Circuit Breakers

An MCB is a compact breaker for final circuits with fixed trip settings and lower short-circuit capacity, usually mounted on a DIN rail. An MCCB handles higher current, has a higher breaking capacity and usually offers adjustable thermal and magnetic settings. MCCBs are used on feeders and motor circuits, MCBs on lighting and small power.

An MCB protects against overload and short-circuit, an RCCB protects against earth leakage only, and an RCBO does both in one device. An RCCB needs a separate overcurrent device ahead of it. Use RCBOs where each circuit needs its own earth leakage protection without nuisance tripping of the others.

A VCB is for medium voltage, typically 3.3 kV to 33 kV, and interrupts the arc in a sealed vacuum interrupter. An ACB is a low-voltage breaker, used up to 1000 V, commonly at 415 V incomers. The system voltage decides: you cannot swap one for the other.

They describe the instantaneous trip threshold as a multiple of rated current. B trips at roughly 3 to 5 times, C at 5 to 10 times, and D at 10 to 20 times. B suits resistive loads and long cables, C suits mixed loads, and D suits motors and transformers with high inrush current.

Find the prospective short-circuit current at the breaker's location from the transformer kVA, impedance and cable length, and select a breaker whose rated breaking capacity is higher. For example, a 1000 kVA transformer at 5 percent impedance gives about 28 kA at its 415 V terminals. A qualified engineer should confirm the fault level calculation.