energised to keep the breaker closed. As soon as the voltage on the coil drops below a certain threshold, opening of the breaker is guaranteed — for an MN release, opening is typically guaranteed once coil voltage has fallen to roughly 0.35× rated control voltage or lower, while above roughly 0.7× of that rated voltage no opening occurs; between those two limits, behaviour is not guaranteed to be unambiguous. In addition, a de-energised MN coil prevents the breaker from being closed — manually or electrically — until coil voltage is again sufficiently high (closing is typically only guaranteed from roughly 0.85× rated voltage upward).

This makes the undervoltage release fail-safe: loss of control voltage — for whatever reason, including a broken cable or a blown fuse in the control circuit — automatically leads to the breaker opening, precisely the behaviour typically required of a safety circuit or emergency-stop function.

Note: the exact voltage thresholds (percentages of rated control voltage at which opening or closing is guaranteed or excluded) are laid down in IEC 60947-2 itself and must be verified per application against the current text of the standard and the rating plate of the specific breaker/coil, rather than assumed from rules of thumb alone.

MX and MN side by side on the same breaker

It is common for a circuit breaker to carry both an MX and an MN coil, each for a different purpose: the MX coil for a targeted, active remote command (for example from a PLC or control panel), and the MN coil as a fail-safe backstop that opens the breaker as soon as control voltage itself is lost — for example linked to an emergency-stop circuit that interrupts the MN coil's control voltage rather than sending a switching command. Confusing these two functions — for instance designing an emergency-stop circuit that energises an MX coil instead of interrupting the supply of an MN coil — undermines the fail-safe property the safety circuit was meant to provide.

Practical relevance

When assessing a switchgear installation during a NEN 3140 inspection (see also the guide on switching provisions and their classification and the guide on emergency-stop categories), it is important to check which type of coil is used for which function: an emergency-stop or safety circuit that depends on a shunt release stops working as soon as control voltage is lost, whereas that same loss of voltage on an undervoltage release actually produces the desired opening.

Common mistakes

Relying on a shunt release (MX) for an emergency-stop or safety function without realising that it can no longer trip once control voltage itself is lost — fail-safe behaviour requires an undervoltage release (MN).
Confusing MX and MN coils in drawings or specifications, since they look mechanically similar but behave in opposite ways.
Assuming every undervoltage release trips within a few milliseconds at any voltage dip, without checking the exact voltage thresholds and time delays of the specific make against its rating plate and the standard.
Fitting an undervoltage release without accounting for its closing behaviour: until coil voltage has sufficiently recovered, the breaker cannot be closed again either — a detail that can cause confusion on restart after a fault if it is not known in advance.