101
Cables
• NAC circuits: The polarity of the ground fault is reversed in the panel report because in
normal supervisory condition, the NAC circuits are driven in the opposite polarity.
• Annunciator and network connections that use electrical industry standard RS-
485 connections: The panel reports a ground fault on either lead of these connections
as negative. This is a result of the low voltage circuitry used for such signaling. Neither
the positive lead nor the negative lead of the RS-485 line is of a high enough voltage
level to accurately differentiate between a positive or negative ground fault, so in the
case of a ground fault on either lead of the RS-485 line, only a negative ground fault is
reported.
12.5 Ground Fault Detection
In certain applications, a large number of wires connected to the same panel can cause a high
capacitance to ground, which can cause the MMX system to report false intermittent ground
faults.
If the system reports intermittent ground faults when there is no ground fault, turn on Alternate
Ground Fault Detection in the Configurator. This compensates for the errors caused by high
capacitance to ground in cables.
To turn on Alternate Ground Fault Detection
1. In the Configurator, open the version of the job that is active on the panel.
2. Select the node that is reporting ground faults.
3. Check Alternate Ground Fault Detection at the bottom of the window.
4. Click Panel > Send Job to send the job to the Fire Alarm Control Panel.
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