How to Read MCB Nameplate Data Rating Printed on It?

How to Read MCB Rating, Marking, and Labels?

The MCB (Miniature Circuit Breaker) is a commonly used protection device in domestic and industrial wiring. Similar to a fuse, it plays a crucial role in electrical safety. However, when purchasing an MCB, it’s important to exercise caution and make an informed choice.

Often, when we visit shops or seek advice from electricians, they recommend a particular brand without considering the technical aspects. For those of us who are not from an electrical background, understanding the technical specifications of an MCB can be challenging.

Many people simply look at the MCB’s rating and make a purchase, but this can be a mistake. While your electrician or shopkeeper may recommend the right product, it’s also important for you to have some understanding of what you’re buying.

It’s advisable to educate yourself about MCBs and their specifications before making a purchase to ensure you select the most suitable device for your needs.

• You may also read: Rated characteristics of Electrical Contactors

Selecting a proper MCB is generally straightforward, as most of the necessary information is typically printed on the device. Only a few local companies might obscure this information, but you can usually ignore them. Let’s go over the printed information you’ll find on an MCB:

• Also read: ELCB, RCB and RCD Circuit Breakers

1. Manufacturer’s Name:

This indicates the company that produced the MCB. For example, Schneider, ABB, Eaton, Siemens, Legrand, and Hager etc.

2. Model Number:

All reputable manufacturers have a specific code for each device type. It makes communication with sellers or manufacturers easier, especially in the case of service complaints.

3. Tripping Curve:

The first letter in the rating (e.g., C20, B25, C63) indicates the characteristics curve or tripping curve of an MCB. There are three common curves: B, C, and D.

• B curve trips quickly for sensitive electronic devices, with a short circuit rating of 3-5 times the standard rated current.
• C curve is for inductive loads like pumps or motors, with a rating of 5-10 times.
• D curve is for highly inductive or capacitive loads, with a rating of 10-20 times.

Choose the curve carefully based on your application. The numeral part indicates the rating of the MCB in amperes. For example, 20A means a 20-ampere rating.

4. Rated Current (I_n):

I_n is the maximum current the MCB can carry continuously without tripping. It’s measured in amperes (A).

Example: The numerical part of the B25 MCB indicates the maximum current rating that the MCB can safely handle is 25A. (As shown in above fig).

In the IEC (International Electrotechnical Commission) and NEC (National Electrical Code) standards, commonly available current ratings for MCBs include:

• IEC Standards: 6A, 10A, 16A, 20A, 25A, 32A, 40A, 50A, and 63A.
• NEC Standards: 15A, 20A, 30A, 40A, 50A, 60A, 70A, 80A. 90A and 100A

These ratings represent the maximum current that the MCB can safely carry continuously without tripping.

It is recommended to use 80% of the load on the rated breaker, meaning the breaker current rating should be 125% of the load circuit.

Example: a 20A breaker should be used for 16A load circuits. (See – How to Properly Size a Circuit Breaker?)

5. Operating Voltage (U_e):

The voltage rating is the maximum voltage at which the current rating is specified. For three-phase systems, it’s usually 400V or 415V. For single-phase systems, it’s 120V, 230V or 240V. Choose the rating according to your application’s voltage requirements.

Example: The operating voltage rating in the above fig is 230V AC.

6. Breaking Capacity (I_cu or I_cn):

I_cu or I_cn is the maximum fault current the MCB can safely clear. It is written in amperes, such as 10000 for 10kA. Choose the breaking capacity based on the possible fault level, ensuring it is higher than the expected fault level.

Example: The maximum short-circuit current in the MCB shown in the above figure is 10000A. This means the MCB can safely handle a fault current of up to 10kA.

7. Number of Poles:

MCBs can be single-pole, double-pole, or triple-pole, or four pole depending on the number of conductors they can interrupt.

Example: The MCB shown in the above figure is 2-pole. This means it can be used for 120V or 230V circuits using Phase and Neutral, or for 240V AC circuits using 2 Hot wires.

8. Short Circuit Protection:

Some MCBs indicate the type of short-circuit protection they provide (e.g., magnetic, thermal).

9. Energy Class:

MCBs limit fault current to prevent it from reaching its peak, but some energy is still generated. This energy, termed let-through energy, should be limited for efficient MCB operation. MCBs are classified into classes based on the amount of let-through energy. Class 3 allows a maximum of 1.5L joules/second.

10. Status Indicator:

A clear ON-OFF indicator is essential for safe operation. Never buy an MCB without a clear status indicator, as confusion can lead to serious damage.

11. Operation Symbol:

This symbol, printed by good manufacturers, illustrates the MCB’s operation mechanism.

12. Catalog Number:

Most MCB manufacturers include the catalog number of their products. This code provides overall information on the manufacturer’s website, such as MCB specifications and datasheets

13. Additional Relevant Information:

Information like impulse voltage, ISI marking, and wiring diagrams is usually printed on the side of the MCB. While there are many parameters to judge the quality of an MCB, this guide aims to make you aware of the printed information on an MCB.

14. Standards Compliance:

Look for markings indicating compliance with relevant standards (e.g., IEC, UL).

• Also read: Air Circuit Breaker (ACB): Construction, Operation, Types and Uses

General MCB Rating, Marking, Labels & Specifications

Below is the typical 2-pole MCB nameplate rating data, which shows the most important information printed on it. These generally include:

• Product Modal No
• Max Current Rating (6, 10, 16, 20, 25, 32, 40, 60, 63)
• Operation Symbol
• Breaking Capacity Type
• Breaking Capacity (Max Short Circuit Current)
• Operating Voltage (120V, 230V, 240V, 400V, 415V 440V, 480V)
• Tripping Curve Type
• Energy Class
• ON-OFF Indication
• Catalog No

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You may also read:

• GFCI: Ground Fault Circuit Interrupter. Types & Working
• Important Terms and Definitions related to Motor Control and Protection
• High Inrush Current in Capacitor Switching and Ways to Prevent It.
• Fault Current Limiter and Their Types
• Air Circuit Breaker (ACB): Construction, Operation, Types and Uses