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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. there are five questions that have to be answered in the selection process:

  • What is to be protected, wires or devices?
  • What is the voltage type, AC or DC?
  • What is the circuit or device current rating?
  • How many poles are required, 1, 2 or 3 pole?
  • What is the inrush current level, and the desired trip curve — short, medium or long?

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.

The branch circuit breakers, often referred to as UL489 circuit breakers are used in most applications for protecting the circuit wiring and preventing a fire. Due to the strict testing requirements by UL, the DIN rail mount branch circuit breakers are not available in as many configurations or sizes. However, they are typically available in one and two pole versions with current ratings of 1 to 25 amps. Branch circuit UL489 circuit breakers are UL listed so these breakers can be used in applications where a listed breaker is required.

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.

The high current circuit breakers are used in applications to protect devices with current requirements higher than 63 amps. Whereas most DIN rail mounted circuit breakers have a maximum rating of 63 amps, the high current circuit breakers are unique to only a few manufacturers and have a current rating up to 125 amps. These circuit breakers are UL1077 approved for use as supplemental protectors and are available in 1, 2 and 3-pole versions with the option of medium (C) or long (D) trip curves. In some applications the high current circuit breakers are a cost-effective solution to replacing expensive molded case circuit breakers (MCCB).

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 breakers have useful accessories that are easy to wire — screw-clamp connections and DIN rail mounting to save valuable time. A shunt release is easily installed on the left hand side of the circuit breaker. It provides remote control tripping and indicates circuit breaker status. An alarm contact and auxiliary contact can be used to indicate if a circuit breaker is open or closed. A helpful accessory is a lockout/tagout. This accessory is a locking lever that can be added to the circuit breaker after installation for those applications that require lockout/tagout. It allows the circuit breaker to be locked in either the open or the closed position. When in place, the lockout accessory does not impede the ability of the circuit breaker to trip. In addition, the lockout/tagout accessory is designed so that up to 3 padlocks can be added to the breaker.

Selecting a proper circuit breaker to match the application can be easy, provided all the circuit specifications are met. It is an important component in the system that cannot be overlooked, because it is providing needed protection for expensive devices and used to prevent system disasters.

Contact: Automation Systems Interconnect, Inc., PO Box 1340, Mechanicsburg, PA 17055 877-658-5160 or 717-249-5581 fax: 717-249-5542 Web:

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