MCB, MCCB or ACB
The three families cover ascending current ranges and ascending levels of adjustability. As the rating rises, protection settings move from fixed to fully adjustable, because larger installations need discrimination between devices in series.
| Device | Typical range | Adjustability | Usual position |
|---|---|---|---|
| MCB | Low current final circuits | Fixed characteristic | Final distribution boards |
| MCCB | Moderate to high current | Adjustable thermal, often adjustable magnetic | Sub-main distribution, motor feeders |
| ACB | High current main incomers | Fully adjustable electronic trip unit | Main incoming and bus-section duty |
Breaking capacity is the safety rating
Breaking capacity is the maximum prospective fault current the device can interrupt safely. It must equal or exceed the fault level available at its point in the installation, which is determined by the supply transformer and the impedance of the cabling between it and the device.
A breaker rated correctly for load current but below the available fault level may fail catastrophically during a short circuit rather than clearing it. This is why fault level at each board has to be established during design rather than assumed, and why a like-for-like replacement should never be downgraded on breaking capacity to save cost.
Tripping curves
A tripping curve defines how quickly the magnetic element operates relative to the rated current. The right curve lets a circuit start normally without nuisance tripping while still clearing genuine faults quickly.
- B curve — trips at a low multiple of rated current; suits resistive loads and long cable runs with low fault current
- C curve — the general-purpose choice for mixed loads with moderate inrush
- D curve — tolerates high inrush; used for transformers, motors and other highly inductive loads
- K and Z curves — specialist characteristics for motor circuits and sensitive electronic equipment respectively
Contactors and overload relays
A contactor switches a load repeatedly under normal operation; a breaker protects it against faults. They are complementary, not alternatives. Contactors are selected on utilisation category — the same contactor carries a lower rating switching a motor than switching a resistive load, because motor starting current and the arc energy at break are far higher.
- AC-1 covers resistive and lightly inductive loads
- AC-3 covers squirrel-cage motor starting and switching while running — the common motor case
- AC-4 covers inching, plugging and reverse braking, which is far more severe duty
- Overload relays protect the motor thermally and must be set to the motor full-load current, not the contactor rating
Discrimination and coordination
In a correctly coordinated installation, a fault on a final circuit operates only that circuit’s device, leaving upstream boards live. Achieving that requires the upstream device to be slower and higher-rated than the downstream one across the whole fault range — which is why adjustable trip units matter at the larger sizes.
What to include in a switchgear enquiry
- Device type, current rating and number of poles
- Required breaking capacity, based on the fault level at that point
- Tripping curve or trip unit settings required
- System voltage and frequency
- Enclosure requirement and ingress protection rating
- Accessories — auxiliary contacts, shunt trips, undervoltage releases, motor operators
- Brand preference, quantity, delivery city and required date