120V Solid-state Circuit Breaker Receives First-ever UL Listing

October 04, 2022 by Mike Falter

A solid-state circuit breaker from Siemens increases safety and reliability in residential, commercial, and light industrial applications.

Siemens has announced that it is the first company to successfully receive UL listing for its 120 V, single pole solid-state circuit breaker. Siemens has been involved in solid-state circuit breaker technology for many years, but this latest product represents the first one developed entirely by Siemens in the United States.

Solid-state circuit breakers offer a much broader set of features than traditional electro-mechanical breakers with moving parts. As opposed to mechanical contacts, solid-state breakers use power semiconductor switches, or transistors, to control current through the breaker. Improvements over traditional breakers include faster response times, connectivity, and advanced digital algorithms.



120 V solid state breaker with UL listing. Image used courtesy of Siemens


One challenge for solid-state breakers, like all solid-state devices, is that their switches generate conduction heat. However, the Siemens breaker is the first UL-listed device of its kind to use passive heat management (natural conduction and convection), allowing it to be used in standard electrical panels.


Traditional Circuit Breaker Designs

Circuit breakers have been in use for over a century, with the first circuit breaker introduced by Thomas Edison in 1879 for use in homes.

The main function of a circuit breaker is to protect downstream electrical circuits and equipment from overload or short-circuit conditions. Traditional circuit breakers use a mechanical switch to create an open, or break, between the power source and load. Common circuit breaker designs include electromagnetic and thermal. Electromagnetic breakers use solenoid power to open a switch when current flows through the solenoid exceeds rated levels. Thermal breakers use a bimetallic switch that opens when the heat generated from current flow exceeds the threshold for safe operation.


Magnetic circuit breaker. Image used courtesy of RSP Supply


Circuit breakers can be designed for various applications, currents, and voltage levels. They have worked great for decades, but the traditional analog-mechanical design has limitations compared to emerging solid-state options.


Solid State Circuit Breakers as “Smart” Devices

Current through a solid-state breaker is conducted through the power switch, an IGBTSiC FET, or some other power FET properly rated for the breaker application (voltage, current, etc.). During normal operation, the switch will remain closed, but during a detected fault condition, the embedded microcontroller will generate a fault condition signal to actuate the gate driver to open the switch.

By using semiconductor switches, solid-state breakers can interrupt power 100 times faster than traditional breakers and enable better arc flash mitigation since no energy is released when the solid-state switch opens.

Solid state breakers are “smart” devices. The onboard microcontroller can process voltage, current, temperature, and other operational inputs, applying sophisticated algorithms for more precise control of the breaker. These breakers also offer programmability for trip and other performance settings, onboard monitoring and diagnostics, and connectivity to the outside world (WiFi, BTLE, etc.) for remote monitoring, system coordination, or industrial IoT applications.  


Solid-state circuit breaker block diagram. Image used courtesy of Infineon


The Future of Solid-state Circuit Breakers

With their enhanced configurability and connectivity capabilities, it is not difficult to envision an expanding role for solid-state breaker technology in smart buildings, grids, and other applications.

These “smart” circuit breakers can adapt to their operating environment and communicate with the broader world, internal and external, to their host system. Further, we can expect advances in device physics for solid-state power switches to help these breakers outpace traditional mechanical breakers with smaller form factors, improved thermal performance, higher ratings, and lower costs.

Other participants in the solid-state breaker market include ABB and Schneider Electric, who are actively developing their next-generation circuit breaker technologies portfolios.


Solid state “smart” circuit breaker. Image used courtesy of ABB