Utilisation categories of electrical systems

When designing a system that is switching an electrical load it is vitally important to choose the correct type of switchgear that will switch the load on or turn the load.

Electrical systems often involve loads being switched on and off for operation of equipment.

In order to select the correct switchgear for the intended application, it is first necessary to determine the operating characteristics of the load to be controlled or switched. In this regard, it is enormously important to assess correctly how much stress is caused by current and voltage on contacts when circuits are made during the switching operation.
 

IEC (International Electrotechnical Commission)


In order to assist with selection of switchgear devices, IEC defined several utilisation categories. Utilization categories are defined in the standards for low-voltage switchgear low-voltage switchgear (IEC 60947-1, -2, -3, -4, -5, -6), they take into account the intended application and hence the associated loading of the various low-voltage switchgear types, such as contactors, disconnectors, circuit breakers and load switches (Tab. 1.1-1).


They serve as an indication, on any electrical device, of the type of electrical load it carries, and what the duty cycle is of the load. This makes the selection of appropriate contactors and relays much simpler, and ensures the maximum safety of electrical equipment so it doesn't overload.

Example:

The utilisation category that governs low-voltage switchgear lays out the operating conditions characteristic of switchgear; contactors, contactor relays, circuit-breakers and their associated fuse units, etc. Such devices are all set up for different operating conditions and electrical loads, which need to be separated into sub-categories for accurate definition.

Utilisation categories defined by the IEC standards:
 

Standard 
IEC 60947-1General requirements
IEC 60947-2Circuit-breakers
IEC 60947-3Circuit-breakers, switch-fuse units and switch-disconnectors
IEC 60947-4Contactors and motor starters:
4-1 Electromechanical contactors and motor starters
4-2 Semiconductor motor controllers & starters for AC voltage
IEC 60947-5Control apparatus and switching elements:
5-1 Electromechanical control apparatus
IEC 60947-6Multi-function switchgear and control gear:
6-1 Mains switches (categories AC-32A/B)
6-2 Control and protective switchgear (CPS)
IEC 60947-7Auxiliary equipment:
7-1 Terminal blocks for copper conductors
7-2 Protective conductor terminal blocks for copper conductors

    



How To Select The Appropriate Rating For Your Device

The rated operational powers or currents are usually listed in the technical data provided with the device. They are generally given for various rated operational voltages. To make these standards universally applicable, each piece of switchgear will usually specify the data for several utilisation categories.
That makes device selection much simpler for project engineers; they just have to compare the ratings of the load with the performance data of the switchgear in the respective utilisation category, and select a device that meets, or even exceeds the load ratings.

The rated operational current Ie and the rated operational voltage Ue are stipulated for a particular utilisation category, defines the necessary making and breaking capacity of a given switchgear item. No further agreements are generally required between users and manufacturers, with device selection and comparison having been facilitated.

There is also IEC/EN 61095 that defines categories governing "household and similar applications". Contact load in amperes is typically stated directly on the contactor, e.g. AC1 for inductive loads such as heaters, and AC3 for motor usage.

For other devices the operational currents or rated operational power can be found in the products technical data. Often manufacturers of electrical switchgear will list data for several utilisation categories for a particular product. Correct switchgear selection can be achieved by ensuring that the electrical load characteristics meet or exceed the ratings laid out in the manufacturer's data.

Test Conditions

Test regulations set out in the IEC standards lay down the parameters within which tests must be carried out by manufacturers, in each individual utilisation category. These standards ensure that all tested devices are suitable for the respective application, and means the user doesn't have to get drowned in technical details. In some circumstances, conditions for the application may differ considerably in practice from the conditions set out in the standards. Such conditions include high frequency of operation, heavy-duty starting, and particularly long lifespan of equipment. In such circumstances, both manufacturers and users must agree what loads are permitted.

Commonly Used Utilisation Categories

he table below gives an overview of the various different abbreviations used for low voltage switchgear and control gear, together with the devices to which each utilisation category applies, and the IEC standard that governs them. 
 

Utilization CategoryTypical ApplicationApplicable IEC norm
AC-1Non-inductive or slightly inductive loads, example: resistive furnaces, heaters60947-4-1
AC-2Slip-ring motors: switching off60947-4-1
AC-3Squirrel-cage motors: starting, switches off motors during running time60947-4-1
AC-4Squirrel-cage motors: starting, plugging, inching60947-4-1
AC-5aSwitching of discharge lamps60947-4-1
AC-5bSwitching of incandescent lamps60947-4-1
AC-6aSwitching of transformers60947-4-1
AC-6bSwitching of capacitor banks60947-4-1
AC-7aSlightly inductive loads in household appliances: examples: mixers, blenders60947-4-1
61095
AC-7bMotor-loads for household appliances: examples: fans, central vacuum60947-4-1
61095
AC-8aHermetic refrigerant compressor motor control with manual resetting overloads60947-4-1
AC-8bHermetic refrigerant compressor motor control with automatic resetting overloads60947-4-1
AC-12Control of resistive loads and solid state loads with opto-coupler isolation60947-5-2
AC-13Control of solid state loads with transformer isolation60947-5-1
AC-14Control of small electromagnetic loads60947-5-1
AC-15Control of A.C. electromagnetic loads60947-5-1
AC-20Connecting and disconnecting under no-load conditions60947-3
AC-21Switching of resistive loads, including moderate overloads60947-3
AC-22Switching of mixed resistive and inductive loads, including moderate overloads60947-3
AC-23Switching of motor loads or other highly inductive loads60947-3
AC-31A
AC-31B
Non-inductive or weakly inductive loads60947-6-1
AC-33A
AC33B
Motor loads or mixed loads including motors, resistors and up to 30 % incandescent lamp load60947-6-1
AC-35A
AC-35B
Gas discharge lamp load60947-6-1
AC-36A
AC36B
Incandescent lamp load60947-6-1
AC-40Distribution circuits of mixed resistive and inductive loads60947-6-2
AC-41Non-inductive or weakly inductive loads, resistance furnaces60947-6-2
AC-42Slip ring motors: Starting, switching off60947-6-2
AC-43Squirrel cage motors: Starting, switching off during operation60947-6-2
AC-44Squirrel cage motors: Starting, counter-current braking or reversing, typing60947-6-2
AC-45aSwitching of gas discharge lamps60947-6-2
AC-45bSwitching incandescent lamps60947-6-2
AC-51Induction-free or slightly inductive loads, resistance furnaces60947-4-3
AC-52aControl of the stator winding of a slip-ring motor: 8-hour operation with starting currents for starting processes, maneuvering, operation60947-4-2
AC-52bControl of the stator winding of a slip ring motor: Intermittent operation60947-4-2
AC-53aControl of a squirrel cage motor: 8-hour operation with starting currents for starting processes, maneuvering, operation60947-4-2
AC-53bControl of a squirrel cage motor: Intermittent operation60947-4-2
AC-55aSwitching of electrical controls of discharge lamps60947-4-3
AC-55bSwitching incandescent lamps60947-4-3
AC-56aSwitching transformers60947-4-3
AC-56bSwitching of capacitor batteries60947-4-3
AC-58aControl of a hermetically sealed refrigeration compressor motor with automatic reset of overload releases: 8-hour operation with starting currents for starting, maneuvering, operation60947-4-2
AC-58bThe control of a hermetically sealed refrigeration compressor motor with automatic reset of the overload trips: Intermittent operation60947-4-2
AC-140Control of small electromagnetic loads with holding current =< 0.2 A; e.g. contactor relays60947-5-2
AProtection of circuits, with no rated short-time withstand current60947-3
BProtection of circuits, with a rated short-time withstand current60947-3
DC-1Non Inductive or slightly inductive loads, resistance furnaces, heaters60947-4-1
DC-3Shunt-motors, starting, plugging(1), inching(2), dynamic braking of motors60947-4-1
DC-5Series-motors, starting, plugging(1), inching(2), dynamic braking of motors60947-4-1
DC-6Switching of incandescent lamps60947-4-1
DC-12Control of resistive loads and solid state loads with opto-coupler isolation60947-5-1
60947-5-2
DC-13Control of D.C. electromagnetics60947-5-1
60947-5-2
DC-14Control of D.C. electromagnetic loads having economy resistors in the circuit60947-5-1
DC-20Connecting and disconnecting under no-load conditions60947-5-1
DC-21Switching of resistive loads, including moderate overloads60947-5-1
DC-22Switching of mixed resistive and inductive loads, including moderate overloads (i.e. shunt motors)60947-5-1
DC-23Switching of highly inductive loads (i.e. series motors)60947-5-1







































































The table may be used to select the most suitable size of component for the respective device type, depending on the rated voltage, the rated current, and the electrical load that requires switching.