Short answer: a shunt trip fires the breaker on a control pulse and holds nothing afterwards; an undervoltage release must be energised for the breaker to close at all, and drops it out when the control voltage sags — it fails safe, which is why it is used where reopening a machine on restored power would be dangerous. An auxiliary contact reports the position of the main contacts; an alarm contact reports only fault trips. They look like the same 9 mm block clipped to the side of the breaker and they are not interchangeable.
We are an independent trading company — not a manufacturer, and not an appointed distributor of any brand — so the codes below come from the range data for the exact ordering code, and the coil voltage printed on the part you receive governs the wiring.
Four jobs, four different devices
People order accessories as one line item, then discover at commissioning that the block reports status instead of tripping. The functions are distinct:
| Device | What it does | Typical use | The trap |
|---|---|---|---|
| Shunt trip / shunt release | An electromagnetic coil that, when energised, releases the latch and opens the breaker | Remote opening from a control panel, fire-alarm interlock, door switch, emergency stop circuit | It is a pulse device. Leaving the coil energised burns it out, and the breaker cannot be reclosed until the pulse is removed |
| Undervoltage release | A coil that must be energised for the breaker to close and stay closed; it releases when the voltage falls below its threshold | Motor circuits where automatic restart after a supply dip is a hazard, safety circuits, unattended plant | Wire it to the same supply as the circuit it guards, or the breaker will refuse to close for reasons nobody can find at 2 a.m. |
| Auxiliary contact | Volt-free contacts that follow the main contact position, open or closed | Status indication on a PLC, interlocking, signalling to a building management system | It reports position, not cause. “Open” tells you nothing about why |
| Alarm contact | Changes state only on a fault trip | Separating a genuine fault trip from someone switching the breaker off by hand | It does not follow manual operation, so it cannot replace an auxiliary contact |
The code families, and what fits what
Accessories are frame-specific: a block that clips onto one series will not fit another. On the NB1-63 family they are XF9 auxiliary contact, XF9J alarm contact, S9 shunt release and V9 undervoltage release. On NXB-63 and NXB-40 the same functions carry other names — AX-X1 and AX-X3 auxiliary contacts, AL-X3 alarm, SHT-X1 and SHT-X3 shunt releases, UVT-X1 and UVT-X3 undervoltage releases, plus OVT-X3 for overvoltage — and AX-1 serves the DZ158 and DZ158LE breakers.
The lesson is boring and expensive to learn late: the accessory code is only valid with the series it was designed for, so both codes go on the same order line. “A shunt trip for the MCB” is not a part number.
What the contact can actually switch
The auxiliary contact’s rating is its own, and much smaller than the breaker’s. On the AX-X1 the published figures are AC 415 V at 3 A, AC 240 V at 6 A and DC 24 V at 6 A, with at least 10,000 operations, side-mounted on the left in a 9 mm module. That is a control-circuit device to IEC 60947-5-1 — not a power contact.
| Rating point | AX-X1 figure | Practical meaning |
|---|---|---|
| AC 415 V | 3 A | Fine for a PLC input or a pilot light at 400 V; not for a contactor coil of any size |
| AC 240 V | 6 A | Sensor supplies, indication circuits, small relays |
| DC 24 V | 6 A | The usual PLC discrete input and interposing relay duty |
| Endurance | ≥ 10,000 operations | Enough for status signalling, marginal for a circuit that is switched several times an hour |
| Mounting | Left side, 9 mm module | Costs you a way in the enclosure and shifts the breaker’s footprint on the rail |
For comparison, the AX-1 used on the DZ158 family is a control-circuit device to IEC 60947-5-1 with a mechanical life of 10,000 operations and an electrical life in the region of 6,000 or more — adequate for signalling, inadequate for switching a motor. If the contact energises a contactor coil, use an interposing relay and let the relay take the inrush.
Shunt trip and undervoltage release side by side
| Behaviour | Shunt trip (S9, SHT-X1/X3) | Undervoltage release (V9, UVT-X1/X3) |
|---|---|---|
| Coil energised in normal service | No — energised only to trip | Yes — must be energised for the breaker to close and remain closed |
| Effect of losing control voltage | None. The breaker stays as it is | The breaker opens. This is the fail-safe direction |
| Effect of a supply dip | No response unless the dip reaches the trip circuit | Drops out below its threshold, typically around 35 to 70 percent of rated voltage, and will not reclose until voltage recovers |
| Close with no control supply? | Yes, by hand, unless interlocked | No — closing is mechanically blocked |
| Coil burden | Zero in normal service | Continuous, adding heat inside the enclosure |
| Typical application | Remote or automatic opening: fire alarm, remote stop, interlock with a door or a second supply | Prevention of unexpected restart after a power failure, safety-related stopping, unattended machinery |
| Common wiring mistake | Coil left permanently energised, then burnt out, or a maintained contact used instead of a pulse | Coil fed from a different source from the protected circuit, so a dead upstream supply looks like a faulty breaker |
The choice is a safety argument, not a convenience one. A shunt trip is something you command; an undervoltage release is something the installation does to itself when the supply fails. If the risk is “the machine must not restart by itself when power returns”, a shunt trip does not address it at all — with a dead control supply it is a breaker that simply stays closed.
Voltage codes, and why the coil is the order line
Every release is built for a control voltage, and the range data states plainly that those specifications are varied. The voltage is not a property of the function; it is a property of the part. A 230 V AC shunt release and a 24 V DC shunt release are different order codes with the same name, so a quotation for “shunt trip for NM1” that does not ask for your control voltage has quoted nothing. Ask for the coil voltage and whether it is AC or DC, then check the label before the part goes into the panel — the mistake is usually found after the wiring is complete.
What the accessory does not change
Fitting a release to a breaker does not change its breaking capacity, its certificate or its curve. The accessory is a control-circuit device assessed to its own standard, IEC 60947-5-1, while the breaker is assessed to IEC 60898-1 or IEC 60947-2 according to type. It also does not turn an MCB into a safety device: a shunt trip wired to an emergency stop is a remote opening means, and whether the arrangement meets the required performance level is a question for the machine’s risk assessment, not the accessory datasheet.
Practical checks before you order
Confirm the breaker series, the control voltage and whether it is AC or DC, how many ways the extra 9 mm module consumes, and whether the contact duty is signalling or switching. Get those right and the accessory line is a five-minute item; get them wrong and it is a return, a rewire and a delayed shipment. Send the breaker code and the control voltage together and the matching accessory code comes back on the same quotation.
If you are still selecting the breaker itself, the MCB range and the MCCB range pages show the families these accessories attach to; selectivity between MCB and MCCB covers how the upstream device interacts with a tripped downstream one, and how to read MCB marking explains the printed ratings you will be comparing against.
FAQ
Q: Can I use a shunt trip as an emergency stop?
A: Only as the opening means, not as the safety function itself. The emergency stop circuit has to remain effective if the shunt coil or its supply fails, which normally means a hard-wired contactor or a breaker with an undervoltage release rather than a pulse device.
Q: Why does my breaker refuse to close after fitting an undervoltage release?
A: The release is not energised. It blocks closing mechanically until its coil sees the rated voltage, so check the control supply, the coil voltage code, and whether the release is wired to the same source as the circuit it protects.
Q: Do I need both an auxiliary contact and an alarm contact?
A: If you want to know the breaker’s position and also whether it tripped on a fault, yes. The auxiliary contact follows the mechanism; the alarm contact distinguishes a fault trip from a manual operation. One contact cannot report two facts.
Q: Will an auxiliary contact switch a contactor coil directly?
A: Usually not. The AX-X1 is rated AC 415 V at 3 A and AC 240 V at 6 A as a control-circuit device, and a contactor coil presents an inrush well above its holding current. Use an interposing relay.
Q: Does fitting accessories change the breaker’s breaking capacity or certificate?
A: No. The breaking capacity belongs to the breaker and its tested configuration. The accessory is a separate control-circuit component with its own standard and its own much lower ratings.
Related: MCB range · MCCB range · How to read MCB marking · Selectivity between MCB and MCCB · MCB vs RCCB vs RCBO · How to size a distribution board · MCCB selection: NM1, NXM, NM8N · How to verify genuine product