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1. GENERAL INFORMATION
The I-Gard SIGMA-MD monitor relay is a combination neutral grounding resistor (NGR) monitor and ground fault
relay. In distribution systems employing high resistance grounding, the SIGMA-MD monitor relay protects against
ground faults and abnormal resistance values of the neutral grounding resistor (NGR).
The SIGMA-MD monitor relay is specifically designed for a variety of system voltages and a neutral grounding
resistor (NGR) limiting the maximum NGR current to the relay’s set let through current.
The SIGMA-MD monitor relay is designed to be used with a TxA type zero sequence current sensor, an NGRS-XX
resistor sensor and an NGR sized to limit ground fault current to that stated in the installation’s specifications.
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.
Hazard of Electrical Shock, Burn or Explosion
All installation, servicing and testing referred to in this manual must be
performed by qualified personnel. All power should be disconnected
prior to removing covers or enclosures and where live conductors
may otherwise be exposed.
Failure to observe these precautions may result in death or severe
personal injury and damage to equipment. Before placing an
intentional ground fault on the power system, check that a fault does
not already exist. Any test ground fault equipment must be rated for
full system voltage and be fused for protection.
DANGER
IMPORTANT
Each SIGMA-MD monitor relay and its auxiliary parts, is carefully inspected before being packed in specially
designed cartons. Any unit received should be examined immediately upon receipt. If damage or indication of rough
handling is apparent, a claim should be filed with the transport company immediately. I-Gard should be notified
promptly if replacements for damaged goods are necessary. If units received are not to be installed immediately
they should be stored in their original containers in an area free of dust and moisture.
