What is a circuit breaker?
A circuit breaker (in low-voltage installations usually a miniature circuit breaker or MCB) is a reusable electrical protective device that automatically interrupts a circuit on overload or short-circuit, and that can be switched back on with a single movement once the fault has been cleared. Unlike a fuse, a circuit breaker does not need to be replaced after tripping. Circuit breakers primarily protect the cable against overheating and fire, and they are standard equipment in distribution boards, motor control centres and control cabinets. For OT teams, the right breaker determines whether a short-circuit in one sensor circuit stays confined to that circuit, or takes down the whole PLC and with it the production line.
π§ How does a circuit breaker work?
A circuit breaker combines two tripping mechanisms in one housing, which together form its tripping characteristic:
- Thermal tripping (overload) - a bimetallic strip bends under prolonged overcurrent and releases the switching mechanism. The higher the overcurrent, the faster it trips: at 1.45 Γ the rated current (In) the breaker trips within one hour (for In up to 63 A), at 2.55 Γ In within 1 to 60 seconds (for In up to 32 A; above that within 120 seconds).
- Magnetic tripping (short-circuit) - a coil pulls in an armature on a high current peak and opens the contacts immediately, typically within about 10 milliseconds and always within 0.1 seconds.
- Arc quenching - the arc that forms when the contacts open is driven into an arc chute with metal plates, split up and extinguished.
- Trip-free mechanism - the breaker trips even if someone holds the handle in the on position, so a fault cannot be βheld inβ.
Once the cause has been resolved, you push the handle back up. A breaker that trips repeatedly points to overload, an insulation fault or a characteristic that does not suit the inrush current; fitting a larger breaker is almost never the answer.
π Which tripping characteristics exist?
For circuit breakers to IEC 60898-1 (household and similar installations), the thermal part is the same for every characteristic. They differ only in the current at which the magnetic part trips instantaneously:
| Characteristic | Magnetic trip | Typical application |
|---|---|---|
| B | 3β5 Γ In | Resistive loads, lighting, long cables with low fault current |
| C | 5β10 Γ In | General default in industry: power supplies, small motors, mixed loads |
| D | 10β20 Γ In | High inrush currents: transformers, motors, welding equipment |
| K (manufacturer, IEC 60947-2) | approx. 8β14 Γ In | Motors and transformers, with a more precise thermal part |
| Z (manufacturer, IEC 60947-2) | approx. 2β3 Γ In | Semiconductors and sensitive instrumentation circuits |
A C16 breaker is therefore guaranteed to trip instantaneously above 160 A, but between 80 and 160 A it may still trip with a delay (thermally). A 24 V power supply in a control cabinet often delivers only a few times its rated current, which is why 24 V DC circuits are increasingly protected by electronic circuit protection modules that trip at a set current, regardless of the limited short-circuit current of the supply.
π What is the difference between IEC 60898-1 and IEC 60947-2?
There are two product standards for miniature circuit breakers, and many industrial breakers are marked to both:
| Feature | IEC 60898-1 | IEC 60947-2 |
|---|---|---|
| Target users | Household and similar installations, operated by ordinary persons | Industrial switchgear and controlgear, operated by skilled persons |
| Range | AC up to 440 V, up to 125 A | Up to 1000 V AC or 1500 V DC |
| Short-circuit rating | Icn (e.g. 6 kA or 10 kA) | Icu (ultimate breaking capacity) and Ics (service breaking capacity) |
| Conventional currents | Non-tripping at 1.13 Γ In, tripping at 1.45 Γ In | 1.05 Γ In and 1.30 Γ In |
| Adjustability | Fixed | Adjustable on larger circuit breakers |
An Icn of 6 kA is therefore not the same as an Icu of 6 kA, because the test conditions differ. In an industrial assembly to IEC 61439, the Icu and Ics at the actual supply voltage are what count.
βοΈ What is the difference between a circuit breaker and a fuse?
| Feature | Circuit breaker | Fuse |
|---|---|---|
| After tripping | Switch back on | Replace the fuse link |
| Breaking capacity | Usually 6β25 kA | Often 50β120 kA (gG fuse links) |
| Current limitation | Limited (energy limiting class) | Strongly current-limiting |
| Operation and isolation | Can act as a switch, lockable with a padlock | Remove the fuse link or open the switch-disconnector |
| Signalling | Auxiliary or alarm contact available | Only with special fuse holders |
| Replaceable element | None, the rating is fixed in the device | Fuse link; with some systems an oversized one can be fitted |
| Cost | Higher purchase price, no spare parts | Lower purchase price, but every operation costs a fuse link |
Where prospective short-circuit currents are high, both are combined: a fuse or circuit breaker upstream acts as back-up protection (cascading) for breakers with a lower Icn. Manufacturers publish tables of permitted combinations.
π Where are circuit breakers used in industrial installations?
- Control cabinet supplies - the primary side of 24 V power supplies, UPS inputs and cabinet lighting
- PLC and I/O circuits - separate circuits for the CPU, O modules and field supply, so a fault in the field does not switch off the CPU
- Motors - small motors and fans; larger induction motors usually get a motor protection circuit breaker in a motor starter with a contactor
- Variable frequency drives - the supply side of a VFD, protected as specified by the manufacturer (sometimes with fuses instead)
- Auxiliary circuits - control voltage for relays, solenoid valves and field components
A circuit breaker protects against overcurrent, not against earth leakage. That requires a residual current device (RCCB, IEC 61008-1) or a combined RCBO (IEC 61009-1), alongside proper earthing. For motor protection against slow overload, adjustable thermal protection is more accurate.
π§ How do you choose the right circuit breaker?
In the Netherlands the choice follows NEN 1010 (based on IEC 60364), supplemented for machinery by IEC 60204-1. A practical approach:
- Determine the design current (Ib) of the load, including diversity.
- Choose the rated current (In) so that Ib β€ In β€ Iz, where Iz is the current-carrying capacity of the cable after correction for temperature and grouping. Because an IEC 60898-1 breaker trips within 1.45 Γ In, the requirement I2 β€ 1.45 Γ Iz is then also met.
- Choose the characteristic based on the inrush current: C as the default, D for transformers, B for long cables.
- Check the disconnection time - in a TN system at 230 V, a final circuit up to 63 A with socket-outlets, or up to 32 A supplying only fixed equipment, must disconnect within 0.4 seconds. A C16 needs 160 A of fault current for that, so a maximum earth fault loop impedance of 230 / 160 = 1.44 Ξ©; a B16 may go up to 2.88 Ξ©.
- Check the breaking capacity - Icn or Icu must exceed the prospective short-circuit current at that point, or there must be back-up protection.
- Ensure selectivity - choose the upstream protection so that on a fault only the nearest breaker trips.
- Document the circuit, In, characteristic and Icn in the schematic and on the circuit chart.
π What role does the circuit breaker play in safe work and monitoring?
Beyond protecting cables, the circuit breaker is part of everyday electrical safety and, increasingly, of the OT data landscape:
- Isolation - under NEN 3140 you de-energise a circuit, secure it against reclosing and verify that it is dead. Lockout clips for breakers exist for Lock-out Tag-out, often as part of a work permit.
- Hazardous areas - breakers are preferably installed outside the hazardous zone. If they sit in an ATEX zone, they must be housed in an enclosure certified to IEC 60079, for example flameproof (Ex d).
- Signalling contacts - an auxiliary or trip-alarm contact wired to a PLC input shows the operator immediately which circuit has tripped.
- Smart breakers - modern variants measure current, energy and temperature and send that data via Modbus or a wireless gateway to an energy management or condition monitoring system, enabling predictive maintenance. That also makes the breaker a networked device that belongs in the OT asset inventory and the segmentation plan.
- Periodic inspection - the NEN 3140 inspection checks, among other things, loop impedance, residual current devices and thermal load (often with a thermal imaging camera).
β Frequently asked questions
Why does my circuit breaker keep tripping?
A circuit breaker that trips repeatedly usually points to overload, excessive inrush current or an insulation fault in the circuit. Tripping after some time indicates the thermal part (overload), instant tripping the magnetic part (short-circuit or inrush peak). Never simply replace a circuit breaker with a larger one, because the cable is then no longer protected.
Which circuit breaker should I use for a motor?
For small motors, a circuit breaker with a C or D characteristic is common. For larger motors, a motor protection circuit breaker is better, because it has adjustable thermal protection and phase-loss protection. Its combination with a contactor must be coordinated according to IEC 60947-4-1.
What does C16 on a circuit breaker mean?
C16 means a circuit breaker with a C characteristic and a rated current of 16 A. The breaker never trips instantaneously below 80 A, always trips instantaneously above 160 A, and on sustained overload trips within one hour at 1.45 Γ 16 = 23.2 A.
Is a residual current device the same as a circuit breaker?
No, a residual current device detects a difference between outgoing and returning current (leakage to earth) and so protects people, but not against overload. A circuit breaker protects against overload and short-circuit. An RCBO combines both functions in one module.
Can a circuit breaker be used as a switch?
Circuit breakers to IEC 60898-1 are suitable for isolation and can therefore be used to de-energise a circuit. They are not intended for frequent switching of loads; contactors and switch-disconnectors are designed for that.
How long does a circuit breaker last?
A circuit breaker to IEC 60898-1 is tested for thousands of mechanical and electrical operations and typically lasts for decades under normal use. After several heavy short-circuits the contacts may be damaged, so replace the breaker preventively and check it during periodic inspections.
π In summary
A circuit breaker is a reusable protective device that protects cables and equipment against overload and short-circuit, with a characteristic (B, C, D) and breaking capacity matched to the load and the installation. In industrial installations, the right choice and selectivity of circuit breakers determines whether a fault stays local or shuts down the entire control system.