Starting a motor and protecting it are different jobs

Hello, colleagues. A contactor controls the power supplied to a motor, but its presence does not establish that every required protective function is in place. IEC 60947-4-1 addresses electromechanical contactors and motor starters. Effective application depends on selecting the load rating, switching duty and protective devices together.

Scope and edition information

The official IEC page identifies IEC 60947-4-1:2023 as the fifth edition, replacing the 2018 edition. The published copy incorporates Corrigendum 1 dated March 2026. Its scope includes electromechanical contactors and starters, motor protective switching devices such as MPSD and IMPSD, specified coil-circuit contacts and dedicated accessories. The relevant circuits have rated voltages not exceeding 1,000 V AC or 1,500 V DC.

Those upper limits do not mean that every product can operate at those voltages. Use the selected model's own voltage, current and application ratings. An AC utilization category also does not automatically establish suitability for switching the same current on DC.

Contactors, overload relays and short-circuit protection

The difference between a relay and a contactor involves the load and operating conditions as well as physical size. A contactor performs normal operational switching. A separate overload relay can detect motor overload and cause the contactor to open through the control circuit. That relay does not, by itself, interrupt a high short-circuit current.

A suitable fuse or circuit breaker is required for short-circuit protection. For devices such as a thermal-magnetic circuit breaker, check which protective functions the particular model provides. Treating motor overload protection and short-circuit protection as interchangeable can leave gaps in protection. The correct combination matters more than simply choosing a larger component.

How AC-1, AC-3 and AC-4 differ

AC-1 concerns non-inductive or slightly inductive loads, such as resistance heating. Its current rating must not be treated as an equivalent motor-switching rating.

AC-3 concerns starting squirrel-cage induction motors and switching them off while running normally. The contactor encounters starting current when closing and interrupts the running motor current during a normal stop. Routine starting and stopping of a pump or fan can be an example of this duty.

AC-4 covers more demanding duties such as inching, jogging and plugging. Reversing a motor after it has stopped imposes different stresses from energising it in the opposite direction while it is still rotating. The permissible operating current and electrical endurance of the same contactor can change with the duty. Operations per hour and the required total number of operating cycles also matter.

Do not confuse motor current with coil voltage

Read the motor's rated current together with the supply voltage and utilization category. For example, consider a motor with a nameplate current of 15.2 A at 400 V. A contactor marked 20 A for AC-1 is not automatically suitable: check its rated operational current Ie at the relevant voltage and motor duty. Ambient temperature and switching frequency must also remain within the manufacturer's limits.

The coil supply is a separate selection. The main circuit might operate at 400 V AC while the coil requires 24 V DC. Check AC or DC operation, frequency, pull-in demand and holding consumption. If a PLC output controls the coil, assess output capacity and suitable suppression; the 24 V power supply must also support the coil's initial demand. Check the applicable IEC 60947-5-1 ratings of auxiliary contacts separately for their control-circuit loads.

What do Type 1 and Type 2 coordination mean?

With Type 1 coordination, a short circuit must not create danger to people or the installation, but the contactor or starter may require repair or replacement before further service. Type 2 coordination additionally requires suitability for further use. Some contact welding is nevertheless recognised, with inspection and maintenance measures specified by the manufacturer. Type 2 does not authorise restarting after a fault without inspection.

Coordination applies to a specified combination of protective devices and starter components. Check the voltage, prospective short-circuit current, contactor, overload relay and protective device together in the manufacturer's table. When selecting a fuse, matching amperage alone is insufficient; the fuse type and permitted combination also matter.

For a concrete example, Eaton's product page for reference 277300 specifies a 125 A gG/gL fuse for Type 1 short-circuit protection at 400 V, compared with 63 A gG/gL for Type 2. Those values belong only to that product and must not be transferred to other contactors or motors. They demonstrate that different coordination targets can require different protective-device selections for the same product.

An overload trip class is not a fixed delay

The overload relay's adjustment range must suit the motor and circuit arrangement. Its trip class describes time-current behaviour under defined test conditions. Class 10, for example, does not mean that every overload causes a trip after exactly ten seconds. Loads with long acceleration times require trip-class selection consistent with the motor's permissible thermal stress.

Increasing the current setting or trip class merely because a motor struggles to start can weaken protection. Check the role of the overload relay's auxiliary contacts, the reset arrangement and the prevention of unexpected restarting after a fault.

Starting arrangements and installation conditions

Contactors perform different duties in direct-on-line, reversing and star-delta arrangements. A motor cannot be converted to star-delta starting simply by adding contactors without checking its star and delta connection ratings. Prevent incompatible contactors from closing simultaneously. Specify interlocking, transition sequencing and protective-device placement according to the manufacturer's verified circuit arrangement.

Panel temperature, terminal tightening torque, conductor size and ventilation matter as much as initial selection. Mechanical endurance does not establish the same number of electrical switching operations under load. Assess contact wear and coil chatter together with wiring and supply conditions.

A product standard reference does not verify the complete panel

An IEC 60947-4-1 reference concerns the relevant product requirements. The completed panel's short-circuit withstand capability, protective coordination and control behaviour require separate verification. Assessments under UL 60947-4-1 must not be treated as direct substitutes for IEC coordination tables; use documentation and combinations appropriate to the intended market.

Related standards

The IEC 60947 series provides the overall framework, IEC 60947-1 covers common product rules, and IEC 60947-2 addresses circuit breakers.

IEC 60947-3 concerns switches, disconnectors and fuse-combination units; IEC 60947-4-2 addresses semiconductor motor controllers and starters.

Product and application guides

The contactor range covers switching devices, while overload relays provide options for protection against motor overload.

Star-delta starting, soft starters and variable frequency drives serve different operating needs. In a semiconductor-based system, establish whether the contactor is used on the line, bypass or output side, and check suitability against the equipment manufacturer's connection instructions.