UNT Controller—Unitary Controller (UNT) 81
IMPORTANT:
When using pilot relays to ensure electrical
isolation between the binary outputs and their
loads, pilot relays must be used on all BOs. This
ensures that electrical noise created by contact closure
or opening of a pilot relay cannot travel back through
the BO wiring, or any other BO, and adversely affect
the UNT’s other binary outputs that are not relay
isolated. A pilot relay to the economizer is an
exception to this because it drives a circuit, not a
relay/contactor load.
Ensure that relays have adequate isolation
between coil and contacts.
Many relays on the market do not provide suitable
isolation. Capacitance between the coil and contacts
must be less than 3 pico-farads. As manufacturers do
not specify this parameter, remember that smaller
relays generally have more capacitance. (RLY100 and
RLY050 relays provide suitable isolation.)
Ensure that relays provide a minimum of 50 mA
load to the UNT binary output.
If the selected relays draw less than 50 mA, you must
add appropriately rated resistors in parallel with the
relay coil to avoid possible relay chattering. We
recommend using 1k ohm, 2 watt resistors.
Rooftop air handling units may be very “noisy” from an electrical
standpoint. This is due to contacts switching inductive loads and electrical
spark ignition devices. You must take precautions to prevent electrical
noise from causing false BO triggering, resetting of UNTs, or interference
with N2 communication.
There is wide variation in relays, contactors, and gas ignition control
devices. Some are much noisier than others. Electromagnetic fields can
radiate from wires carrying spark or coil current, as well as from the spark
or contact arc itself. Electromagnetic fields are also present around wires
carrying power to the ignition control. These are “noisy” wires.
For more information, refer to the Power Line Wiring Transient Noise
Precautions section of this technical bulletin.
Electrical Noise
in Rooftop Units
Summary of Contents for UNT Series
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