Choosing the right standard and protection for a controlled motor start

Hello, UL 60947-4-2 is a product standard for AC semiconductor motor controllers and starters. Soft starters are a familiar example of equipment within its scope. It is not a standard defining motor protection circuit breakers: controlling a motor start and interrupting fault current are separate functions.

Scope and relationship to IEC

The scope includes equipment with or without bypass arrangements, connected to circuits rated no higher than 1000 V AC. This limit does not mean every product can operate at 1000 V. The individual product’s voltage, current and application ratings remain decisive.

The UL publication adopts IEC 60947-4-2 with U.S. national differences. IEC compliance alone therefore does not establish UL certification. As checked on October 17, 2026, the official record lists the second edition as published on May 31, 2022, with a latest revision dated May 29, 2026. Review the project’s specified edition alongside the product’s certification record.

What does a voltage ramp change?

A typical soft starter adjusts the motor’s RMS voltage by changing the thyristor firing angle. This can limit starting current and mechanical shock. The supply frequency remains unchanged. If variable speed is required throughout operation, consider a variable frequency drive; a soft start alone does not provide continuous speed control.

In a simplified induction motor example, torque varies approximately with voltage squared at the same frequency and slip. Reducing voltage to 60% of its full value can reduce torque to approximately 36%. This does not mean the motor will run at 36% speed. Setting the current limit too low can prevent acceleration and cause heating during a prolonged start.

Select for the load, motor and starting duty

Matching an electric motor to a starter requires more than a kW rating. Consider motor nameplate current, starting time, starting current, starts per hour and ambient temperature together. A fan with a long acceleration time and a conveyor starting under load impose different demands. Enclosure cooling and the manufacturer’s heavy duty ratings also matter.

A programmed voltage ramp time does not guarantee that the motor will reach rated speed within that time. Check actual current, acceleration and temperature during commissioning. For pumps, an appropriate soft stop setting can help reduce pressure transients; it should not be assumed to provide braking or an emergency stop function for every load.

Overload protection, short-circuit protection and SCCR

Motor overload protection monitors the thermal effects of sustained overcurrent. Not every controller includes this function; a suitable thermal or electronic overload relay is required where necessary. In UL’s combination component terminology, a motor starter includes overload protection, while a motor controller may be identified without it. Check the actual model’s equipment and settings.

A semiconductor starter is not normally designed to interrupt short-circuit current on its own. The circuit needs the manufacturer’s specified fuse or suitable circuit breaker. Information about IEC 60947-2 circuit breakers does not replace the UL product standards and approvals required for a U.S. project.

SCCR means short-circuit current rating; it is not the continuous current or motor power rating. A stated SCCR may depend on a specified protective device and connection arrangement. Substituting another fuse or breaker with the same ampere rating does not automatically preserve it. One device’s SCCR also does not establish the entire panel’s rating; compare the panel rating with the available fault current at the installation.

Bypass contactors and electrical isolation

A run bypass routes current through mechanical contacts after starting, reducing losses in the thyristors. Apply internal or external contactors according to the manufacturer’s circuit diagram. Normal run bypass and an arrangement that starts the motor directly while bypassing a failed starter require different design and sizing decisions.

Control circuits and related standards

A PLC output must match the starter’s input voltage and input type. Size the necessary control power supply separately. Start and stop pushbuttons, fault relays and auxiliary contacts belong to the control circuit; a status output must not be used as a motor power output.

UL 60947-1 addresses general rules. Distinguish UL 60947-4-1 for electromechanical contactors and motor starters, UL 60947-4-3 for semiconductor control of AC non-motor loads, and UL 60947-5-1 for the relevant control circuit devices. Belonging to the same numbered family does not make these products interchangeable.

Product assessment covers subjects such as operating behaviour, insulation, temperature, overload, endurance and applicable short-circuit conditions. Compliance with a device standard alone does not validate a machine’s safety function. Risk assessment also requires the relevant frameworks, such as EN ISO 13849, and product-specific safety documentation where needed.

Related pages

For the wider standards family, see the UL 60947 guide. For other products used in the control system, visit the controllers main page.