Littelfuse MPS - Overcurrent; Auxiliary Overcurrent; Reduced Overcurrent; Jam

Page 5-5
MPS Motor Protection System Rev. 6-F-022117
Protective Functions
The asymmetrical-current multipliers for RMS and
DFT measuring methods are shown in Fig. 5.2. X/R is
the ratio of system reactance to system resistance.
Typical X/R values are 6.6 for a low-voltage system, 15
for a medium-voltage system, and can be as high as 25 for
a high-voltage system. As shown by the graph, the DFT
filters the dc component so that the overcurrent setting
can be set closer to the symmetrical fault value.
FIGURE 5.2 Asymmetrical-Current Multipliers.
5.4 AUXILIARY OVERCURRENT
OPI Menu: Setup | Protection | Aux Overcurrent
Auxiliary overcurrent is the same as overcurrent
protection. This function is intended to be used when
backup protection for the overcurrent function is required.
Trip Level ............................. 1.00 to 15.00 x CT-Primary
Rating (I
p
)
Trip Delay ............................. 0.00 to 10.00 s
(See Tables 5.1 and 5.2)
Protection .............................. Enable/Disable Trip1, 2, 3
Measurement Method ........... DFT
5.5 REDUCED OVERCURRENT
OPI Menu: Setup | Protection | Reduced OC
Reduced overcurrent is used to reduce the overcurrent
set point when performing maintenance while a motor is
running. This mode of operation can be used to meet
NEC 240.87 Arc Energy Reduction (B) (3) Energy-
reducing maintenance switching with local status
indicator.
Reduced overcurrent is controlled by a digital input
assigned to Reduced OC. When the digital input is not
applied, this set point is operational and when the digital
input is applied, this set point is not operational. When
reduced overcurrent is selected, ROC:ON is displayed in
the Metering | System State menu and a relay assigned to
Reduced OC will be energized.
The Protection selection must include Trip1, Trip2, or
Trip3. If Disable is selected, Reduced OC mode is
disabled.
Trip Level ............................. 1.00 to 15.00 x CT-Primary
Rating (I
p
)
Trip Delay ............................. Fixed at 0.00 (Instantaneous)
See Tables 5.1 and 5.2
Protection .............................. Enable/Disable Trip1, 2, 3
Measurement Method ........... DFT
5.6 JAM
OPI Menu: Setup | Protection | Jam
A trip or alarm occurs if a jam condition is detected.
Jam protection is active when the motor is in the Run
mode, allowing protection to be set below motor-starting
current.
Trip Level ............................. 1.00 to 10.00 x FLA
Trip Delay ............................. 1.00 to 100.00 s
Alarm Level .......................... 1.00 to 10.00 x FLA
Alarm Delay ......................... 1.00 to 100.00 s
Protection .............................. Enable/Disable Trip1, 2, 3
Enable/Disable Alarm1, 2, 3
Measurement Method ........... DFT
5.7 EARTH FAULT
OPI Menu: Setup | Protection | Earth Fault
Earth-fault protection is based on the fundamental-
frequency component of zero-sequence current.
Trip Level ............................. 0.05 to 1.00 x EF-CT-
Primary Rating (I
e
)
Trip Delay ............................. 0.00 to 100.00 s
(See Tables 5.1 and 5.2)
Alarm Level .......................... 0.05 to 1.00 x I
e
Alarm Delay ......................... 0.00 to 100.00 s
Protection ............................. Enable/Disable Trip1, 2, 3
Enable/Disable Alarm1, 2, 3
Measurement Method ........... DFT
5.8 CURRENT UNBALANCE
OPI Menu: Setup | Protection | Unbalance (I)
OPI Menu: Setup | System Config | I
p
Threshold
Positive-sequence current (I
1
) and negative-sequence
current (I
2
) are used to determine current unbalance (I
2
/I
1
).
The unbalance display range is 0.00 to 1.00 where 1.00 is
100% unbalance—a single-phase condition. Negative
unbalance will be indicated if current inputs are connected
B-A-C. Severe unbalance may be indicated if phase-CT
polarity is incorrect.
Lightly loaded systems, especially with synchronous
motors, may cause false current unbalance trips. The I
P
Threshold sets the current level where unbalance
protection becomes active. The I
P
Threshold is based on
the maximum of the three-phase currents.
For example, if the CT-primary rating (I
p
) is 100 A, and
I
p
Threshold is set to 0.25, current unbalance protection
will be active above 25 A.