Combining motor circuit control and protection

Control and protective switching devices combine normal circuit control with protection against overcurrent conditions. They are commonly described as Control and Protective Switching Devices, abbreviated CPS. Their verified international reference is IEC 60947-6-2. For North American applications, the product's actual certification record and its classification under a standard such as UL 60947-4-1 must be checked separately.

This function is different from transferring a load between utility power and a generator. In the IEC series, transfer switching equipment is addressed by IEC 60947-6-1. CPS focuses on coordinated switching and protection for motors and other circuits. A complete motor control center is also a different product from an individual CPS device.

Which functions does CPS combine?

In a conventional motor feeder, a contactor may perform normal switching, an overload relay provides motor overload protection, and fuses or an appropriate circuit breaker provide short-circuit protection. A CPS coordinates these functions within a device or modular device assembly defined by its manufacturer. Simply placing several components together does not automatically create a CPS.

IEC 60947-6-2:2020 covers CPS whose main contacts connect to circuits rated up to 1000 V AC or 1500 V DC. These scope limits do not mean that every product can operate at both voltages. Suitability for an electric motor depends on the actual operational voltage, current range, load characteristics and manufacturer data. The main control function is operated otherwise than by hand, although local manual operating means may also be provided.

Depending on the product, additional functions can include isolation, communication or monitoring. Isolation capability must not be assumed for every CPS. Where electrical isolation is required for maintenance, check the product's relevant markings and instructions; the distinction between these duties can be explored in the switches and disconnectors guide.

Continuity of service does not mean uninterrupted power

For CPS, continuity of service means the ability to return to service after specified overcurrent conditions. It does not mean that the motor will never stop during a fault or that its power supply remains uninterrupted. Protection opens the circuit when required. Before restarting, eliminate the fault cause and follow the manufacturer's instructions for inspection, checks and resetting.

Conformity assessment addresses operating behavior, dielectric properties, constructional safety and short-circuit performance. Rated operational current and short-circuit performance are different characteristics. A device being able to carry the motor current does not establish suitability for the prospective fault current at the installation.

Type E and Type F in North American applications

Type E identifies a self-protected combination motor controller meeting specific performance requirements. Type F can comprise a specified combination of a manual self-protected motor controller and a contactor. The IEC CPS designation does not automatically equate to either UL construction type; a product's assessment under each system must be verified separately.

Combination approval can depend on exact component references, voltage, motor current or horsepower rating, and required accessories. A specified phase barrier, connecting module, terminal adapter or enclosure condition forms part of the evaluated construction. Substituting a different contactor with the same ampere rating may invalidate the documented short-circuit rating of the combination.

A motor branch circuit must clearly provide the functions of disconnecting, short-circuit protection, motor control and overload protection. A standard reference printed in a catalog is insufficient on its own: check which functions the product has been evaluated to perform and which additional components are required. A contactor without protective functions does not replace appropriate overload and short-circuit protection.

Reading SCCR alongside motor current

SCCR is the short-circuit current rating of a device or combination under specified conditions. Compare it with the available fault current at the connection point. For example, assume a motor nameplate current of 12 A, a selected device with a continuous current rating of 16 A, and a documented SCCR of 10 kA. If the available fault current is 25 kA, the fact that 16 A exceeds the motor current is insufficient.

An appropriately evaluated combination with a higher SCCR, or a suitable protective solution, is needed. A manufacturer's higher combination rating applies only when the specified components and conditions are satisfied. A high SCCR for one motor branch does not automatically establish the same SCCR for the complete panel; the other power circuit components must also be evaluated.

Control circuits and installation details

Check the control voltage separately from the power circuit. A control unit operating at 24 V DC needs an appropriate power supply and sufficient control capacity. A PLC output can directly operate the control input only if their electrical characteristics are compatible. Define start and stop pushbuttons, enable conditions and the restart behavior following a fault clearly in the circuit diagram.

Auxiliary contacts can report operating or trip status; they are not expected to carry the same current as the main power contacts. References such as UL 60947-5-1 for control devices and IEC 60947-7-1 for terminal blocks under the IEC system must be applied within their respective scopes. Conformity of these parts does not automatically certify every function of the main device.

Consider the general rules of UL 60947-1 together with the relevant product-specific requirements. Select conductor size, terminal tightening torque, ambient conditions, mounting orientation and accessories according to the manufacturer's instructions. An arrangement used upstream of a variable frequency drive must not automatically be assumed suitable downstream: IEC 60947-6-2:2020 does not cover CPS used downstream of a frequency drive.

The presence of a CPS alone does not establish the required safety performance of an emergency stop or guard circuit. Safety functions assessed under EN ISO 13849 require separate consideration of architecture, fault behavior and component reliability data. Preserve the distinction between normal motor control and a safety-related stopping function.

Related technical information

For further motor-starting information, see IEC 60947-4-1. The UL 60947 series guide explains distinctions between North American references. Return to the control devices main page to browse the product groups.