15
8.4 GROUND FAULT TRIP CURRENT LEVEL
The SIGMA-MD monitor relay can be programmed for a number of ground fault trip levels expressed as a
percentage of the NGR let-through current setting. Eight ground fault trip level settings are available and are: 5%;
10%; 15%; 20%; 25%; 30%; 40%; 50%.
I-Gard recommends that the ground fault trip level setting be set as low as possible to provide maximum protection
without having the SIGMA-MD monitor relay report ground faults falsely.
Refer to TABLE 4 GROUND FAULT TRIP CURRENT LEVEL SETTINGS to determine the DIP switch settings for
the application.
Caution: CAN/CSA-M421-00 Use of Electricity in Mines Paragraph 3.5.5 states, in part, that “Where ground-fault
protection is used, the supply shall be … de-energized in less than 1s if ground-fault current exceeds 20% of the
prospective ground-fault current”. Therefore, I-Gard recommends that the ground fault trip level be set as low as
possible and not higher than 20% when complying with this CSA Mine Safety Standard.
8.5 SYSTEM FREQUENCY
The SIGMA-MD monitor relay has two settings for selecting the AC frequency of the system it monitoring for ground
faults. Refer to TABLE 5 for DIP Switch settings for the AC frequency setting.
8.6 NGR SET LET-THROUGH CURRENT
The SIGMA-MD monitor relay has sixteen settings for the NGR let-through current. Refer to TABLE 7 for DIP switch
settings for the NGR let-through current.
The NGR let-through current settings in Table 7 show that the SIGMA-MD monitor relay can be configured for let
through currents ranging from 50 amperes through 800 amperes using a single standard I-Gard TxA type zero
sequence current sensor.
The resistance tolerance DIP switch can be used to toggle the NGR resistance tolerance between 66%-150%
(recommended when used with I-GARD neutral grounding resistors) and 66%-300% (recommended to be used
when the resistor alloy used in the NGR has a high coefficient of resistance/temperature). Refer to TABLE 6 for
setting the NGR resistance tolerance.
9. SIGMA-MD MONITOR RELAY CONFIGURATION SWITCHES
