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6.7 Protective Coordination with Motor Starters
Motor starters comprise a magnetic contactor and a
thermal overload relay, providing the nesessary
switching function for control of the motor, plus an
automatic cutout function for overload protection.
Mitsubishi Electric’s excellent line of motor starters
are available for a wide range of motor applications
and are compatible with Mitsubishi MCCBs.
Magnetic contactors are rugged switching devices
required to perform under severe load conditions with-
out adverse affect. They are divided into Classes A
through D (by capacity); Class A, e.g., must be able
to perform 5 cycles of closing and opening of 10 times
rated current, followed by 100 closing operations of
the same current after grinding off 3/4 of the contact
thickness.
Current ratings of contactors usually differ according
to the circuit rated voltage, since voltage determines
arc energy, which limits current-handling capability.
Thermal overload relays (OLRs) employ bimetal ele-
ments (adjustable) similar to those of MCCBs.
For compatibility with the magnetic contactor, the OLR
must be capable of interrupting 10 times the motor
6.7.2 Levels of Protection (Short Circuit)
In some cases it may be advantageous to allow the
starter to be damaged in the event of a short circuit,
provided that the fault is interrupted and the load side
is properly protected.
IEC standards defines 2 types of coordination, sum-
marized as:
1. Type “1” coordination 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 re-
pair and replacement of parts.
2. Type “2” coordination 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 con-
tact welding is recognized, in which case the
manufacturer shall indicate the measures to be
taken as regards the maintenance of the equip-
ment.
Fig. 6.18 Protective Coordination; MCCBs and Motor Starters
MCCB
Magnetic contactor
and thermal overload
relay
Protection function
MCCB
Starter
combination
Protects the motor against overcurrents up
to 10 times rating.
Protects circuit wiring
'
control devices
'
and
OLR against fusion.
1 . Motor normal starting
current
2 . OLR-MCCB curve
intersection
3 . Transient peak of
motor current
Time
Motor overheat/burnout curve
MCCB trip. curve
OLR trip. curve
Current-time limitations of motor wiring
Intersection of MCCB and OLR trip curves
OLR heater fusion
Current-time limitations of
MCCB-to-starter wiring
2
1
3 4
5
6
Current
Key
4 . MCCB inst. trip current
5 . Protection limit; the
possible short-circuit at
the motor terminals must
be less than this value.
6 . MCCB rated interruption
capacity
Motor starting
current
Fig. 6.19 Protection Coordination Criteria for MCCBs and
Motor Starters
full-load current without destruction of its heater ele-
ment. Mitsubishi Type TH OLRs are normally capable
of handing 12 to 20 times rated current; in addition
there is available a unique saturable reactor for par-
allel connection to the heaters of some types, giving
a fusion-proofing effect of 40~50 times.
6.7.1 Basic Criteria for Coordination
It is necessary to ensure that the MCCB does not trip
due to the normal starting current, but that the OLR
cutout curve intersects the MCCB thermal delay-trip-
ping curve between normal starting current and 10
times full-load current. The MCCB instantaneous-trip-
ping setting should be low enough to protect the OLR
heater element from fusion, in a short-circuit condi-
tion.
The above criteria should ensure that either the MCCB
or the OLR will interrupt an overload, to protect the
motor and circuit wiring, etc. In practice it is desirable
for the MCCB instantaneous tripping to be set for about
15 times full-load current as a margin against tran-
sients, such as in reclosing after power failure, Y-delta
switching, inching, etc.
