I-Gard SIGMA 3 - 1. INTRODUCTION; 2. DESCRIPTION; 3. OPERATION; 3.1 System Components

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1. INTRODUCTION
The I-Gard Sigma 3 monitor relay is a combination ground fault relay and Neutral Grounding Resistor (NGR)
monitoring relay. The Sigma 3 monitor relay protects against ground faults and abnormal resistance values of the
NGR.
The Sigma 3 monitor relay is specically designed for a variety of system voltages and an NGR limiting the
maximum NGR current to the relay’s set let-through current.
The Sigma 3 monitor relay is designed to operate with a TxA or Rx-yA zero sequence current sensor, an NGRS#
sensing resistor with NGRX lter, and an NGR sized to limit ground fault current.
The let-through current measured by the zero sequence current sensor will be the vector sum of any leakage
currents or charging currents normally in the system and any ground fault currents that may be present.
2. DESCRIPTION
The Sigma 3 monitor relay measures the current through the NGR, the transformer neutral-to-ground voltage, and
the NGR resistance. The relay compares the measured values with the eld settings of the relay and provides relay
outputs and LED indications when an abnormal condition is detected.
Ground fault current is measured using the zero sequence current sensor. The ground fault trip level 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.
Neutral-to-ground voltage is measured by means of the NGRS-. The NGRS- sensing resistor is used by the Sigma
3 monitor relay in combination with the zero sequence current sensor as part of a comparator to monitor the NGR
resistance. This NGR resistance monitoring complies with the requirements of CSA C22.1 section 10-302.
The Sigma 3 monitor relay provides a current source output for connection to a 1mA full scale ammeter. The output
signal is proportional to the measured current and is expressed as a percentage of the NGR let-through current
setting.
A watchdog failsafe normally open contact is also provided.
Pulsing signal terminals are provided to facilitate pulsing of the NGR as a ground fault locating technique where the
NGR is outtted with a solid state relay pulsing contactor.
The Sigma 3 can be used with the Sigma3-TDM touchscreen display module to provide an advanced HMI.
3. OPERATION
3.1 System Components
The Sigma 3 is used with voltage and current sensors to monitor the system. The NGRS-## sensing resistor is
used to sense the voltage over the NGR. The zero sequence current sensor, which is used to sense the current
through the NGR, is an i-Gard type T2A, T3A, TxA or any Rx-yA zero sequence current sensor. In addition, the
Sigma 3 operates in injection mode when the current through the NGR is low, whereby a high frequency signal
is injected into the system to measure the zero sequence impedance. A lter is used in conjunction with the high
frequency injection signal called the NGRX. Combination NGRS / NGRX are also available, called NGRXS-##.