Utilization categories for low voltage switchgear

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.

Utilization categories

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 VolumesDescription
1General requirements
2Standard for Circuit-breakers
3Circuit-breakers, switch-fuse units and switch-disconnectors
4-1Electromechanical contactors and motor starters
4-2Semiconductor motor controllers & starters for AC voltage
5-1Electromechanical control apparatus
6-1Mains switches (categories AC-32A/B)
6-2Control and protective switchgear (CPS)
7-1Terminal blocks for copper conductors
7-2Protective conductor terminal blocks for copper conductors

IEC 60947 – Utilization categories

Utilization
Category
Typical ApplicationIEC standard
AC-1Non-Inductive or slightly inductive loads, for example, resistive furnaces, Heaters60947-4
AC-2Switching of Slip-ring motors60947-4
AC-3Switching and running of squirrel cage induction motors:60947-4
AC-4Squirrel-cage motors: starting, plugging (1), inching (2)60947-4
AC-5aSwitching of electric discharge lamps60947-4
AC-5bSwitching of incandescent lamps60947-4
AC-6aSwitching of transformers60947-4
AC-6bSwitching of capacitor banks60947-4
AC-7aSlightly inductive loads in household appliances:  examples: mixers, blenders62947-4
AC-7bMotor-loads for household appliances: examples: fans, central vacuum62947-4
AC-8aHermetic refrigerant compressor motor control with manual resetting overloads62947-4
AC-8bHermetic refrigerant compressor motor control with automatic resetting overloads62947-4
AC-12Control of resisitive loads and solid state loads with opto-coupler isolation60947-5
AC-13Control of solid state loads with transformer isolation60947-5
AC-14Control of small electromagnetic loads60947-5
AC-15Control of A.C. electromagnetic loads60947-3
AC-20Connecting and disconnecting under no-load conditions60947-3
AC-21Switching of resistive loads, including moderate loads60947-3
AC-22Switching of mixed resistive and inductive loads, including moderate overloads60947-3
AC-23Switching of motor loads or other highly inductive loads60947-3
AProtection of circuits, with no rated short-time withstand current60947-2
BProtection of circuits, with a rated short-time withstand current 60947-2
DC-1Non Inductive or slightly inductive loads, resistance furnaces, heaters60947-4
DC-3Shunt-motors, starting, plugging(1), inching(2), dynamic breaking of motors60947-4
DC-5Series-motors, starting, plugging(1), inching(2), dynamic breaking of motors60947-4
DC-6Switching of incandescent lamps60947-4
DC-12Control of resistive loads and solid state loads with opto-coupler isolation60947-4
DC-13Control of D.C. electromagnetics60947-5
DC-14Control of D.C. electromagnetic loads having economy resistors in the circuit60947-5
DC-20Connecting and disconnecting under no-load conditions60947-5
DC-21Switching of resistive loads, including moderate overloads60947-3
DC-22Switching of mixed resistive and inductive loads, including moderate overloads (i.e. shunt motors)60947-3
DC-23Switching of highly inductive loads (i.e. series motors)60947-3

Below are most commonly used utilization categories.

AC-1

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

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.

AC-4

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.

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