Selecting the right circuit breaker for your application
Most designers have a pretty good idea of what a circuit breaker is and what it does; if you need a refresher, here’s a quick definition: A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. It detects a fault condition and interrupts current flow.
Specifying the right breaker early in the design process can lead to a more robust design, avoid redesigns, decrease development costs, and add value to the end product. Seems pretty simple, right? But selecting the best circuit breaker for your application is a multi-step process. The basic functional parameters—voltage, current, and number of poles—are just the beginning. To refine your selection, you then have to consider mechanical and environmental factors, regulatory and safety issues, size and cost constraints, and more.
Circuit Breakers 101
Let’s begin with a review of key circuit breaker parameters. A number of technology options are available to accomplish the circuit breaker function, each with slightly different performance characteristics. Table 1 gives an overview of five; depending on your precise requirements, several types may be suitable for your application. Circuit breakers using different technologies will exhibit subtly different performance characteristics under overload conditions. Figure 1 shows these variations. The horizontal axis (current) shows multiples of the continuous current rating for the breaker; the vertical axis (time) shows how long it will take the circuit breaker to trip at the given current. The widths of the curves indicate typical tolerances. It’s recommended that you consult a data sheet for more information.
Derating Factors
The specifications of a circuit breaker are only valid under the conditions quoted in the data sheet; a frequency of 60 Hz in an open-air environment at 40ºC ambient, for example. If the real-world conditions differ from those under which the specifications were derived, derating factors must be applied
Packaging
Circuit breakers come in a variety of sizes and mounting configurations for use on printed circuit boards, racks, and panels, all the way up to highly specialized installations for high-voltage electrical grid use.
Selecting the Right Circuit Breaker
A circuit breaker, to most design engineers, is thought of as a low-cost commodity component that usually doesn’t get much attention when designing a new piece of equipment or system. In fact, on many occasions I have heard someone say, “It doesn’t matter; I just need a circuit breaker.” Yet a circuit breaker is an extremely important system component, and should not be handled in such a cavalier fashion. They are used to protect valuable equipment or devices from damage or more importantly they are used to prevent a life-threatening system disaster such as fire. It is important to take the time and make sure that the circuit breaker you are selecting is not only the right type for your application but it is also of high quality
Today, there are many circuit breaker choices to fit virtually any industrial DIN rail application. However, even with so many choices available, selecting the proper circuit breaker can be an easy task
Supplementary Protection
Supplementary circuit breakers, often referred to as UL1077 circuit breakers, are protectors that are typically used to provide protection to a device. Usually a branch UL489 circuit breaker is already installed up stream of the supplementary circuit breakers to protect the circuit wiring. Most supplementary circuit breakers are available in one, two and three poles with the option of short (B), medium (C) and long (D) trip curves. They typically can handle a minimum rating of 1A and a maximum amp rating of 63A. For convenience and ease of use the circuit breakers selected should have a reliable wire connection and have an on/off indicator to minimize troubleshooting time.
DC Protection
The DC circuit breakers are used in applications to provide protection where direct current or DC voltage is used. DC circuit breakers are available in both UL1077 and UL489 versions. Generally the DC circuit breakers are available in one or two poles with the short (B) or medium (C) trip curves. They are available with current ratings ranging from 1 to 63 Amps
Ground Fault Protection
The ground fault circuit breaker is used to provide protection to equipment that is located in wet or damp environments. The way this device works is when a ground fault or earth leakage greater than 30mA is detected the unit trips and opens the circuit breaker. On most ground fault circuit breakers a visual trip indicator and push to test button are standard features.
Circuit Breaker Selection Tips
In power plants and substations, they protect the main equipment from overloading, short circuit and thus, partial or total damage which costs very much
In branch circuits, they protect mainly the cables from overloading and breakdown also they protect the load from overloading in some cases.
They protect you from leakage current in case of earth leakage circuit breakers. As in case you touched a live wire, the breaker senses the leakage current though your body to ground and then disconnects the circuit.
As we briefly showed the common types of circuit breakers and where they are being used, now we will discuss how to select a circuit breaker on the scope of medium and low voltage
Dynamic Load: the unique aspect regarding this type is the electro-magnetic field to operate. So obviously we are talking about motors and transformers which draw higher current than rated at starting.
How to Determine Amperage of Circuit Breaker
Each circuit breaker has a specific rated amperage, or amount of current. When that amperage is exceeded, the circuit breaker shuts down the flow of current in that circuit to prevent damage to the wiring and appliances. Learn how to calculate the actual amperage of devices on the circuit and compare it to the rated amperage, so you can avoid unnecessary power interruptions and fire hazards.
Examine the electric panel. Each circuit breaker should have its amperage marked on the handle. This is the maximum amperage that the circuit can take before the circuit breaker trips.
Multiply the amperage by 0.8. For everyday use, it’s a good idea to expose the breaker to a maximum of 80% of the rated amperage. It’s fine to exceed this for short periods of time, but continuous current above this amount could cause enough heat to trip the breaker
Understand double-pole breakers. Some high-voltage devices may be wired to a double-pole circuit breaker — two standard circuit breakers sharing a handle. Do not add together the amperage of the two breakers. Both circuits will be tripped simultaneously by the amperage displayed on one circuit breaker handle
Compare this to the current on the circuit. Now you know how much amperage your wiring and circuit breaker can handle. To find out whether your circuit exceeds this amperage, continue to the next method.
Choosing the Right Circuit Breaker
Determine the Circuit Breaker Type
First, you need to determine whether you need a standard breaker, a Ground Fault Circuit Interrupter (GFCI), or an Arc Fault Circuit Interrupter (AFCI).
Identify the Manufacturer and Brand of the Breaker and Panel
Not all circuit breakers are interchangeable.
Check the Electrical Specifications
Next, you will need to find out a range of specs. If you are replacing an old breaker, you’ll find all the info you need on the unit itself
When you’re trying to determine amperage, but you do not have an old circuit breaker for comparison, you can use the wire gauge to guide your decision. You’ll find the gauge printed on the wire that will connect to the new breaker. The lower the gauge, the higher the breaker amperage. For example, a 10-gauge wire will need a 30-amp breaker, while a 14-gauge wire requires a 15-amp unit.
Confirm Any Special Requirements
For developers or contractors, you may need to fulfill certain special requirements for specific codes, localities, or client preferences. For example, you may need to use only new circuit breakers as opposed to reconditioned circuit breakers, you may need high vibration specifications for industrial facilities, or you may need to choose between a manual or automatic reset.