Shunt Trip and Undervoltage Release: Circuit-Breaker Accessories Explained
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A circuit-breaker on its own reacts to only two things: overload and short circuit. Everything else that should make it open, such as an emergency stop button, a fire alarm signal or a supply that has dropped out, reaches the breaker through accessories bolted to its side. The two that actually open the breaker are the shunt release (shunt trip) and the undervoltage release. The two that only report its state are the auxiliary contact and the alarm contact.
The names vary between catalogues. Some manufacturers use MX for the shunt release and MN for the undervoltage release, others say ST and UVT, and North American literature almost always says “shunt trip”. The functions are the same.
Shunt release: trip on command
A shunt release is a solenoid that mechanically trips the breaker when you apply voltage to its coil. Remove the voltage and nothing happens; the breaker stays where it was until someone or something resets it. It is a remote off switch for a protective device.
Typical reasons to fit one:
- an emergency stop or emergency power off (EPO) button in a plant room, server room or workshop;
- a fire alarm panel output that has to isolate ventilation, gas solenoid supplies or loads that must not run during an alarm;
- a firefighter’s switch arrangement or a building management system shutdown;
- tripping a breaker from a separate protection relay, for example an earth fault relay or a differential relay on a larger feeder.
Under IEC 60947-1 a shunt release has to operate reliably anywhere between 70 % and 110 % of its rated control voltage. In practice this means it will still trip when the control supply sags, which is useful, because emergency signals tend to arrive at the worst moment.
Why the coil needs a cut-off contact
Most shunt coils are rated for momentary duty. They draw a heavy current for a few tens of milliseconds and are not designed to stay energised. If the trip command is a maintained signal, a latched fire alarm relay for instance, the coil stays energised after the breaker has opened and will eventually burn out. Many shunt releases have a built-in cut-off contact that opens with the breaker. If the one you are fitting does not, wire an auxiliary contact of the breaker in series with the coil so that the coil is disconnected as soon as the breaker opens.
The weakness of a shunt trip
A shunt release needs energy to work. If the wire to the emergency button is cut, the control fuse has blown or the control transformer has failed, pressing the button does nothing, and nobody finds out until the day it is needed. For that reason a shunt trip used for an emergency function should be supplied from a reliable source, often the line side of the breaker it trips or a monitored supply, and the circuit should be tested at every periodic inspection.
Undervoltage release: trip when the voltage goes
An undervoltage release works the other way round. Its coil is energised all the time, and while it is energised it holds a latch that allows the breaker to stay closed. When the voltage falls below a threshold or disappears, the latch drops and the breaker trips. The breaker cannot then be closed again until the voltage has returned.
IEC 60947-2 gives the operating window. The release has to open the breaker when the supply falls to somewhere between 70 % and 35 % of its rated voltage, it must prevent closing below 35 %, and it must allow closing when the voltage is at 85 % or more. A 230 V undervoltage release will therefore trip somewhere between about 161 V and 80 V, depending on the product.
The main uses:
- Restart protection. When the supply returns after an outage, machines should not start by themselves. IEC 60204-1 requires that for most machinery, and an undervoltage release on the machine’s supply breaker is one simple way of meeting it.
- Fail-safe emergency stop. Wire normally closed emergency stop buttons in series with the undervoltage coil. Pressing any button, cutting the cable or losing the control supply all trip the breaker. This is the opposite behaviour of a shunt trip, and it is why undervoltage releases are often preferred for emergency stopping.
- Protecting equipment that dislikes low voltage. Large motors stall and draw high current on a deep sag. Tripping them prevents overheating and a messy restart of everything at once.
Time-delayed versions
An instantaneous undervoltage release will trip on a short voltage dip that the rest of the installation would have ridden through, for instance when a large motor elsewhere starts or when the network recloses after a transient fault. A delay unit, usually adjustable from a fraction of a second to a few seconds, stops these nuisance trips while keeping the protection against a real outage. If an installation has a history of unexplained trips during storms, a missing delay unit on an undervoltage release is one of the first things to look at.
Shunt trip or undervoltage release?
| Shunt release | Undervoltage release | |
|---|---|---|
| Coil normally | De-energised | Energised |
| Trips when | Voltage is applied | Voltage falls or is removed |
| Broken control wire | Remote trip no longer works, silently | Breaker trips |
| Supply outage | Breaker stays closed | Breaker trips and stays open until reset |
| Typical use | Fire alarm shutdown, EPO where an outage must not trip the load | Machine restart protection, fail-safe emergency stop |
| Continuous consumption | None | A few watts or VA, all the time |
The choice usually comes down to one question: what should happen if the control circuit fails? If losing the control circuit must not switch off the load, for example on a supply to a sprinkler pump or smoke extraction, a shunt trip is the right device. If losing the control circuit should leave things safe, choose an undervoltage release.
Auxiliary and alarm contacts
These do not trip anything. They tell a control system or an indicator lamp what the breaker is doing.
- Auxiliary contact (often marked OF or AUX) changes state whenever the main contacts open or close, whatever the reason. Use it for “breaker open” indication, for interlocking and for the series cut-off of a shunt coil.
- Alarm contact (often SD, or “trip indication”) changes state only when the breaker has tripped, not when it was switched off by hand. That distinction lets a building management system raise an alarm for a fault trip while ignoring a deliberate switch-off for maintenance.
On many miniature circuit-breakers the accessories clip onto the left-hand side and add half a module or one module of width each, so leave space in the enclosure when you design the board.
Wiring and commissioning checklist
- Match the coil voltage and type (AC or DC) to the control supply. A 24 V DC coil on a 230 V supply will not survive the first test.
- For a shunt release, confirm there is a cut-off contact, built in or wired, if the trip signal can be maintained.
- For an undervoltage release, decide whether it is supplied from the line side of its own breaker or from a separate source. Supplied from the load side, it would trip the breaker and then lose its own supply, which may be what you want, but make sure it is deliberate.
- Label the control wiring and the remote buttons with the circuit they trip.
- Test the real function: press the emergency button, operate the fire alarm output, switch off the control supply. Do not test only the breaker’s own test button, which checks the mechanism and nothing else.
For how fast the breaker itself opens once the release has acted, see tripping time of a circuit-breaker. For reading the rest of the nameplate, see circuit-breaker markings and labels, and for the undervoltage condition itself, protection against undervoltage.
References
- IEC 60947-1, Low-voltage switchgear and controlgear, general rules: operating limits of shunt releases.
- IEC 60947-2, Circuit-breakers: operating limits of undervoltage releases.
- IEC 60204-1, clause 7.5, protection against supply interruption or voltage reduction and subsequent restoration.