VAV Controller—Variable Air Volume (VAV) Controller 77
This section assists you in measuring field wiring terminated to the VAV.
If you are confident that the field wiring has no earth grounds, you may go
to the following section called Power Transformer Isolation - VAV100/101.
A field device that is earth grounded becomes apparent when you test the
entire controller.
Before you terminate the field wires to the VAV, measure the
resistance of each wire to earth ground using an LED test circuit
(preferred) or a DVM. For the LED test circuit, assemble the test
circuit illustrated in Figure 38. Then, connect the circuit from each
input to earth ground and then to each output to ground. For the DVM
test, connect the DVM from each input to earth ground and then to each
output to ground (Figure 38).
If the LED turns On (or you read a value less than 1 Megohm at any
input or output), the circuit is improperly isolated. Replace or repair
the wiring or the field device; or, for a binary output, add an isolation
relay. Repeat the DVM test only.
If the LED does not turn On (or you read a value greater than
1 Megohm), the circuit may be properly isolated. (It will not be
isolated if there are earth grounds that exist at higher voltages.)
A value of infinite ohms indicates a completely isolated circuit at
approximately zero volts. Connect this wire to the appropriate VAV
terminal and check the next wire.
Field Device
IN/OUT
IN/OUT COM
DVM
ISOCIRCT
470 ohm
1/4 watt
9V
Battery
(+)
(-)
10 mA
Red
LED Test Circuit (Preferred Method)
or
VAV
Figure 38: Testing for an Isolated Circuit
Field Device
Wiring Isolation –
VAV100/101
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