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SEL Application Guide 2020-05 Date Code 20200324
ENABLE FASTER TRIPPING DURING MAINTENANCE MODE
The arc-flash function in the SEL-751 provides the fastest tripping during a fault. It uses light and
a fast overcurrent detector to detect faults within 2–5 ms [2] and can remain enabled at all times.
This application guide shows an alternative method for faster tripping when the relay has not been
ordered with arc-flash detection. We set up an instantaneous phase overcurrent element for faster
tripping and enable it only during maintenance mode.
The SEL-751 has four levels of instantaneous phase overcurrent elements. Set one such element
(Element 2 in this example) with sensitive pickup settings that are temporarily activated during
maintenance mode conditions. When determining the pickup setting, the element must be set high
enough to ensure that the relay does not nuisance trip under heavy load conditions. A typical set-
ting is 1.5 times the expected maximum load current [1]. In this example, we use a 50P2P pickup
setting of 2 A secondary, as shown in Figure 3. Set the 50P2TC torque-control equation to NOT
LT01 so that the sensitively set 50P2P overcurrent element is enabled only under maintenance
mode conditions. If you want instantaneous tripping on a ground element, you can also set a
ground instantaneous overcurrent element in a similar manner.
Figure 1 Relay Word Bits Associated With Each Operator Control Pushbutton
Figure 2 Logic to Enable and Disable Maintenance Mode
PB2A_LED
PB02, PB02_PUL
PB2B_LED
PB3A_LED
PB03, PB03_PUL
PB3B_LED
PB4A_LED
PB04, PB04_PUL
PB4B_LED
PB1A_LED
PB01, PB01_PUL
PB1B_LED
AUX4
AUX3
AUX1
AUX2
PB6A_LED
PB06, PB06_PUL
PB6B_LED
PB7A_LED
PB07, PB07_PUL
PB7B_LED
PB8A_LED
PB08, PB08_PUL
PB8B_LED
PB5A_LED
PB05, PB05_PUL
PB5B_LED
BLOCK CLOSE
BREAKER CLOSED
BREAKER OPEN
AUX
In reclosing models only
Not available in models with four pushbuttons
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