Shunt Trip (MX) and Undervoltage Release (MN): Which One and Where?
shunt trip coil: A shunt trip coil (MX) opens the breaker when voltage is applied and suits emergency power off, fire detection and remote control, while an undervoltage release (MN) opens the breaker when voltage disappears and is used wherever the circuit must fail to a safe state on loss of supply.
Contents
Whenever a circuit breaker has to be opened independently of its manual handle, two auxiliary devices come into play: the shunt trip coil and the undervoltage release. Both open the breaker, yet they work on exactly opposite logic, and choosing the wrong one can leave an emergency stop chain silently inoperative. This article explains the operating principles, supply requirements, typical scenarios and wiring rules within the framework of IEC 60947-2.
How the Shunt Trip Coil (MX) Works
A shunt trip coil is an electromagnet that releases the mechanism and opens the breaker when rated voltage is applied to its terminals. The command usually comes from a pushbutton, a relay contact or a volt-free contact from the fire detection system, and it is issued only as a pulse. Because the coil is not designed for continuous energisation, the circuit must be interrupted by an auxiliary contact once the breaker has opened, otherwise the coil overheats and fails permanently. The arrangement is reliable as long as control voltage is present, but no tripping occurs at all if the supply is lost.
How the Undervoltage Release (MN) Works
The undervoltage release works the other way round: it is permanently energised and the presence of voltage is a condition for the breaker to stay closed. When the voltage falls to roughly 35 to 70 percent of rated value the coil drops out and the breaker opens, and closing is inhibited until the voltage rises above about 85 percent. This behaviour prevents motors from restarting by themselves after a supply interruption and creates a fail-safe emergency stop chain. A broken control wire also causes the coil to drop out, so a wiring fault cannot hide as a silent failure.
Choosing Between Them Scenario by Scenario
For ordinary functions such as remote tripping, power cut-off driven by the fire alarm system or SCADA control, the shunt trip coil is sufficient and economical. For emergency stop buttons, door safety switches, interlocking with a busbar disconnector and any circuit with machinery safety requirements, the undervoltage release is preferred because a break in the control loop automatically falls to the safe side. Critical installations often use both: the MN for the safety chain and the MX for operational remote tripping. The table below compares their behaviour side by side.
Supply, Voltage Tolerance and Auxiliary Contacts
| Feature | Shunt trip (MX) | Undervoltage release (MN) |
|---|---|---|
| Trips when | Voltage is applied | Voltage disappears |
| Continuous consumption | None, pulse command | Yes, permanently energised |
| Broken control wire | Unnoticed, no tripping | Trips automatically |
| Typical use | Remote control, fire alarm | Emergency stop, machinery safety |
| Reclosing | Unrestricted | Not possible until voltage returns |
| Fail-safe behaviour | Low | High |
Coil supply is typically specified as 24, 48, 110 or 220 V DC, or 230 V AC, and must match the auxiliary supply architecture of the assembly. In critical installations a battery-backed DC supply guarantees tripping even when the mains fails, so a DC source is strongly recommended whenever an MX coil serves a fire scenario. Breaker status should be monitored through auxiliary contacts (OF), the trip indication contact (SD) and the ready contact. Although the conductor cross section is small, voltage drop over long runs must never bring the coil below its pick-up threshold.
Commissioning, Testing and Common Mistakes
The most frequent mistake is leaving a shunt trip coil permanently energised through a maintained contact, which burns it out within minutes. The second is assuming that a breaker with an undervoltage release is faulty because it will not close during commissioning, when in reality the coil simply has not been energised. Both coils should be tested through the real scenario at commissioning, verifying tripping time and signalling when the emergency stop button is pressed. A functional test at least once a year is the only method that proves the coil still works in the field.
Key takeaways: shunt trip coil
- Use an MN coil in safety chains — A broken control wire still opens the breaker, which gives genuine fail-safe behaviour.
- Drive the MX coil with a pulse — An auxiliary contact must cut the circuit once the breaker opens so the coil is never left energised.
- Choose DC supply for critical tripping — A battery-backed DC source guarantees the trip command even when the mains has failed.
- Schedule functional testing — Testing the real scenario at least annually is the only proof that the coil still operates.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
What happens if a shunt trip coil stays energised?
The coil is designed for short pulses only, so it heats up quickly and the winding is usually damaged within a few minutes. The control circuit must therefore be interrupted by an auxiliary contact as soon as the breaker opens. Some manufacturers integrate that contact inside the coil, but this has to be verified for each project.
Which coil should an emergency stop button drive?
In applications with machinery safety requirements, the emergency stop button must be wired in series with the undervoltage release. The breaker then opens both when the button is pressed and when the control wire breaks. A shunt trip coil cannot detect a broken wire, so the emergency stop could remain silently inoperative.
Why will a breaker with an MN coil not close?
An undervoltage release mechanically prevents closing until it is energised. The voltage must be above roughly 85 percent of rated value and every contact in the emergency stop chain must be closed. This is often mistaken for a fault during commissioning, although it is exactly the intended behaviour of the design.
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Devpan designs tripping coil selection, auxiliary supply architecture and safety chain wiring around your project requirements and verifies them with factory testing. Let us plan your emergency stop and remote tripping scenarios together. Related Devpan solution: electrical panel design services.
