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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.
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