Johnson Controls METASYS VAV110 Technical Manual page 28

N2 Bus Isolation
VAV 100/101
Power
Transformer
Isolation
VAV100/101
Load Isolation
VAV100/101
VAV110/111 and
3 6 F 3 7 F 3 8 F
VAV140/141
28 VAV Controller—Variable Air Volume Controller
All VAV100/101 connections must be isolated from earth ground to protect
the CBLPRO, laptop PC, or other system components from damage. Due
to OEM system designs and/or electrical codes, the VAV may become
earth grounded unless you take isolation measures. Typically, the VAV
box manufacturer supplies a step-down isolation transformer as part of
their mounting service.
!
CAUTION: Potential Equipment Damage and Improper
Functioning. Do not earth ground the 24 VAC high
side of the VAV's transformer or any of the VAV's
BO terminals. If you do, the VAV's N2 Bus
communications will cycle online and offline. You
will also damage all of the interface components that
are connected, including the CBLPRO, laptop PC,
Companion PC, and MM-CVT101.
Depending on the primary voltage of the power transformer and the
prevailing electrical code, the 24 VAC secondary might be earth grounded.
Earth ground on one side of the secondary destroys hardware by creating a
short circuit path through the CBLPRO and laptop PC serial card during a
download. Earth ground on either side of the secondary can disrupt the
isolation required for N2 communication integrity.
The box manufacturer supplies fan or electric heat relays that might also be
earth grounded. This also disrupts the isolation required for N2
communication integrity.
Note: The Troubleshooting section of this technical bulletin contains a
detailed process that will reduce installation errors by ensuring
proper isolation.
If the secondary of the power transformer is grounded, you must use a
separate 24 VAC to 24 VAC isolation transformer such as the Y65GS
(see Figure 8).
If VAV output loads are grounded, such as a driven contactor or solenoid
coil, you must use a separate isolation relay for each load (see Figure 8).