It is necessary to select the correct rating of switchgear for any application. Therefore, during selection, it is necessary to know the characteristics of the load. The International electrotechnical commission in their standard for low voltage switchgear (IEC 60947) has defined utilization categories to simplify the selection of devices.
The chosen switchgear must be capable of safely make and break the load currents. The making and breaking current for a resistive load is the same as that during continuous operation while an induction motor draws current multiple times its rated current during when it is switched on. Therefore, loading of contacts while making, breaking as well as continuous operation is significant.
IEC 60947-1, IEC 60947-2, IEC 60947-3, IEC 60947-4, IEC 60947-5 and IEC 60947-6 defines application categories for various switchgear such as contactors, circuit breakers, switch disconnectors and load switches. In order to simplify the selection of suitable switchgear, the rated operational voltage – Ue, the rated operational current- Ie and making and breaking capacity of the device are defined well defined for each utilization categories. This facilitates the correct selection of a suitable device for any application, irrespective of manufacturers.
IEC 60947 volumes and description
|IEC 60947 Volumes||Description|
|2||Standard for Circuit-breakers|
|3||Circuit-breakers, switch-fuse units and switch-disconnectors|
|4-1||Electromechanical contactors and motor starters|
|4-2||Semiconductor motor controllers & starters for AC voltage|
|5-1||Electromechanical control apparatus|
|6-1||Mains switches (categories AC-32A/B)|
|6-2||Control and protective switchgear (CPS)|
|7-1||Terminal blocks for copper conductors|
|7-2||Protective conductor terminal blocks for copper conductors|
IEC 60947 – Utilization categories
|Typical Application||IEC standard|
|AC-1||Non-Inductive or slightly inductive loads, for example, resistive furnaces, Heaters||60947-4|
|AC-2||Switching of Slip-ring motors||60947-4|
|AC-3||Switching and running of squirrel cage induction motors:||60947-4|
|AC-4||Squirrel-cage motors: starting, plugging (1), inching (2)||60947-4|
|AC-5a||Switching of electric discharge lamps||60947-4|
|AC-5b||Switching of incandescent lamps||60947-4|
|AC-6a||Switching of transformers||60947-4|
|AC-6b||Switching of capacitor banks||60947-4|
|AC-7a||Slightly inductive loads in household appliances: examples: mixers, blenders||62947-4|
|AC-7b||Motor-loads for household appliances: examples: fans, central vacuum||62947-4|
|AC-8a||Hermetic refrigerant compressor motor control with manual resetting overloads||62947-4|
|AC-8b||Hermetic refrigerant compressor motor control with automatic resetting overloads||62947-4|
|AC-12||Control of resisitive loads and solid state loads with opto-coupler isolation||60947-5|
|AC-13||Control of solid state loads with transformer isolation||60947-5|
|AC-14||Control of small electromagnetic loads||60947-5|
|AC-15||Control of A.C. electromagnetic loads||60947-3|
|AC-20||Connecting and disconnecting under no-load conditions||60947-3|
|AC-21||Switching of resistive loads, including moderate loads||60947-3|
|AC-22||Switching of mixed resistive and inductive loads, including moderate overloads||60947-3|
|AC-23||Switching of motor loads or other highly inductive loads||60947-3|
|A||Protection of circuits, with no rated short-time withstand current||60947-2|
|B||Protection of circuits, with a rated short-time withstand current||60947-2|
|DC-1||Non Inductive or slightly inductive loads, resistance furnaces, heaters||60947-4|
|DC-3||Shunt-motors, starting, plugging(1), inching(2), dynamic breaking of motors||60947-4|
|DC-5||Series-motors, starting, plugging(1), inching(2), dynamic breaking of motors||60947-4|
|DC-6||Switching of incandescent lamps||60947-4|
|DC-12||Control of resistive loads and solid state loads with opto-coupler isolation||60947-4|
|DC-13||Control of D.C. electromagnetics||60947-5|
|DC-14||Control of D.C. electromagnetic loads having economy resistors in the circuit||60947-5|
|DC-20||Connecting and disconnecting under no-load conditions||60947-5|
|DC-21||Switching of resistive loads, including moderate overloads||60947-3|
|DC-22||Switching of mixed resistive and inductive loads, including moderate overloads (i.e. shunt motors)||60947-3|
|DC-23||Switching of highly inductive loads (i.e. series motors)||60947-3|
Below are most commonly used utilization categories.
These are primarily non-inductive loads or loads having power factor greater than 0.95. Making and breaking for these loads are easier. Example: Resistive heaters,.
AC-3 category primarily applies to squirrel cage induction motors. These motors have an inrush current of 5 to 8 times its rated current during start. At that instant the terminal voltage may drop to 20% of line voltage. Therefore contactors manufactured for AC-3 loads should be capable of such high current. The breaking can be easy.
Like AC-3 category, AC-4 also applied to start and breaking of squirrel cage induction motors. But in this case, the breaking is severe than that of AC-3 category. This category applies to induction motors that requires inching or plugging operations.