The current in thermal and magnetic circuit breakers passes through both a detection mechanism and a set of electrical contacts.  The contacts are generally spring-loaded and latch restrained.  When triggered by the overcurrent detection mechanism, the latch will release a movable contact arm.  The arm then withdraws from the fixed contact at a rate determined by spring loading and electromagnetic forces due to the contact current.

When the contacts are closed, or “latched”, current flows between them only at very small physical contact points, due to roughness on the surfaces of the contact faces.  The actual area of electrical contact is only a small fraction of the facing surfaces of the contacted pair, typically < 1%.  Current flowing in the contacts is constricted at these contact points, much like fluid flowing through a pipe with an insert containing very small holes.  The resistance created by these contact “spots” is referred to as the contact resistance.  The voltage drop across this resistance is then commonly referred to as the contact drop, which in most cases does not exceed more than 0.1-0.2 volts.

Our next article will examine the Parting Dynamics of a pair of contacts when the circuit breaker switches to the open position.

Over the course of the next several upcoming articles, we will present the basics of the behavior of the contact mechanism and the physics of the arc which is present in all electromechanical overcurrent protection devices.

Our discussion of contacts will consider both the electrical and mechanical characteristics of contacts in circuit breaker mechanisms.

Following our review of contacts, we will discuss the development of a simple dynamic thermal model which we will use to consider arc behavior within AC and DC electrical circuits.  

In this context, our next article will consider contact resistance and contact parting dynamics.

Circuit protection devices, like most items involving product or public safety, are regulated and/or tested by some agency.  This is done to confirm compliance with industry or legislative standards.  The agency, depending on the country involved, may be a government department or an independent organization which may or may not have close ties to the government.  While there are several independent laboratories in the United States, the most dominant for circuit protection devices (circuit breakers and supplementary protectors) in the commercial and industrial marketplace is Underwriters Laboratories, Inc. (UL).  Headquartered in Northbrook, IL, UL works closely with several government agencies to establish standards to assure product and public safety.

With origins dating back to 1893, UL now maintains more than 1,000 standards for safety.  Test labs are located throughout the country, with product safety reviewed under major categories such as Electrical; Burglary Protection & Signaling; Casualty & Chemical Hazards; Fire Protection; Heating, Air Conditioning & Refrigeration; and Marine.  Under these standards, UL’s engineering investigations and studies are carried out following strictly defined procedures.  The standards are usually developed through joint agency/industry committees, and submitted to the industry for review and comment prior to adoption.  Depending on the complexity of the standard, several iterations of this process, and a period of several years may occur before a standard is adopted.

For more information on Underwriters Laboratories go to www.ul.com 

Thermal overcurrent protection devices are a versatile, space saving and economical circuit protection option available to the application engineer – a.k.a. thermal circuit breakers and supplementary protectors.  Thermal circuit breakers and supplementary protectors are offered in a wide variety of standard “off the shelf” specified/qualified VAC and VDC ratings ranging from tenths of an Amp to more than 200 Amps.  And, for more unique/challenging applications, certain circuit breaker manufacturers will work closely with the OEM to provide engineered solutions to customize an overcurrent protection device specific to the application requirements.

Thermal circuit breakers are available in a variety of packages, bezel mounts (including snap in) and actuator configurations.  They are available in automatic reset configurations and in a wide selection of switch and/or indicator configurations including rockers, push/pull, toggle, illuminated and push to reset.  The usual number of poles are up to 2.  Terminals can be wires, quick connect blades, screw terminals, Edison base plug type, or fuse clip terminals.  Shunt trip, relay trip and alarm circuit devices are also available. 

Additional design flexibility is available because many thermal circuit breakers are offered compliant to a wide variety of agencies and requirements such as UL489, UL1077, UL1500, CCC, CSA 22.2 No.235-04, IEC, ABYC, SAE J553 and SAE J1625.   

Given the range of options offered in thermal overcurrent protectors, these robust, space saving, economical devices should warrant serious design consideration for the protection of numerous low voltage electrical circuits.