Category of duty
Switching devices are classified according to Utilization Category, or Category of Duty, thus:-
| Utilisation Category |
Typical Applications |
| AC20/DC20 |
Connecting and disconnecting under no-load.
Assumes all switching operations are carried out by other capable devices before this device is operated.
|
| AC21/DC21 |
Switching of resistive loads including moderate overloads.
Suitable for purely resistive type loads. Device can switch 150% of its rated current under fault conditions. |
| AC22/DC22 |
Switching of mixed resistive / inductive loads, including moderate overloads.
Suitable for mixed resistive / inductive loads. Device can switch 300% of its rated current under fault conditions. |
| AC23/DC23 |
Switching of highly inductive loads.
Devices complying with AC23/DC23 are provided mainly as back-up to other means of switching, e.g. Contactors. In the event of failure of the functional device, an AC23/DC23 type device can safely interrupt a stalled motor current. Where devices are the only means of controlling individual motors, they should comply with the requirements of appendix A of the standard. |
Switch Disconnectors for AC21/DC21, AC22/DC22 and AC23/DC23 also have to meet the requirements of AC20/DC20.
For specific and special applications such as switching of capacitors and tungsten lamps, the manufacturers’ advice should be sought. |
