Tag Archives: MCB

What are Digital Circuit Breakers?

We need protection from fires resulting from an electrical overload caused by a faulty device or an accidental short circuit. The huge current from the overload heats up wires and their insulation may go up in flames. There are several ways to activate this protection.

The oldest method consists of a fuse wire. Usually, this is a thin wire enclosed in a casing. The material of the fuse wire is carefully chosen to heat up and melt (blow) when a certain current level is exceeded. Melting of the wire disconnects the circuit and interrupts the current, preventing heat buildup. Once a fuse wire blows, it has to be replaced by a similar wire to continue protection and reestablish electrical operation.

Nowadays, it is common to see switchboards where the fuse holder has been replaced by a miniature circuit breaker (MCB). The device has a bi-metallic spring holding pair of mechanical contacts, which can establish connection by throwing an external switch. An electrical overload causes the bi-metallic spring to trip and the contacts open up, disconnecting the fault from the rest of the circuit. Once the fault has been cleared up, the MCB can simply be rearmed by flipping the external switch.

Although simpler to operated compared to the fuse wire, MCBs have their own disadvantages of being slow to react and expensive, with their cost going up proportional to their trip current. Over time, the bimetallic strip tends to deform, reducing the current capacity of the breaker and its accuracy. The mechanical construction of an MCB makes it prone to wear and tear.

Opening mechanical contacts to interrupt high currents often causes an arc flash to jump across the contacts. It is necessary to quench the arc flash within a short time to prevent incidence of fires.

For overcoming the above problems, using a digital circuit breaker offers the most convenient solution. The device has an all-electronic construction involving an electronically controlled automatic switch. There are no mechanical components involved, no bi-metallic strips, and no electromagnetic coils inside.

Atom Power is proposing a solid-state digital circuit breaker to replace the traditional types and thereby avoiding the related problems. Currently awaiting approval from the Underwriters Laboratory (UL), Atom Power has two models, one each for AC and DC circuits.

So far, Atom Power was producing only a few numbers of their digital circuit breakers, using their in-house 3-D printers for producing the plastic parts of the housing. With increase in production, they will use the resources of an external rapid manufacturing company, and will move to injection molding for higher volumes of commercial operations.

The Atom Switch, within the breaker, responds to a digital signal generated whenever the current exceeds a certain level, whether due to overload or short-circuits. With tripping speeds exceeding 16,000 times those of its mechanical counterparts, the arc flashes simply do not happen.

Another technique used to prevent arc flashes is to switch the device off when the AC voltage passes through zero. This is called zero voltage switching or ZVS, and is a very useful technique to prevent arcing across the open ends of the circuit.

What are Miniature Circuit Breakers?

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.