UNT Controller—Unitary Controller (UNT) 65
If the field wires are not yet connected (the Field Device Wiring
Isolation--UNTs without an Earth Ground Connection section was
skipped), disconnect one 24 VAC wire and terminate the field wires.
Reconnect the 24 VAC wire.
•
Connect the leads of the DVM in parallel with the 100k ohm resistor
from the DC power supply (+15 VDC) output terminal of the UNT to
earth ground (see Figure 37). This tests whether the field devices, the
UNT, and the transformer “assembly” are properly isolated.
If you read 5 VDC/VAC or greater indicates the circuit is
improperly isolated. Follow these steps to correct this:
Note: Binary outputs are often the cause of ground loops; therefore, we
recommend that you test the binary outputs of the UNT before
testing other points.
1. Remove all the field wires and N2 Bus wires from the UNT,
but leave the transformer wires attached.
2. 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. 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 UNT and measure again. If the
value is still 5 VDC/VAC or greater, replace the UNT.
3. 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.
If you read less than 5 VDC/VAC, the circuit is properly isolated.
A value of less than 0.10 VDC/VAC indicates a completely isolated
circuit.
Load Isolation--
UNTs without an
Earth Ground
Connection
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