MCBs, or miniature circuit breakers, are a type of switch used to automatically turn off electrical circuits during abnormal conditions. Often replacing fuses in low voltage electrical circuits as they are more reliable, more sensitive and are resettable. Miniature circuit breakers are designed to protect electric circuits from overcurrent.
Typically mounted on DIN Rail and used in consumer units and distribution boards in a range of industrial, commercial, and domestic applications. You can learn more in our MCB guide.
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When continuous overcurrent flows through an MCB, a bimetallic strip is heated and causes it to bend. This bending of the bimetallic strip releases a mechanical latch opening the contacts and turning off the MCB, thereby stopping the current to flow in the circuit.
To restart the flow of current the MCB must be manually turned on. This mechanism protects the MCB from faults arising due to overcurrent or overload and 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 using 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.
There are three common types of electrical circuit protection in residential, commercial, and industrial applications. The use is classified by a curve type, which determines the tripping characteristics of the MCB:
Curve B - trips between 3 and 5 times the load current
Type B will trip instantly at a rate of three to five times its rated current. These are normally used for resistive or small inductive loads where switching surges are very small. Therefore, these are suitable for residential or light commercial installations.
Curve C - trips between 5 and 10 times the full load current
Type C will trip instantly at a rate of five to ten times its rated current. These are normally used for high inductive loads where switching surges are high such as small motors and fluorescent lighting. In such cases, type C MCBs are preferred to handle higher values of short circuit currents
Curve D - trips between 10 and 20 times the full load current
Type D will trip instantly at a rate of ten to twenty-five times its rated current. These are normally used for very high inductive loads where high inrush current is very frequent. Applications include x-ray machines, UPS systems, industrial welding equipment, large winding motors, etc.