Protection of motor circuits

Low voltage current limiting cartridge fuselinks are used normally in conjunction with air break contactors to protect 3-phase ac induction motors. The fuses provide the protection against short circuits and must therefore have adequate capacities. The lower currents are cleared by the overload protection associated with the contactors. In these circumstances the rated current of the fuselinks does not need to correspond to the motor rating and certainly when motors with direct on-line starting are to be protected, the choice of fuse current rating is dictated by its ability to withstand the motor starting current surge, typically 5-6 times the full load current. This usually results in the use of gG fuselinks with rated currents up to twice the motor full load current. Such fuses thus carry up to about three times their rated current during starting periods.

The surges are not so great when other methods of starting are employed, and therefore fuses with lower current ratings are used, but these levels again are exceeded during motor starts. Allowance may also have to be made for the high transient currents which flow, with some methods of starting, when transitions are made from one connection to a succeeding one, as occurs, for example, when a star-delta or a rotor-resistor starter is used.

As stated earlier, the fuselink provides short circuit protection and does not provide low overcurrent protection. Compact aM back-up fuses or full range gM fuselinks are thus supplied. These also give economies of size in the associated equipment and are therefore commonly used where large numbers of motor starters, or motor control centres are installed.

For low voltage applications the requirements for contactors and motor starters are given in IEC 60 947-4-1. This includes the co-ordination requirements with short circuit protective devices (SCPD’s). The rated conditional short circuit currents of the contactors and starters backed up by the short circuit protective devices are specified. Two types of co-ordination are permissible: Type ‘1’ and type ‘2’.

Type ‘1’ co-ordination requires that, under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and may not be suitable for further service without repair and replacement of parts.

Type ‘2’ co-ordination requires that, under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and shall be suitable for further use. The risk of contact welding is recognised, in which case the manufacturer shall indicate the measures to be taken as regards the maintenance of the equipment.

Clearly type ‘2’ is the preferred co-ordination. In the past decade there have been developments in contactors and motor starters which have required the short circuit protection device to have relatively low values of let-through I2t and cut-off current characteristics. Co-ordination recommendations are made by the manufacturers of motor starters in accordance with IEC 60947-4-1.

The low voltage fuse standards committee of the IEC has undertaken a study into the co-ordination of fuselinks with motor starters and contactors and has found from a survey of tests, that type ‘2’ co-ordination is achieved by using aM, gG or gM fuselinks which have pre-arcing I2t values towards the bottom of limits specified in IEC 60269-1.

This IEC committee produced a technical report IEC 61459 on an Application Guide on co-ordination between fuses and contactors/motor starters. The figure shows the important parameters in the region of the crossover current for successful co-ordination between fuses, overload relay and contactors.

The strong current limiting feature of the fuselink prevents thermal damage to the contactor and its associated overload relay. It also prevents mechanical damage because of the low peak let-through, or cut-off current, of the fuselink. If the contactor/relay were not protected by the fuselink, the electromagnetic forces associated with the fault current would be more likely to cause damage and possible welding of the contacts. The motor starter manufacturers recommend suitable gG or gM fuselinks that provide type ‘2’ co-ordination.