Johnson Controls METASYS VAV110 Technical Manual page 79

Load Isolation –
VAV100/101
If the field wires are not yet connected (because you skipped the section
called Field Device Wiring Isolation - VAV100/101), disconnect
one 24 VAC wire and terminate the field wires. Reconnect the 24 VAC
wire.
Note: If you ground the contactor or solenoid coils to be driven by the
VAV, you must use a separate isolation relay for each load.
•
Connect the leads of the DVM in parallel with the 100k ohm resistor
from the DC power supply (+15 VDC) output terminal of the VAV to
earth ground (Figure 40). This tests whether the field devices, the
VAV, and the transformer assembly are properly isolated.
Line
24 VAC
Voltage
24 VAC COM
Field Wiring*
*AICOM, BICOM, ZBUS COM, AOCOM, BOCOM
Figure 40: Testing for Ground Loops
•
If you read 5 VDC/VAC or greater, the circuit is improperly
isolated. Follow these steps:
1. Remove all the field wires and N2 Bus wires from the VAV, but
leave the transformer wires attached. With the DVM 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.
2. 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 DVM.
3. 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
VAV and measure again. If the value is still 5 VDC/VAC or
greater, replace the VAV.
Note: Binary outputs are often the cause of ground loops. Therefore, we
recommend that you test the binary outputs of the VAV before
testing other points.
VAV
24 VAC
15 VDC
DVM
100k ohm
1/4 watt
VAV Controller—Variable Air Volume (VAV) Controller 79
DVM < 5 VDC/VAC = OK (Isolated)
DVM > 5 VDC/VAC = Earth Grounded
GRDLOOP