Johnson Controls METASYS ASC, Networking And Troubleshooting Manual

Page: 39 / 48

Introduction—ASC and N2 Bus Networking and Troubleshooting Guide 39
In most installations, reliable controller operation is ensured by a separate
isolation transformer or step-down transformer to provide 24 VAC to a
UNT or VAV Controller. However, in environments where transient
electrical noise is present on power lines, you must take further
precautions to prevent unwanted BO cycling or other possible controller
malfunctions. Also, the switching of inductive loads can generate
transients that can be conducted and/or radiated into the circuits
controlling those loads, as well as into other circuits nearby.
While the practices described in a previous section Rules for Using
Shielded Cable are intended to lessen the effects of such transients on field
wires, the transients on the power line can be significantly reduced in
either of these ways:
●
Connect an Metal Oxide Varistor (MOV) across the UNT supply
transformer primary. The MOV must be rated appropriately for the
line voltage. For your application, refer to Table 7 and select the
correct MOV.
●
Or, use a Split-Bobbin transformer, rather than a standard non-isolated
transformer, to attenuate electrical noise. For 120 VAC line
applications, we recommend the AS-XFR100-1 as a step-down
transformer.
Note: For the N2 Bus, the recommended suppression device is the
Transient Eliminator, model TE/JC04C12, made by Advanced
Protection Technologies (APT). For details, refer to the N2
Communications Bus Technical Bulletin in the Metasys Network
Technical Manual.
The most effective location for the suppression device is directly across
the load, since it lessens the propagation of transient energy into connected
wiring (which, in turn, becomes a source of noise to adjacent wiring).
Difficulties in getting access to the load, however, may sometimes make it
necessary to locate the suppression device at the ASC.
Suppression
Devices