A current rating alone is not enough to select a control contact

Hello, a pushbutton or auxiliary contact may have a modest role, but an unsuitable contact can fail to transmit a command or weld closed. IEC 60947-5-1 addresses electromechanical control circuit devices and switching elements used for control, signalling and interlocking in switchgear and controlgear. Selection considers load type, operating voltage, switched current and contact function together.

The official 2024 edition applies to rated voltages up to 1,000 V AC, at frequencies up to 1,000 Hz, or 600 V DC. These are the standard's scope limits, not a statement that every product is suitable for all those values. The product's own operating ratings and installation conditions govern its application.

Which devices and which duties?

Pushbuttons, selector switches, mechanically operated limit switches and suitable auxiliary contact arrangements are assessed within this framework. Applicable requirements for signalling devices are also included. An auxiliary contact on a contactor can provide status feedback or perform electrical interlocking; its duty differs from that of the main contacts carrying motor current.

NO and NC describe a contact's position in the device's defined normal state. An NO contact is normally open, while an NC contact is normally closed. Read the definition of the normal state alongside the device's operating principle and circuit diagram. An NC designation alone does not establish direct opening action or all the characteristics required for a safety application.

Why AC-15, DC-13 and thermal current differ

AC-15 and DC-13 are utilization categories used for electromagnetic control loads in AC and DC circuits respectively. With a load such as a contactor coil, making and breaking conditions matter alongside steady-state current. An AC contact rating must not simply be transferred to a DC load at the same voltage.

For example, Eaton's technical record for the M22-AK01 contact element lists 6 A at 230 V AC, AC-15, and 0.3 A at 230 V DC, DC-13. These are ratings for that product under the manufacturer's stated conditions. The example shows how substantially the usable current can change when the voltage type and utilization category change.

Ith is the conventional free-air thermal current; Ie is the rated operational current at the specified operational voltage and conditions. Carrying current within a thermal limit and reliably making and breaking a particular load are different selection considerations. Specify Ue, Ie, utilization category and contact arrangement together instead of ordering only a “10 A contact”.

Direct opening, mechanically linked contacts and mirror contacts

Direct opening action concerns mechanically opening the relevant NC contact under defined travel and force conditions. Contact separation is not left solely to spring return; the way actuator movement reaches the contact is important. Neither an NC designation nor snap action automatically establishes this feature. Verify the required characteristic through appropriate product documentation and marking.

Mechanically linked contact elements concern the relative behaviour of specified NO and NC auxiliary contacts; this topic is associated with Annex L of IEC 60947-5-1. A mirror contact concerns the defined relationship between power contacts and an auxiliary contact and is addressed by IEC 60947-4-1. A device can have both features, but the terms are not interchangeable.

A PLC input may need more than a high current rating

A contact switching a PLC input may operate at very low current. A high ampere rating alone does not establish reliable small-signal transmission on every operation. Check minimum switching voltage and current, contact material and small-signal suitability in the manufacturer's data. Special tests for low-energy contact performance are addressed separately by IEC 60947-5-4.

For example, in a control circuit supplied by a 24 V DC power supply, the current drawn by a PLC input differs from that drawn by a contactor coil. If the same pushbutton type is used for both duties, assess each load under its own conditions. Successful operation in one circuit does not establish suitability for another.

Protection, durability and maintenance

A control contact is not inherently a short-circuit protective device. Select the manufacturer's specified fuse or other short-circuit protection arrangement for the applicable product conditions. An IEC 60947-5-1 marking does not mean that every control device contains overload protection.

Mechanical durability and electrical durability must also be read separately. A no-load mechanical operating-cycle figure is different from the number of operations achievable under a specified electrical load. Voltage, load current, utilization category and switching frequency matter when assessing electrical life. Maintenance checks include loose connections, correct seating of contact blocks, mechanical movement and signs of overheating.

Finding the right part within the standards family

IEC 60947-1 general rules provide the common framework, while IEC 60947-2 circuit-breakers and IEC 60947-3 switches and disconnectors address their respective product duties.

For load switching, IEC 60947-4-1 covers electromechanical contactors and motor-starters, IEC 60947-4-2 semiconductor motor controllers and soft-starters, and IEC 60947-4-3 semiconductor devices for non-motor AC loads.

On the sensor side, IEC 60947-5-2 proximity switches, IEC 60947-5-3 proximity devices with defined fault behaviour, IEC 60947-5-6 NAMUR interfaces and IEC 60947-5-7 proximity devices with analog output address different requirements. IEC 60947-5-8 concerns three-position enabling switches.

IEC 60947-6-1 addresses transfer switching equipment and IEC 60947-6-2 control and protective switching devices (CPS). Within the IEC 60947-7 family, 7-1 covers terminal blocks for copper conductors, 7-2 protective conductor terminal blocks and 7-3 fuse terminal blocks. IEC 60947-8 concerns PTC thermal protection control units for rotating electrical machines.