A major safety concern is the loss of neutral path, i.e. broken wire from source neutral to resistor or between resistor and ground or even a bad or loose connection.
The result is changing from a high-resistive grounded system to either an ungrounded or solidly grounded system without anyone knowing it! This would cause severe safety hazards, such as transient over-voltages, unlimited fault current, escalating arcing faults etc. With the Sigma monitoring relay the neutral path from neutral to ground (including resistor) is continuously monitored for integrity. If an open or short circuit occurs, the relay will alarm.
The I-Gard SIGMA monitoring relay is a combination neutral grounding resistor (NGR) monitor and ground fault relay. In distribution systems employing resistance grounding the SIGMA monitoring relay protects against ground faults and abnormal conditions in the path between system neutral and ground possibly caused by loose or improper connections, corrosion, foreign objects or missing or compromised ground wires.
The SIGMA monitoring relay measures the current through the NFR, 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.