About MCB Selection
MCB (Miniature Circuit Breaker)
1. Rated current not more than 100 A.
2. Trip characteristics normally not adjustable.
3. Thermal or thermal-magnetic operation.
The 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.
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 32Amps 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.