The Complete Guide to Circuit Breakers

As much as electricity is a basic necessity in everyday life, it comes in doses that can kill. Protecting yourself and loved ones means all residential, commercial, and industrial buildings will be fitted with some sort of electrical protection device, that cut off the current when they sense an electrical fault, such as overheating or electrical surges. This keeps anyone near exposed wiring, sockets, or any part of the circuit safe. Protection devices also protect all equipment and appliances. And while there are different types used for different purposes and different settings, basic to all circuits is the humble circuit breaker. 

What is a Circuit Breaker?

circuit breakers

A circuit breaker is a protection device that interrupts (or ‘breaks’) the flow of current in an electrical circuit, thereby protecting all surrounding electrical components and wiring from damage in cases of electrical overload and short-circuiting. The most common type of breakers and ones found in very low current environments, averaging 100 amps or less, like homes and offices, are called Miniature Circuit Breakers (MCBs). These are what you’d find in switchboards and distribution boards. For high current settings (in the vicinity of 2500 amps) like industrial and manufacturing facilities, look for Molded Case Circuit Breakers or MCCBS. The two types of breakers serve a similar purpose but are used in different areas. 

Parts of a Circuit Breaker 

A circuit breaker is rated for the current it can safely carry and the current it can safely interrupt. To understand how this works, let’s first break down the parts that make up a circuit breaker. 

Housing – the housing or frames in breakers are made of thermoset plastics in low-current applications, while those used in medium and high loads have metal frames.  Both materials are tasked to protect the components inside.
Terminals – these connect the circuit breaker to the circuit it has to protect. They consist of pieces of conductive metals.  

Switch – allows for the opening and closing of the circuit. The switch can be in three possible positions: ‘on’, ‘off’, or ‘tripped’. The tripped position results when the current exceeds that for which the circuit breaker is rated. The switch can also be set to the off position, for instance during maintenance.

Contacts – these are two metallic pieces that move to close or open (or ‘break’) the circuit, such as when there is a current overload. One contact is connected to the actuator mechanism and the other to the main panel or switchboard. 

Actuator mechanism –  is the component consisting of a metal arm with one end connected to the circuit breaker switch and the other to a contact. Flipping the switch upwards in the ‘on’ position forces the contacts together. Flipping it down in the ‘off’ position causes the actuator mechanism to pull the contacts apart. 

Trip Unit – this is what activates the actuator mechanism when there is an overload, short circuit, or other electrical faults. There are two basic types. In low-current breakers, the trip unit is electromechanical, consisting of a bi-metal and electromagnet that work together to determine when to trip the circuit. Electronic trip units measure and time the current in the circuit and initiate a trip signal when needed. 

So, How Does a Circuit Breaker Work? 

Turning off the electricity from the circuit breaker

With the switch in the ‘on’ position, electricity can freely flow from the power source and switchboard and through the contacts, the trip unit, and out of the upper terminal to power whatever is connected. When there is a current overload, the bi-metallic strip in the trip unit overheats and bends and automatically opens the circuit to cut off power. During an arc and ground fault or short circuit, the contacts are separated by the increase in electromagnetic force, and the circuit is tripped and power cut off. 

Types of MCBs

While there are several kinds of MCBs on the market, the most common are type B, type C and type D breakers. They differ in their ‘trip curve’ or how the breaker trips a circuit based on the amount of excessive current and the time needed to open the circuit. Understanding the trip characteristics of different circuit breakers makes it easier in choosing the right type for the intended application. 

Type B MCBs are designed to trip if the current flowing through it reaches three to five times its recommended maximum or its ‘rated load’. These are the most common types found in homes and low-current settings. 

Type C MCBs trip the circuit if the current is between five and ten times the rated load. These are found in high-current environments, such as commercial and industrial facilities. 

Type D MCBs trip circuits that undergo extreme current surges, typically ten to twenty times the rated load. Common uses include circuits that power transformers and welding equipment. 

Choosing the Right Circuit Breaker

Choosing the Right Circuit Breaker

Number of Poles

MCBs can also be divided by the number of poles they take. Single pole MCBs have one switch and protect a single phase of the circuit. A double pole circuit breaker has two switches and protects two phases and a neutral connection. Three-pole variants can break the current on all three circuits, and four-pole MCBs are used in unbalanced circuits and contain four switches, three phases, and a neutral. 

Ampere Rating

This is the maximum current circuit breakers can handle without tripping. Specifying a breaker will depend on the loads and equipment connected. Ampere ratings in B and C-type low-current MCBs range from 2A to 125A. Keep in mind that breakers will safely handle up to 80% of their overall amperage. 

Breaking Capacity 

This is defined as the maximum amount of current a breaker can interrupt during overloads, surges, and short circuits without damage. The breaking capacity needs to roughly match the strength of potential surges in the vicinity of the circuit. The maximum for B and C-type breakers tops out at 10KA or 10,000 Amperes.