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Utilisation Categories. Utilisation categories indicate the type of load to be switched. It is important to understand these categories and ensure that the correct category is specified when selecting contactors. The utilisation categories appropriate to the majority of motor applications are :
Multiples of FLC
Normal Operation
AC1 Non inductive, slightly inductive or resistive loads.
AC2 Starting or plugging of slip ring motors.
AC3 Starting and switching off during running of squirrel cage motors.
AC4 Starting, inching or switching off during running of squirrel cage motors.

Rated Operational Voltage (Ue)
. These are values of voltage at which the contactor is capable of switching the associated values of rated operational current. They may not be greater that the rated insulation voltage except in the case of rotor resistance switching contactors.

Rated Insulation Voltage (Ui). This is the value of voltage assigned to the contactor which determines the required creepage, clearance, dielectric and impulse voltage test levels necessary.

Rated Thermal Current (Ith). Values of thermal rated current may be given unenclosed and/or enclosed. These values are determined by testing to the requirements of IEC947 and BS EN 60947 such that, under the test conditions specified, temperature rise limits are not exceeded. The tests are conducted until thermal equilibrium is reached or for a minimum of 8 hours.

Rated Operational Current (Ie). The current value that a contactor will operate at taking into account the supply voltage, intermittent duty and utilisation category. The maximum operational current may not exceed the thermal rated current.

Mechanical Life. The number of no load operations that can be made before it is necessary to replace any part.

Contact Life (Electrical Life). The number of on load operations performed at a given load without any repair or replacement of contacts.

Short Circuit Coordination. In the event of a short circuit on the load side of a contactor (or starter) the heavy current resulting could completely destroy the device and possibly cause danger to personnel. By including a correctly rated protective device i.e. an HRC fuse or moulded case circuit breaker, the damage can be limited to one of two categories. The permissible damage is briefly summarised below :


Type 1 All internal parts of the starter may have to be replaced. The case or enclosure must not be damaged, or become live during the short circuit.

Type 2 Damage is limited to welded contacts which must be separable.

When tested in accordance with IEC947 and BS EN 60947


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