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8.1.2 Undervoltage Trip (Failsafe) Mode
When programmed for the UNDERVOLTAGE TRIP MODE and the SIGMA-MD monitor relay is operating normally the
trip relay energizes approximately 1 second after control voltage is applied. The trip relay de-energizes (trips) under
any of the following conditions:
• The measured values of the ground fault current or NGR resistance exceed the threshold settings for the time specified.
• The SIGMA-MD monitor relay is reset.
• Control voltage is removed.
The trip relay remains energized after a trip, providing control voltage is present, until the SIGMA-MD monitor relay
is reset whether or not the fault that caused the trip remains present on the system.
When control voltage is applied the trip relay will at first be energized (no-trip) and the SIGMA-MD monitor relay will
start operating normally. If the trip memory option is OFF the trip relay will remain energized (no-trip). However, if
the trip memory option is ON and a trip state is stored in the non-volatile memory the trip relay will be de-energized
(trip) approximately 1 second after control voltage is applied to the relay and will remain de- energized (tripped),
regardless of whether a fault is present on the system.
The trip relay will be energized (no-trip) if the SIGMA-MD monitor relay is reset through a local or remote reset. When
reset, the SIGMA-MD monitor relay will resume monitoring of the system and, if a fault remains on the system, will
detect the fault and will re-trip, de-energizing the trip relay.
8.2 TRIP MEMORY SETTING
The SIGMA-MD monitor relay stores the states of the trip relay and auxiliary fault relays in non-volatile memory and
can, if programmed to do so by means of the TRIP MEMORY Setting restore the state of the trip relay when control
power is applied.
The SIGMA-MD monitor relay can be set for trip memory ON (DIP switch #2 UP) or trip memory OFF (DIP switch
#2 DOWN).
Refer to TABLE 2 TRIP MEMORY SETTING.
When the trip memory option is OFF the trip relay and auxiliary fault relays are returned to their Idle operating states
when control power is applied.
When the trip memory is ON the trip relay and auxiliary fault relays are restored to the state these relays had prior to
the loss of control power. If the SIGMA-MD monitor relay was indicating a fault condition when control power was
lost, the states of the trip relay and auxiliary fault relays will be maintained until the relay is manually reset.
When the trip memory is ON, the SIGMA-MD monitor relay retains the states of the trip relay and auxiliary fault
relays stored in non-volatile memory until the relay is manually reset.
8.3 GROUND FAULT TRIP TIME DELAY
The SIGMA-MD monitor relay can be programmed for ground fault trip delays ranging from 40-60 milliseconds to
3.15 seconds.
Refer to TABLE 3 GROUND FAULT TRIP TIME DELAY SETTINGS to determine the DIP switch settings for the application.
