Circuit Breakers- How they work and the different types

What is a CB?

Circuit breakers are safety devices that stop the flow of electricity when its specifically rated parameters are passed. These can come in a few forms, the first is an Overcurrent Protection Device or OCPD, which you can learn all about in Article 240 of the NEC. These circuit breakers do exactly what they say they do, using various methods to prevent a branch circuit from receiving too much current or amperage. Another option is a fuse. Fuses are tubes with metal filaments inside that melt and disintegrate when too much current travels through them, breaking the connection and opening the circuit. Unlike breakers, fuses no longer function after the first short and must be replaced. 

How does it work?

Thermomagnetic circuit breakers work with two elements, these are the most common breakers and the type we usually use in homes and small businesses. The first element is a bi-metal strip. When too much current passes through this bi-metal strip, it heats up causing the strip to warp and change shape, releasing the spring inside of the breaker, opening the circuit to stop the flow of electricity. This slow build-up of heat is caused by an Overload, which happens when the circuit is demanding more current than the breaker is rated for. The amount of time it takes for this breaker to trip is directly related to how much amperage it receives. This mechanism is what classifies this as an Inverse-time breaker. The more amperage they receive the quicker they trip, but if the overload is minimal, say two or three amps, it will take much more time to trip than if the breaker experienced a short. Google an inverse breaker timetable and get a good look, it’s pretty cool stuff.

The second element of thermomagnetic breakers is magnetic! When a short occurs in a circuit, an intense amount of electricity is released in the form of light and heat. This huge flow of electricity creates a magnetic field that instantaneously separates the magnetic strips inside the breaker, causing it to release the internal spring that trips the lever and opens the circuit. This type of tripping classifies these breakers as Instantaneous. Because of how many amps these short circuits can generate, most standard residential circuit breakers are rated to withstand 10,000 amps before completely blowing up, melting and dying a terrible, fiery death. 

MCB with 10kA Short-Circuit rating

Some breakers come with extra components that allow for more versatile control, like the Shunt-Trip breaker, which communicates with other important systems like security and fire suppression. These systems tell the breaker when to trip without triggering either of the thermomagnetic elements and is essential for some of the emergencies that can happen in restaurants and factories etc. Another type is Electronic or “smart” breakers, which are designed to fit into a panel that is connected to a computer system so it can be controlled remotely.


There are two styles of circuit breakers, varying vastly in ratings and sizes. The first and more broadly used are Mini Circuit Breakers or MCBs. Smaller in size, these breakers fit into residential services and distribution panels of commercial businesses to protect standard branch circuits. These breakers have a set amperage rating with no interchangeable parts, usually meant for smaller current loads between 10a and 125a.

Example of a miniature circuit breaker (MCB)

The second style is the Moulded Case Circuit Breaker or MCCB. This breaker is typically quite large and used mostly in commercial applications where more amperage is needed. Most MCCBs are adjustable so you can customize the trip ratings, this helps coordinate specific trip events in industrial electrical distribution systems. Labeled MCCB because the big brick case is rated for varying currents, while the interior mechanisms are interchangeable for versatile factory production.

Example of a molded-case circuit breaker (MCCB)

Discontinued  panels and their breakers

One problem our field faces, especially in service work, is coming across panels that have been outlawed and discontinued but still need that one replacement breaker to fix a customer’s problem circuit. The most common brands you might have heard of are Federal Pacific, Zinsco, Challenger plus a few others. As of right now, the industry’s only solution comes from Connecticut Electric, a company that essentially manufactures new breakers for old panels. While this may not seem like big news, it is vital for homeowners and electricians alike to have an alternative to replacing panels, which may not always be possible due to time or financial constraints. 

Discontinued Zinsco circuit breaker

Products used in Video

Square D Homeline 20A breaker

Square D QO 20A breaker

Square D QO 20A AFCI plug-on neutral breaker

Zinsco 20A Breaker

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to top button