How to select the right MCB?

Introduction

MCB (miniature circuit breaker) is an electrical switching device, designed to protect wiring installation from over current faults. Its basic function is to detect a fault and interrupt continuity of supply immediately. Unlike fuse, which operates once and then has to be replaced, a circuit breaker can be reset (manually) to resume normal operation. MCB works on a bi-metal principle that protects against overload current and solenoid short circuit current.

Thermal operation of miniature circuit breaker is achieved with a bimetallic strip whenever continuous over current flows through MCB, the bimetallic strip is heated and deflects by bending. This deflection of bimetallic strip releases mechanical latch. As this mechanical latch is attached with operating mechanism, it causes to open the miniature circuit breaker contacts.

During short circuit condition, sudden rise in electrical current causes electromechanical displacement of plunger associated with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing immediate release of latch mechanism consequently open the circuit breaker contacts.

This is working principle of MCB. The inverse time delay characteristics of all MCBs complying with IEC 60898 must operate within the limits specified below.

Now, the question arises how to select MCB? We will study it in detail in subsequent sections of this article.

Did you know: L&T Electrical & Automation has comprehensive portfolio of MCBs with current rage 0.5-125A in single, double, three & four pole configurations with breaking capacities 6kA & 10kA.

How to Select MCB

Selection of MCB depends on the following three parameters

1. Current Rating of the MCB
2. Characteristic Curve as per the application
3. Short circuit breaking capacity in line with fault level

Let us study these criteria in depth.

1. Selecting Current rating of an MCB

In order to achieve overload protection, the rated current (In) of an MCB should not be less than the design current or rated current (Ic) of the circuit and rated current of MCB (In) should not exceed the current carrying capacity of the conductor or wire (Iw). It can be summarized as below:

Ic < In < Iw

As per the thermal characteristics table of MCBs, the current carrying capacity of wire (Iw) should be minimum 1.45 times of rated current of MCB (In).

Let us consider an example:

For a resistive load of 1100W, 220V supply, the load current comes to 5A. It can be calculated from

P = V I (For resistive load); Where,

P = Power in Watt, V= Voltage in Volts, I = Current in Ampere

Therefore, I = 5A Hence MCB rating to be selected should be equal to or greater than 5 A and the nearest MCB rating available is 6A.

Now, for 6A MCB, minimum trip current will be 1.45 times its rated current. Therefore, 6 A x 1.45 = 8.7A will be minimum trip current. Accordingly, the wire size should be selected with a current carrying capacity of more than 8.7A. So the nearest wire size available with current carrying capacity of more than 8.7 A is 1 sq mm.

Did you know: L&T E&A MCBs are IEC 60898 & IEC 60947 compliant. These MCBs possess advanced features and come in all types of characteristics curves.

2. Selecting Characteristic curve of an MCB

We are aware of different curves of MCBs. Let us consider some of them as listed below.

When normal resistive loads are being fed, the B characteristic MCB is sufficient. But in case of industrial application such as motors, special lightings (Inductive) and data equipment are being fed through the same B curve MCB may lead to unwanted tripping at the starting time. This makes it necessary to use C curve MCBs for such applications. In some application like UPS, Compressor motors (highly inductive), even these C curve MCBs may lead to unwanted tripping. Therefore, D characteristics of MCBs should be used for such applications. It is advised to discuss with a qualified person before selecting an MCB as per the application.

3. Selecting Short circuit breaking capacity of an MCB

The third selection parameter is the short circuit breaking capacity. Short circuit breaking capacity of MCB is the current that a breaker is able to interrupt without being destroyed. Short circuit faults are generally of higher magnitude. The ability of an MCB to operate under these conditions gives breaking capacity to it. Fault level mainly depends on the following parameter

I. Transformer rating - It is directly directional proportionally to the fault level

II. Fault impedance - If the distance of fault from the source (transformer in this case) is less than the fault impedance is less therefore the fault level is more and vice versa.

Fault level is dependent on transformer rating and impedance. Knowing this maximum fault level can be calculated. Accordingly breaking capacity of MCB I to be selected.

Other factors such as mechanical & electrical life expectancy, number of poles, energy class and operating voltage help in finalizing MCB selection.

Did you know: L&T E&A portfolio also brings in DC MCBs for various applications with current rating ranging between 0.5-63A.

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