Most places that earlier used fuses in low voltage electrical networks now commonly use miniature circuit breakers (MCB) instead. Although an MCB is much larger than a fuse holder is, the MCB has several advantages when compared to fuses.
Any abnormal condition of the network, meaning overload or fault conditions, causes the MCB to sense and automatically switch off the electrical circuit. While a fuse does not sense it, the MCB senses the abnormality in a more reliable way. Additionally, the MCB is a more sensitive device for sensing an overload than the fuse.
A blown fuse can only be confirmed by opening the fuse grip or cutout from the fuse base. For an MCB that has tripped, the switch-operating knob comes down to its off position, and this is easily visible from a distance. That allows the faulty zone of the electrical circuit to be identified easily.
Restoring the operation of supply after repair of fault takes time as the blown fuses have to rewired or replaced with the proper type. With an MCB, restoration is very quick, as it involves only switching on operation.
Deciding on a fuse for protecting an electrical circuit is not easy, as it involves selecting the proper wire gauge and material. In most cases, people use any readily available thin wire as a fuse. Comparably, deciding on an MCB is much easier, as the manufacturer offers all the specifications that help in taking a decision.
An MCB is more expensive than the cost of a simple fuse wire and base. However, the benefits of the MCB more than compensate for the increase in expenses. Although very robust and maintenance free, the MCB has to be periodically switched off and on for the spring inside to retain its tension. A non-working MCB has to be simply replaced by a new one when it malfunctions.
The MCB protects an electrical circuit from two major faults—overcurrent and short circuits. The mechanism of the MCB tripping differs for the two faults. Long time overload current causes thermal heating, which affects a bimetallic strip. As the bimetallic strip bends, it ultimately disconnects the circuit.
Shorts in the circuit are more dangerous, as they cause a huge rush of current within a very short time. The circuit may catch fire before the bimetallic strip can heat up sufficiently to trip. To overcome this, MCB has an electromagnetic plunger associated with a tripping coil. A sudden increase in the current levels trips the electromagnetic plunger, opening the circuit breaker.
Some MCBs have three positions of operation. The means of manual opening and closing operation of the MCB are designated as ON and OFF. The third position is marked as TRIPPED. This makes it easy to determine the condition of the MCB whether it has been manually closed, tripped, or has been manually switched off.
For an MCB, the trip unit is its main part. It is responsible for the proper working of the MCB. As with the fuse, for an MCB also, the important factor is the time it takes to trip. The manufacturer of the MCB gives this as the I2t figures.