In short. Choose the device by current, breaking capacity at the installed point, trip characteristics and the need to coordinate with devices upstream and downstream. Breaking capacity must exceed the maximum fault level at that point, including the contribution of generators and motors.
Device families
| Device | Typical range | Typical use |
|---|---|---|
| ACB | Larger currents, high short-time withstand | Incomers, couplers, generator breakers |
| MCCB | Medium currents | Feeders, motors, sub-distribution |
| MCB | Small currents | Final circuits, controls |
| Fuses | Wide range, fast current limiting | Motor circuits, back-up, switch-fuse |
Key ratings
- Rated current (In): continuous current, with enclosure and temperature derating.
- Breaking capacity: the highest fault current it can interrupt safely. Check ultimate and service values.
- Short-time withstand: for ACBs used with time-delayed selectivity.
- Trip unit: thermal-magnetic or electronic. Electronic units offer adjustable long-time, short-time, instantaneous and earth-fault settings.
- Poles: three or four pole, which matters for neutral switching and generator changeover.
Selection steps
- Calculate load current and apply derating.
- Find the maximum fault level at the device location. See fault-level engineering.
- Choose a device with a breaking capacity above this level.
- Choose the trip unit to give the settings range for coordination.
- Check selectivity with neighbouring devices. See coordination and selectivity.
- Check cable protection by the device.
Generator-specific points
- A generator breaker should coordinate with the alternator’s damage curve.
- Low generator fault current may be below the instantaneous pickup of a large feeder breaker. Check that the device trips on the generator alone.
- Four-pole breakers may be required when switching between sources with a solid neutral. See ATS engineering.
Illustrative example
A feeder to a 160 kW motor group has a load current near 300 A and a fault level of 30 kA at the board. An MCCB with an electronic trip unit and a breaking capacity above 36 kA is chosen. Its short-time setting is set to allow downstream devices to trip first. On generator supply the fault current is lower, and the setting is verified to still trip within the time the cable can withstand.
Selecting a breaker: the parameters
| Parameter | Meaning |
|---|---|
| Rated current (In) | Continuous current at a stated ambient |
| Rated voltage | System voltage the device is designed for |
| Breaking capacity (Icu, Ics) | Maximum fault current the breaker can interrupt; service value Ics is the more meaningful for repeated duty |
| Short-time withstand (Icw) | Ability to carry fault current for a time, for selectivity |
| Trip unit | Thermal-magnetic or electronic with adjustable settings |
| Poles and neutral rating | 3P, 4P, with a neutral sized for harmonics where needed |
Choosing between types
- MCB: final circuits and small loads, fixed settings, limited breaking capacity.
- MCCB: feeders and motors, adjustable settings on many models.
- ACB: main incomers and bus couplers, high breaking capacity, short-time withstand, good for selectivity.
Check against the fault level
The breaker’s breaking capacity must exceed the prospective fault current at its location. Compute the fault level from both utility and generator sources and use the higher. See fault-level engineering.
Generator incomers
Generator breakers are selected for the generator’s rated current, the lower fault level and its decaying shape. Choose a trip unit that will actually trip at the generator’s fault current, and check coordination with downstream devices.
Derating and installation
- Ambient temperature inside enclosures higher than outside; apply derating.
- Terminal temperature limits for cable size.
- Grouping in a board raises temperature.
- Altitude and humidity per maker.
Maintenance considerations
Draw-out ACBs allow testing and replacement without long shutdowns. Moulded-case devices are usually replaced rather than repaired. Record trip tests and operation counters.
Common mistakes
- Selecting a device by current only.
- Ignoring motor contribution to the fault level.
- Using the same settings on grid and generator without checking.
What to send us
Send the single-line diagram and fault data. Send your application details. See also fault current basics.
Questions engineers ask
When do I need an ACB rather than an MCCB?
For higher currents and where high short-time withstand is needed for selectivity, such as incomers and couplers.
What is breaking capacity?
The maximum fault current the device can interrupt safely at a given voltage.
Why four-pole breakers?
To switch the neutral when changing between sources, which supports correct earth-fault protection.
Do fuses still have a role?
Yes, for fast current limiting and motor circuits, often with a contactor and relay.
This guide gives general principles. Check manufacturer data for ratings.