Know Your MCB – Working, Trip Curves & Replacement

In every living space, miniature circuit breakers (MCBs) are an essential part of protecting the electrical circuits. They prevent appliances from being damaged by the power supply, and they prevent fires that arise from short circuits and overload currents. But they are a bit of a mystery, and property owners often need a crash course in their functioning to make day-to-day life easier. 

How does this electro-mechanical contraption work? What are their different trip curves? How do you know if you have to replace an MCB? 

These questions have answers that can be moderately complex. In this blog post, we will take the time to clear up the doubts surrounding them. They are the last line of defense between your cherished household appliances and the external power supply. Therefore, it is worth noting their intricacies to know what to buy and install on one’s properties.    

1. How Does an MCB Work?

The idea is that once a certain current-related problem occurs, the circuit “trips” i.e. is broken to safeguard the wiring and the living space. A miniature circuit breaker is mainly targeted to solve two current-related problems.

i. Overload Current:

A higher load of current that the MCB’s current rating causes a thermal effect on the MCB. A bi-metallic strip in the MCB bends after overheating due to this. This releases the mechanical switch to separate the MCB contacts. And that stops the current from damaging the circuits.

ii. Short Circuit:

An unusual magnitude of current causes a considerable electromagnetic effect on the MCB. Due to the concomitant magneto-motive force, an electromagnet (a solenoid) with a plunger gets displaced, and the plunger strikes a trip lever, which releases the latch mechanism to causes the contacts to separate. This stops the current.   

Each appliance and circuit have a current rating, meaning that if a current supersedes this current rating, the MCB will trip to safeguard it. 

2.  What are the Types of MCBs in Terms of Trip Curves? 

Trip curves are the graphical representation of the behavior of an MCB based on the current and tripping times. As a thumb rule, the larger the current, the shorter the time it takes to trip.

Each MCB trips based on a certain range of current. There are 6 main types of MCBs based on their trip curves:

i. Type A Curve: This MCB trips between 2-3 times its full load current. This means it is highly sensitive and can safeguard sensitive appliances with resistive elements and negligible inrush current at startup: lighting, computers, and other electronic equipment. This makes it ideal for residential applications.

ii. Type B Curve: This MCB trips between 3-5 times its full load current. This also makes it ideal for residential spaces as well as for light commercial applications, ideal for cable protection and lighting.   

iii. Type C Curve: This MCB trips between 5-10 times its full load current. This higher resistance makes it ideal for a wide range of residential, commercial, and industrial applications. Can be used for small transformers, where inrush current isn’t very large at startup.    

iv. Type D Curve: This MCB trips between 10-20 times its full load current. This makes it ideal for heavier industrial and commercial applications for transformers, motors, and heavier machines, where inrush current can be quite high at startup.  

v. Type K Curve: This MCB trips between 8-12 times its full load current. Like D Curve MCBs, they are ideal for heavier motors, where inrush current rises at high rates during startup. Able to be used in residential, commercial, and industrial applications.   

vi. Type Z Curve: This MCB trips between 2-3 times its full load current. Given their high sensitivity, they are ideal for semiconductor devices. 

3. How do you know if you must Replace an MCB?

MCBs cannot be repaired. They are built for performance until replacement.

Before you can replace a miniature circuit breaker, you must know if it is malfunctioning or damaged. If it is frequently tripping and cannot reset then it is likely that either the appliances, plugs, circuit wiring, switches, or the miniature circuit breaker are faulty. How do you find out which? There are two general ways. 

i. Use the Process of Elimination:

To find out if the MCB is damaged, turn off all devices and switches, unplug everything, and reset the MCB. Then put on each appliance one by one until it trips again. If the MCB trips after you turn on a certain appliance, it means that the appliance is faulty. If not, and the MCB still remains tripped, then the fault is on the MCB or the wiring. That way, you know that the issue has to be fixed by a qualified electrician.

ii. Use a Multimeter Voltage Tester:

Some damaged MCBs give out burning smells or discolorations due to scorching, but many a time their damage is not overt. In such a case, after turning off all appliances connected to a circuit, attach a multimeter voltage tester (with a capacity of at least 230 V) to the terminal screw and neutral bar of the MCB. The multimeter will read 230 V if the miniature circuit breaker is working well. If not, it will show 0 V. In the latter case, contact an electrician to have it replaced.  If you want to do it yourself, you can take care to consider how the faulty MCB is connected to the distribution board, and then disconnect it. Find out the type of MCB that needs replacing and buy one from the many MCB manufacturers in the region. Then fix it in the same way you saw the faulty MCB connected to the distribution board.   

Despite property owners and real estate developers being able to hire qualified electricians to decide the type of MCB for installation, it is useful to know all these concepts. Finding the appropriate switchgear, such as MCBs, will serve a building’s electrical supply with fewer risks of damage. To make an educated choice, you can contact our MCB manufacturing company, Precision Electricals, where we have a variable range of miniature circuit breakers for residential and commercial applications.