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UNT Controller—Unitary (UNT) Controller
This section will assist you in determining if the supply transformer is
properly isolated.
•
Before connecting the transformer to the UNT, connect the input
power to the primary leads of the 24 VAC transformer.
•
Measure the voltage of each secondary transformer lead to earth
ground with the DVM in parallel with a 100k ohm 1/4-watt resistor
(see Figure 33; if using a double banana plug, insert the plug into the
DVM).
If you read 5 VAC/VDC or greater, the transformer is earth
grounded. You need an isolation transformer in order to isolate the
connections from earth ground and protect system components.
Wire a separate 24 VAC to 24 VAC isolation transformer (such as
the Y65G13) to the UNT.
If you read less than 5 VAC/VDC, the circuit is properly isolated.
Usually a value less than 0.10 VAC/VDC indicates a completely
isolated circuit.
Line
Voltage
24 VAC
transfm1
100k ohm
1/4-watt
DVM
100k ohm
1/4-watt
DVM
Figure 33: Testing the Transformer
•
Determine the polarity of the transformer’s leads by using a DVM
referenced to earth ground without the 100k ohm resistor (see
Figure 33). Connect the transformer’s secondary lead with the higher
voltage potential to the 24 VAC terminal on the UNT. Connect the
transformer’s secondary lead with the lower potential to the 24 VAC
common terminal on the UNT.
Power
Transformer
Isolation—
UNTs without an
Earth Ground
Connection
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