MCB tripping again and again? Or not tripping at all?
Before replacing it, you should test whether the MCB is actually faulty.
MCB is a type of protection device. MCB is used to be connected at the back end; before the electrical load. MCB stands for “Miniature circuit breaker“. MCB is a type of “switch cum circuit breaker“. MCB provides protection form “overload “& “short circuit” conditions.
In fault conditions, When fault current crosses the particular current level under certain time limit, It disconnect(trips) the circuit.
This post further will discuss the MCB specifications, Its working & method of testing.
Table of content :
Mcb functions
MCB can be used in a multiple ways such as :
- Overload protection
- Short circuit protection
- As an isolator switch
- As a Terminal connector
Working mechanism of Mcb
MCB has 2 terminals; Input & output. As name suggest, Input power is connected across input terminal.
When MCB operator get lifted up, Input terminal get connected to output terminal. Potential get available to output terminal. While when MCB operator get lifted down or get trip, Input terminal get disconnected from output terminal.
MCB provides protection from “overload “& “short circuit” conditions. “Overload “is slow process phenomenon While “short circuit” is rapid phenomenon. For both the phenomenon, MCB has 2 different working mechanisms :
- Bi-metallic thermal tripping for overload
Inside the MCB , When current exceeds the limit through a Bi-metallic thermal strip, strip get heated up. Due to the heating, Bi-metallic thermal strip produces the bending movement. The bending movement further produces a trigger to trip circuit.
- Electromagnetic tripping for short circuit
In short circuit situation current exceeds the limit rapidly. This sudden excess current produces an sharp Electro-magnetic field which is used to trigger the trip circuit.
Both the trip mechanism are shown in an image below. Both the mechanism, Whether it is Thermal bimetallic or Electromagnetic coil, triggers the same trip circuit. Some times in some faults, Both the mechanism work together.
Mcb specifications
The choice of MCB depend upon multiple criterion’s. Some of the most important criterion’s discussed below:
- Current rating
- Curve type
- Voltage level
- Power type
- Pole
An image of C 32 2 pole MCB is shown below. Different sections of the MCB are marked Which explained next :
1. Terminal Blocks
2. MCB details like Make, Type, Tripping Curve, Current Rating & Voltage Rating
3. Switching Capacity
4. Switching Symbol
5. Operator for manual actuation
Method of testing of Mcb
There are two situation of MCB testing. Such as :
Testing on de-energized Mcb
1. Continuity Testing through Multi-meter
In this method, Continuity option available on multi-meter is utilized to test the working of MCB .
- In connected position, There should be continuity in between both the terminals.
- In disconnected position, There should not be continuity in between both the terminals.
2. Continuity testing through series test method
In series test method, Potential difference & a bulb are utilized to test the MCB . In Series test method phase/voltage directly passes through the bulb. While neutral or negative is directly connected to one of the terminal of the test specimen.
When these 2 test terminals of series test method are connected with each other, Bulb starts glowing. Bulb only glows when circuit is continuous otherwise bulb do not glows.
- If MCB is good, At connected position, Series test bulb glows across the MCB terminals.
- If MCB is good, In disconnected position, Series test bulb do not grow across the terminals of MCB .
Testing on energized MCB
Safe points to consider before testing the energized MCB :
- MCB surface should be clean & free form chemicals & acids.
- The test leads of multi-meter should be cleaned & free from wear & tear.
- The test leads should be well insulated from rated system voltage.
- Gear up the PPE such as gloves, shoes, safety shield according the voltage level.
- Before testing the MCB , Confirm the working of multi-meter & its test lead through continuity option.
- Before testing the MCB , Confirm the system voltage level & its type AC or DC. Set the multi-meter accordingly.
To test the energized MCB , Multi-meter is utilized to detect the potential difference present across the terminals of MCB . Further there are 2 methods to test the energized MCB .
1. Potential drop across MCB
In this method, Potential difference is been checked across both the terminals.
- When MCB is disconnected, Than there will be some potential difference across the terminals.
- When MCB is connected, Potential difference across both the terminals will be zero.
- If both the above said conditions are true Than MCB is working fine otherwise MCB is faulty.
2. Potential at output terminal
Potential/Voltage is been checked across the output terminal of MCB . Voltage is been checked with respect to “neutral “or “Earthing“.
- When MCB is connected, Voltage on output terminal is equal to voltage at input terminal.
- When MCB is disconnected, No voltage appear across output terminal.
If Both the above said conditions are true than MCB is working fine. Otherwise, MCB is faulty.
FAQs for selection of MCB
1. Can I test an MCB without removing it?
Yes, but ensure power is OFF for safety.
2. What is the normal resistance of an Mcb ?
It should be very low (close to 0 ohms).
3. Why does my Mcb trip again and again?
Possible reasons:
- Overload
- Short circuit
- Faulty MCB
4. Is it safe to test Mcb with power ON?
Only for voltage testing and only if you are experienced.
5. Can a multimeter detect a faulty Mcb completely?
It helps identify basic faults, but not all internal issues.
🧾 Conclusion
Testing an MCB using a multi-meter is a simple yet essential skill for maintaining electrical safety. By performing continuity, resistance, and voltage tests, you can quickly determine whether your MCB is functioning properly or needs replacement.
Always follow proper safety precautions and avoid working with live circuits unless you are confident in your skills.