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Application Briefing: Electrical Contact Assemblies in Automatic Transfer Switches
September 3, 2021 at 4:00 AM
by Keith Hoge
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An automatic transfer switch is an electromechanical device that redirects the source of electrical power from a primary power supply to a backup power supply in the event of a power outage. These switches are often used in conjunction with a generator, connecting the generator’s electrical output to the electrical input of the system to be powered (i.e. hospitals, commercial buildings, homes, etc.). Figures 1 and 2 show a simple diagram that outlines the role of an ATS in this type of electrical system.

Figure 1: Simplified schematic of an Automatic Transfer Switch when the electric utility is running properly.

Figure 2: In the event of a power outage, the ATS will connect the backup generator to the electrical system, automatically preventing a sustained blackout.

Automatic transfer switches are commonly used in safety-critical applications and thus, must operate with a high degree of reliability. Hospitals, for instance, cannot afford to lose power for even brief periods of time. As such, they are often connected to massive industrial backup generators by means of highly engineered automatic transfer switches. Within these switches, one can find electrical contacts that are responsible for the safe and effective transmission of power.

ELECTRICAL CONTACT ASSEMBLY DESIGN CONSIDERATIONS

In a typical ATS, there are stationary contact assemblies and moveable contact assemblies. As the name implies, stationary contacts are held in place while moveable contacts have at least one degree of freedom that allows them to make and break the electrical connection by means of an actuating arm. This actuating arm is typically spring-loaded, which minimizes risk of arcing. This concept is explained more fully later in this section.A stationary contact is typically brazed to a copper or bronze backing. The same is true of moveable contacts, but that subassembly is then brazed or mechanically connected to flex connectors that are attached to a ferrule. Within the category of “moveable contacts”, contact assemblies can be further classified into arcing contacts and main (current carrying) contacts. Arcing contacts are responsible for transmitting higher power levels and thus are more prone to arc degradation. As such, these contacts typically have higher levels of refractory in their composition (this concept is further explained in the next section of this briefing titled “Electrical Contact Material Selection”). Figure 3 shows a typical moveable arcing contact assembly, main contact assembly, and a stationary contact assembly, placed next to a quarter for size reference.