MCB (Miniature Circuit Breaker)

• CHARACTERISTICS

• Rated current not more than 100 A.

• Trip characteristics normally not adjustable.

• Thermal or thermal-magnetic operation.

Air Circuit Breaker• Rated current up to 10,000 A.• Trip characteristics often fully

adjustable including configurable trip thresholds and delays.

• Usually electronically controlled—some models are microprocessor controlled.

• Often used for main power distribution in large industrial plant, where the breakers are arranged in draw-out enclosures for ease of maintenance.

Vacuum Circuit Breaker

• With rated current up to 3000 A,

• These breakers interrupt the arc in a vacuum bottle.

• These can also be applied at up to 35,000 V. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers.

RCD (Residual Current Device / RCCB(Residual Current Circuit Breaker)

• Phase (line) and Neutral both wires connected through RCD.

• It trips the circuit when there is earth fault current.• The amount of current flows through the phase (line)

should return through neutral .• It detects by RCD. any mismatch between two currents

flowing through phase and neutral detect by -RCD and trip the circuit within 30Miliseconed.

• If a house has an earth system connected to an earth rod and not the main incoming cable, then it must have all circuits protected by an RCD (because u mite not be able to get enough fault current to trip a MCB)

• RCDs are an extremely effective form of shock protection.• The most widely used are 30 mA (milliamp) and 100 mA

devices. A current flow of 30 mA (or 0.03 amps) is sufficiently small that it makes it very difficult to receive a dangerous shock. Even 100 mA is a relatively small figure when compared to the current that may flow in an earth fault without such protection (hundred of amps)

• A 300/500 mA RCCB may be used where only fire protection is required. eg., on lighting circuits, where the risk of electric shock is small.

MCCB (Moulded Case Circuit Breaker)

• Rated current up to 1000 A.

• Trip current may be adjustable.

• Thermal or thermal-magnetic operation

MCB Selection• he first characteristic is the overload which is intended to prevent the accidental

overloading of the cable in a no fault situation. The speed of the MCB tripping will vary with the degree of the overload. This is usually achieved by the use of a thermal device in the MCB.

• The second characteristic is the magnetic fault protection, which is intended to operate when the fault reaches a predetermined level and to trip the MCB within one tenth of a second. The level of this magnetic trip gives the MCB its type characteristic as follows:

• Type Tripping Current Operating Time Type B 3 To 5 time full load current 0.04 To 13 Sec Type C 5 To 10 times full load current 0.04 To 5 Sec Type D 10 To 20 times full load current 0.04 To 3 Sec• The third characteristic is the short circuit protection, which is intended to protect against

heavy faults maybe in thousands of amps caused by short circuit faults.• The capability of the MCB to operate under these conditions gives its short circuit rating in

Kilo amps (KA). In general for consumer units a 6KA fault level is adequate whereas for industrial boards 10KA fault capabilities or above may be required.

Fuse and MCB characteristics

• Fuses and MCBs are rated in amps. The amp rating given on the fuse or MCB body is the amount of current it will pass continuously. This is normally called the rated current or nominal current.

• Many people think that if the current exceeds the nominal current, the device will trip, instantly. So if the rating is 30 amps, a current of 30.00001 amps will trip it, right? This is not true.

• The fuse and the MCB, even though their nominal currents are similar, have very different properties.

• For example, For 32Amp MCB and 30 Amp Fuse, to be sure of tripping in 0.1 seconds, the MCB requires a current of 128 amps, while the fuse requires 300 amps.

• The fuse clearly requires more current to blow it in that time, but notice how much bigger both these currents are than the ’30 amps’ marked current rating.

• There is a small likelihood that in the course of, say, a month, a 30-amp fuse will trip when carrying 30 amps. If the fuse has had a couple of overloads before (which may not even have been noticed) this is much more likely. This explains why fuses can sometimes ‘blow’ for no obvious reason

• If the fuse is marked ’30 amps’, but it will actually stand 40 amps for over an hour, how can we justify calling it a ’30 amp’ fuse? The answer is that the overload characteristics of fuses are designed to match the properties of modern cables. For example, a modern PVC-insulated cable will stand a 50% overload for an hour, so it seems reasonable that the fuse should as well.

Motor Protection Circuit Breakers