Page 339 - Industrial Power Engineering and Applications Handbook

la318 Industrial Power Engineering and Applications Handbook
On a fault, they would also trip long before the prospective
peak of the fault current, and have characteristics similar
to Figure 12.17 and limit the let-through energy (Z2Rt) of
the fault. Although a costlier proposition, they eliminate
the above deficiencies of a fuse and ensure a three-phase
interruption, on the one hand and a prompt reclosing,
after a trip on fault, on the other. The contactor, however,
may still be essential in the circuit to permit frequent
switching operations. A separate OCR may also be
essential to closely match the thermal characteristics of
the motor, as the characteristics of the releases of the
MCBs or MCCBs may underprotect the motor. Such an
exercise may also be time consuming initially, as the
manufacturers of MCBs and MCCBs may, at best, provide
their characteristics and coordination with their brand of
components. The coordination of MCBs and MCCBs
with components of other brands may have to be done
individually, by the design engineer, more so initially,
until all in the field become acquainted with this
philosophy and characteristics of all brands of MCBs
and MCCBs.

12.11.1 Motor protection circuit breakers (MPCBs)
Figure 12.59 HRC fuses (Courtesy: L&T) A few manufacturers have provided an improvised version
of the above in the form of a motor protection circuit
breaker (MPCB). This can match closely the thermal
characteristics of a motor. In this case one MPCB will be
sufficient to replace the HRC fuses and the thermal relay
1000 and a separate OCR may not be necessary. Since the
range of MPCBs is limited presently, so are the motor
ratings that can be protected. With the use of MPCBs no
individual component matching will be necessary as in
100 the above case. Figure 12.60 illustrates a typical
t characteristic of an MPCB. Notice that the basic difference
between an ordinary OCR and an MPCB is the withstand
10 capability of MPCB on a fault. An MPCB is now able to
8
P take care of fault currents and switching transient currents
0
.$ and isolate the circuit much more quickly on an actual
.? 1 fault. An ordinary relay cannot withstand fault currents
g and requires backup protection through HRC fuses.
9
The characteristic shown also suggests that the MPCBs
can withstand current transients up to 100 times the rated
current (or the current setting) and hence are capable of
switching a capacitor and protecting them against
overloads.
12.11.2 Component ratings
0 In both the above cases, besides saving on the cost of the
t /,I = 71, L switch and fuses, one can also economize on the cost of
(= 14/,-201,) during an
Ir Current "''- IIranse"1 UP to 2 x 1st other equipment, such as main power cables, contactor
open transient condition
and the internal wiring of the starter (particularly from
Figure 12.60 Coordination of MPCB characteristics with the the busbars to the MCB, MCCB or MPCB). The rating
motor characteristics (eliminating the use of HRC fuses) of all such equipment can now be lower and commensurate
with that of the MCB, MCCB or MPCB and, hence, can
that is gaining preference is a fuse-free system, particularly be very close to the full load current of the motor and
on an LT system. It is possible to achieve such a system thus economize on cost. In conventional fuse protection,
through miniature and moulded case circuit breakers their rating was governed by the rating of the fuses, and
(MCBs and MCCBs) designed especially to match the the fuses had to be of higher rating than the rated current
Z2 - t Characteristics of a motor. They are also designed of the motor to remain immune from momentary transient
to be fast acting and current limiting, like HRC fuses. conditions and also to allow for a minimum fusing time