Johnson Controls METASYS ASC, Networking And Troubleshooting Manual

Page: 26 / 48

26 Introduction—ASC and N2 Bus Networking and Troubleshooting Guide
b. Repeat Step 3a for the other two N2 Bus wires.
c. At the ASC that you suspect is causing the ground loop, measure
its +15 VDC (or +30 VDC) terminal to earth ground using the
DMM in parallel with a 100K ohm resistor.
If the value is greater than 5 VDC/VAC, disconnect the N2 Bus at
the ASC. Measure the VDC terminal again. If the value is still
greater than 5 VDC/VAC, a ground loop is present at that ASC.
Locate and correct the isolation problem and measure again. If the
value is less than 5 VDC/VAC, the ground is corrected. Reconnect
the N2 Bus and measure again. If the value is greater than 5
VDC/VAC, ground sources are present at other ASCs or the
N2 Bus.
If the value is less than 5 VDC/VAC, the N2 Bus and ASC are
properly isolated.
For this test, you’ll need a red LED test light, a 470 ohm resistor, and a
9 volt battery. Follow these steps, referring to Figure 13:
1. Disconnect the N2 Bus wires from the Companion PC or Panel/LTD.
2. Assemble the test circuit as shown in Figure 13 or purchase one with
a power supply of 9 volts or greater.
3. Connect the ground lead of the battery to earth ground.
4. Connect the positive lead from the test light to one of the N2 Bus
wires.
If the LED lights when you connect the N2 Bus wire, that particular
wire is grounded or improperly isolated. Subdivide the N2 Bus in
half and repeat Steps 3 and 4. Continue to subdivide the bus until you
find the location of the problem. Correct the grounded condition and
measure again.
If the LED does not light when you connect the N2 Bus wire, the bus
is properly isolated from earth ground.
5. Repeat Step 4 for the other two N2 Bus wires.
LEDTEST
470 ohm
1/4-watt
N2+
N2-
REF
9 V
Battery
(+) (-)
Red LED
(15/20 mA)
Test Circuit
Figure 13: Ground Loop Test
TEST 2:
Ground Loop
(Isolation)