How to Choose the Right Circuit Breaker
Circuit breakers are a very important part of electrical safety. They control the amount of electricity that flows through a building’s electrical wiring system. If your home suffers an electric overload or a short circuit, a properly functioning circuit breaker will detect the issue and cut off the electrical supply. This will protect your wiring and appliances until you take care of the problem and restart the electricity. However, in order for a circuit breaker to do its job properly, you have to match the right circuit breaker with your exact needs. Read the following simple explanations and you’ll be ready to choose the appropriate circuit breaker for your home.
Capacities of Circuit Breakers
Circuit breakers are available in 3 different voltage capacities. Each is made to accommodate a specific amount of electricity.
Low-voltage thermal magnetic circuit breakers are best suited for most single-family homes. They allow for electrical currents that measure only up to 1000 amps.
Medium voltage circuit breakers are used in larger buildings, such as apartment complexes and businesses, that use up to 72,000 volts on a regular basis
High-voltage circuit breakers are used alongside power lines and in other places that use more than 72,000 volts regularly.
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
Typical temperature-derating table
Similarly, operation at higher or lower frequencies than specified may also require modification of the specification. On the one hand, eddy currents and iron losses at the higher frequency cause greater heating within the thermal-trip components, requiring the breaker to be derated. At the low end, even DC operation can cause issues with extinguishing the arc that occurs when the contacts are separated in high-current applications.
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
DIN rail circuit breakers are available in different package configurations ranging from a high-density space-saving size of 13mm per pole to 26mm per pole for a high-current type. Over the last few years, the price of a high quality circuit breaker has dropped below $10.00 per pole. This transition has allowed many more DIN rail mount circuit breakers to be used in place of fuses and fuse holders. In addition to being a cost savings, the circuit breaker is much more convenient; when a fuse blows, it has to be replaced by a new one; when a circuit breaker trips, it simply has to be reset.
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
Circuit Breaker Selection Tips
Circuit breakers are used in the following:
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.
There are many types of CBs, here are some of them:
Miniature Circuit Breaker: for low power applications and low short circuit level.
Molded Case Circuit Breaker: can bear higher power up to 630 A and also can reach 100 kA as short circuit level.
Air Circuit Breakers: used in many application in low voltage systems and is called Air as the insulating medium is Air.
Vacuum Circuit Breaker: can bear higher voltages than the Air as it implements vacuum as the insulating medium and is used in medium voltage systems.
Oil Circuit Breaker: used in medium and high voltage as oil is very robust insulating medium and has good capabilities in arc quenching.
SF6 Circuit Breaker: the most common type and used in medium and high voltage due to the high dielectric strength of SF6, thermal stability and thermal conductivity.
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. But to properly do such a thing you need to be aware of some aspects which are shown below.
Load Classification:
1. 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.
2. Static Load: normally it draws the rated current when operating at full power and never draws more than it. It’s mainly the resistive load, such as heaters.
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.