Contactor-Fuse Coordination - Eaton Cutler-Hammer AMPGARD Technical Data Manual

Contactor-Fuse Coordination

The AMGARD starter provides ensured coordination between its
fuses, contactor, current transformers, protective relays, and the
motor it is controlling.
One of the most critical coordination issues is between the contactor
and the starter fuses. The fuses must interrupt faults greater than the
interrupting rating of the contactor. The AMPGARD 400 ampere
contactor (SL400A) has an 8-cycle dropout time factory setting as
standard and will interrupt at 8500 amperes. The maximum size fuse
used with an SL400A contactor is a 450-24R. By comparing the fuse
curve with the contactor rating, it can be observed that for faults
greater than 8500 amperes, the fuse will open before the contactor.
With faults less than 8500 amperes, the contactor may clear the fault
before the fuse blows, depending on the settings of the protective
relays. Refer to Figure 3 for an illustration of AMPGARD coordination.
Other vacuum contactors available today may have lower interrupting
ratings than the AMPGARD Type SL vacuum contactors. Dropout
times also vary, and may be as short as two cycles on other starter
designs. Lower interrupting ratings and shorter dropout times can
result in fault current levels where the contactor may be required to
interrupt a fault greater than its rating. This can result in equipment
failure. Refer to Figure 4 for an illustration of an improperly coordi-
nated starter.
AMPGARD starters also ensure coordination between other starter
components. The current transformers and protective relays are
selected to work properly with each other, and to protect the motor.
Protective relays like the Cutler-Hammer MP-3000 provide optimal
motor protection, while also rapidly opening the contactor during fault
conditions. This rapid opening signal cannot open the contactor in less
than its set dropout time, but it will take the motor off-line in the
shortest possible time. This will help minimize mechanical damage to
the motor and may prevent the starter fuses from blowing by allowing
the contactor to clear the fault (only if the fault is less than the contactor
interrupting rating).
6 EATON CORPORATION Cutler-Hammer Chiller Starter (AMPGARD) Technical Data TD02003003E Effective: October 2006
Contactor
Dropout
Contactor
Interrupting Rating
FIGURE 3. PROPER CONTACTOR FUSE COORDINATION FOUND IN
AMPGARD STARTER
Contactor
Dropout
Contactor
Interrupting Rating
Fault Current Range
where Contactor can
Attempt to Open on a Fault
Greater than its Rating
FIGURE 4. CONTACTOR FUSES THAT ARE NOT PROPERLY
COORDINATED
24R Fuse
Curve
Current
24R Fuse
Curve
Current