The green LED indicates two functions: When slow flashing it means control power is applied to terminals N- and L+;
when fast blinking it denotes the relay has sensed a ground fault current higher than the trip level threshold selected
for a period longer than the tripping time delay and that the output contacts have operated.
The mGARD-100 operates on any control voltage from 40 to 240V AC or DC, and contains an isolated power supply.
The seven position terminal block is pull-apart type that eases the wiring in the field.
A three position, pull-apart terminal connects the relay to the mGARD-SYM remote indicator (with modbus
connectivity) for remote test / reset and monitoring.
Special consideration must be taken in the selection of the neutral grounding resistor in a high resistance grounded
system as the capacitance-to-ground charging current on a system will vary depending on: the overall length of the
cables; the types of loads; the quality of insulation on the phase conductors; the surrounding equipment grounding,
cable trays, junction boxes, the type of sensor, etc. A “Rule-of-Thumb” for systems 600V and lower: The charging
current is 0.5 Amps per 1000 kVA of sensor capacity. In electrical systems of 600V and below it is common to use a
5 Ampere continuous rating for the neutral grounding resistor and setting the pickup current at 20% of that value (1
Ampere) for all relays and coordinating between upstream and downstream relays by selecting different time delays.
3. OpERATION
3.1 OPERATION AND GLOSSARY OF TERMS
Auto Reset:
After a trip, the relay will automatically reset itself three seconds after the ground fault current drops below the trip
current set point. The auto reset feature can be used in both failsafe and non-failsafe modes.
Chassis Ground:
Chassis ground is the ground to which all of the non-current carrying metal equipment is connected/bonded.
Typically, equipment grounding is provided by means of a ground bus. A solid connection is to be made from
terminal 7 of the mGARD-100 to the nearest chassis ground to ensure the relay complies with the specified
electromagnetic compatibility (EMC) standards. If using an external zero sequence current sensor, do not ground
any of the ZSCS secondary wires.
Interposing Current Sensor:
Interposing Zero Sequence Current Sensors (ZSCS) are required for any of the following applications:
• The ground fault trip current setting levels are higher than the 30 mA –12 Amp range available with the built-in
2” ZSCS.
• The size of the power conductors on which the mGARD-100 is being applied is too large for the 2” built-
in ZSCS.
• The system primary phase current on which the mGARD-100 is being applied exceeds 250 Amps continuous.
Failsafe:
The output relay changes state when control power is applied to terminals N- & L+. The N.O. contact closes and
the N.C. contact opens. When a trip occurs or control voltage is removed, the N.O. contact is opened and the N.C.
contact is closed.
