Johnson Controls METASYS ASC, Networking And Troubleshooting Manual

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8 Introduction—ASC and N2 Bus Networking and Troubleshooting Guide
5. Connect the leads of the DMM in parallel with the 100K ohm resistor
from the DC power supply (+15 or +30 VDC) output terminal of the
ASC to earth ground (Figure 4). This tests whether the field devices,
the ASC, and the transformer “assembly” are properly isolated.
Figure 4: Testing for Ground Loops
If you read 5 VDC/VAC or greater, the circuit is improperly
isolated. Follow these steps:
a. Remove all the field wires and N2 Bus wires from the ASC, but
leave the transformer wires attached. With the DMM still
connected, reconnect each set of field wires one at a time until you
read 5 VDC/VAC or greater. At this point, you have discovered
one cause of the ground loop. Correct the problem.
b. Continue to reconnect each set of field wires until all ground loops
are found and corrected. You’ll know that all grounds are
corrected when you read less than 5 VDC/VAC on the DMM.
c. However, if you check all the field wires and you still read greater
than 5 VDC/VAC, the transformer secondary is earth grounded. In
this case, wire a 24 VAC to 24 VAC isolation transformer to the
ASC and measure again. If the value is still 5 VDC/VAC or
greater, replace the ASC.
Note: Binary outputs are often the culprits of ground loops. Therefore,
we recommend that you test the binary outputs of the ASC before
testing other points. Remember, a single earth ground is allowed
on the common terminal of the LCP, AHU, and UNT110/111
Controllers, not on the 24 VAC terminal.
If you read a value less than 5 VDC/VAC, the circuit is properly
isolated. A value of less than 0.10 VDC/VAC indicates a completely
isolated circuit.
Line
Voltage 24 VAC
VAV/UNT100
24 VAC
24 VAC COM
GRDLOOP
DMM < 5 VDC/VAC = OK (Isolated)
DMM > 5 VDC/VAC =
100K ohm
1/4-watt
DMM
Field Wiring*
*AICOM, BICOM, ZBUS COM, AOCOM, BOCOM
15 VDC
(30 VDC on AHU)
Earth Grounded