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2. DESCRIPTION
The SIGMA-MD monitor relay measures the current through the NGR, the transformer neutral-to-ground voltage and
the NGR resistance. The relay compares the measured values against the field settings of the relay and provides
relay outputs and LED indications when an abnormal condition is detected.
NGR current is measured using the zero sequence current sensor. The trip level of the ground fault circuit is DIP
switch selectable as a percentage of the NGR let-through current setting at 5%, 10%, 15%, 20%, 25%, 30%, 40%
and 50%. Trip time is DIP switch selectable from 60 milliseconds to 3.15 seconds.
Transformer neutral-to-ground voltage is measured by means of the NGRS-XX resistor sensor (NGRS), connected
between the NGR’s connection to the transformer neutral and the relay’s NGRS input terminal. The NGRS-XX
resistor sensor is used by the SIGMA-MD monitor relay as part of a comparator to monitor the NGR resistance. The
NGRS-XX resistor sensor contains a voltage suppressor which limits its output voltage to a safe level.
When the NGR current measured by the ZSCS is above 1% of the NGR let-through current setting, an NGR fault
will be detected if the measured current and voltage indicates that the NGR resistance has increased to more than
150% in the standard mode or 300% when the corresponding DIP switch is set for the extended tolerance mode, or
has decreased to less than 66% of its nominal value.
When the ZSCS current is below 1% of the NGR let-through current setting, the SIGMA- MD monitor relay simply
monitors the NGR resistance for continuity, i.e. whether the NGR is open or presents some resistance.
The SIGMA-MD monitor relay has four output relays:
• The trip relay can be programmed for shunt trip operation (non-failsafe) or undervoltage operation (failsafe) in a
main breaker trip circuit. The trip relay will operate on either an NGR fault or a ground fault.
• The NGR fault auxiliary trip relay can be used to give a door/panel mounted or remote indication of an NGR fault.
• The ground fault auxiliary trip relay can be used to give a door/panel mounted or remote indication of a ground fault.
• The SIGMA-MD Healthy state alarm relay.
The SIGMA-MD monitor relay provides a current source output for connection to a 1mA full scale ammeter (either
analog or digital). The output signal is proportional to the measured current and is expressed as a percentage of the
NGR let-through current.
3. METHOD OF OPERATION
3.1 COMPONENTS OF THE SYSTEM
The zero sequence current sensor is an I-GARD TxA type.
The NGRS-XX resistor sensor is selected to match the line-to-line voltage of the system in which the SIGMA-MD
monitor relay will be used.
The NGR will be sized for a let-through current according to the specifications of the system in which it will be installed.
A DIP switch array on the SIGMA-MD monitor relay permits selection of trip relay operational mode, trip memory,
ground fault trip time delay, ground fault trip current level, NGR resistance tolerance and the NGR let-through current.
