UNT Controller—Unitary (UNT) Controller
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This section assists you in measuring field wiring which will be terminated
to the UNT. If you are confident that the field wiring has no earth grounds,
you may go to the following section, Power Transformer Isolation—UNTs
without an Earth Ground Connection. A field device that is earth
grounded may become apparent when you test the entire controller.
•
Before you terminate the field wires to the UNT, 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 32. Then, connect the circuit from each
input to earth ground and then from each output to ground. For the
DVM test, connect the DVM from each input to earth ground and
then from each output to ground (see Figure 32).
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 test until the LED does not turn on.
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 UNT
terminal and check the next wire.
Field Device
IN/OUT
IN/OUT COM
DVM
isocirct
470 ohm
1/4-watt
(+)
(-)
10 mA
Red
LED Test Circuit (Preferred Method
or
9V
Battery
Figure 32: Testing for an Isolated Circuit
Field Device
Wiring Isolation—
UNTs without an
Earth Ground
Connection
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