Fuse，MCB and MCCB
Fuse characteristic
Fuse circuit protection
Fuse can protect the cable as the design based on the I²t (let through energy) similar to cables characteristic as above. The current limiting fuse charateristic as follow。
Current limitation
Most electrical equipment has a withstand rating that is defined in terms of an RMS symmetrical-short-circuit current，and in some cases, peak let-through current。 These values have been established through short circuit testing of that equipment according to an accepted industry standard.。Or，as is the case with conductors，the withstand rating is based on a mathematical calculation and is also expressed in an RMS short-circuit current。
If both the let-through currents (IRMS and Ip) of the current-limiting fuse and the time it takes to clear the fault are less than the withstand rating of the electrical component, then that component will be protected from short circuit damage。
MCB characteristic
MCB circuit protection
MCB has a minimum trip time 0.01s to 0.02s as below figure and limited breaking capacity generally it is not greater than 9kA (M9)。
Based on the above constaint, it will affect the selection between fuse and MCB for protecting cables。
The terms for MCB and MCCB about the maximum short circuit current：
Breaking capacity is defined as the maximum short circuit current that a breaker can interrupt without damage to it。
Making capacity correspond to peak values of fault current while breaking values are the rms values so making capacity is greater than breaking capacity。
MCCB chararcteristic
MCCB circuit protection
MCCBs are same as the MCBs but have an adjustable characteristic as shown above and with a higher breaking capacity，the characteristic attached below for your reference。
General rules
A protective device (circuit breaker or fuse) functions correctly if：
- Its nominal current or its setting current In is greater than the maximum load current IB but less than the maximum permissible current Iz for the circuitIB ≤ In ≤ Iz
which corresponding to Zone a - Its tripping current I2 “conventional” setting is less than 1.45 Iz
I2 ≤ 1.45 Iz
which corresponding to Zone b - Its 3-phase short-circuit fault-current breaking rating is greater than the 3-phase short-circuit current existing at its point of installation。
which corresponding to Zone c
The “conventional” setting tripping time may be 1 hour or 2 hours according to local standards and the actual value selected for I2。
For fuses，I2 is the fusing (melting level) current If which will operate the fuse in the conventional time equal to k2 x In (k2 ranges from 1.6 to 1.9) 。
I2 ≤ 1.6 to 1.9 In
depending on the particular fuse concerned。
For In < 16 A ≈ In ≤ 1.31 Iz
For In ≥ 16 A ≈ In ≤ 1.10 Iz
Moreover, both the short-circuit current breaking capacity of the fuse ISCF and the short-circuit current breaking capacity of the circuit breaking ISCB must exceed the level of 3-phase short-circuit current at the point of installation of protection device(s)。
The amount of energy allowed to pass through the upstream device is less than that which can be withstood without damage by the downstream device and all associated cabling and appliances。
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