53
controllers associated with that contact input. Each controller may be associated with one or
more groups. Refer to the Load Shedding Section in the
GCC manual for details on setting up the
load shedding features of the
HTC when using a GCC.
When power is applied to the controller, it determines if load shedding mode has been enabled.
If enabled, the controller immediately enters load shedding operation (holding its output off) and
waits to see if the
GCC or an external communicating device has initiated a load shedding com-
mand. If no command is present the controller resumes normal operation. If a load shedding
command is present, the controller will continue to hold the output
OFF, until one of three condi-
tions occurs:
1. The
GCC contact input or zone definition bits of an external communicating device which ini-
tiated load shedding clears and the command to terminate load shedding mode is issued.
2. Communications are interrupted between the controller and its communicating device, as in
the case of a damaged communications wire. Approximately 30 seconds after communica-
tions ceases the controller will return to normal operation.
3. Communications between the controllers and the external communicating device goes off-
line for approximately two minutes, as occurs when the 760 Hand Held Programmer is used
to communicate with the controller.
Note: The controller will return to normal operation if communications between the 780 or exter-
nal communicating device and the controller are disrupted in any way. This will return tempera-
ture control to the
HTC. Also, the HTC does not perform a periodic autocycle test while operating
in load shed mode. When using a
GCC, it must be configured for load shedding operation before
the controller can be set up for load shedding control.
Three parameters must be set in the controller to configure it for load shedding operation:
1. The load shedding feature must be enabled.
2. The
FAIL SAFE MODE parameter must be enabled or disabled depending on the application
requirements. If
FAIL SAFE MODE is enabled, then at least one LOW TS ALARM (of a TS used in
the
TS CONTROL MODE) must be enabled. If the alarm temperature exceeds the CONTROL SET-
POINT
temperature, fail-safe mode will be disabled.
3. The
GCC contact input(s) or zone definition bits of an external communicating device that are
to be associated with the load shedding action for the controller must be defined.
These parameters can only be configured using an external communicating device or the Model
780/GCC-9000 Group Communications Controller. Refer to the Model 780/GCC-9000 user man-
ual for details on how to set up these options. The optional 920 Operator Console can be used to
enable or disable the load shedding feature, but not to set any other load-shedding parameters.
Notes:
• Fail-safe mode is always disabled if the
SWITCH CONTROL MODE is set to either of the two pro-
portional ambient control modes, or the
TS CONTROL MODE = EXT INPUT, FAIL OFF/ON
• Fail-safe mode is disabled if the TS CONTROL MODE is set to either EXT INPUT, FAIL TS 1 or EXT
INPUT
, FAIL TS 2 and a control temperature failure occurs.
• The
HTC will turn on its output switch when the control temperature becomes less than the
highest
LOW TS ALARM temperature if the following conditions are met:
– Fail-safe mode is enabled
– Load shedding is active
– The
TS CONTROL MODE uses both TS 1 and TS 2
– Both TS 1 and TS 2 have their LOW TS ALARMS enabled
•A
FORCE ON override signal has higher priority than a load shedding signal. An INHIBIT signal
has higher priority than fail safe mode.
5.4 TEMPBUS™ Control Mode
TEMPBUS™ is short for Temperature Bus. This refers to a connection that allows one “master”
HTC to share its control temperature with a number of “slave” HTCs. Up to 25 “slave” HTCs can
be connected to this bus. Refer to the wiring diagrams in Appendix C for example connection
details.
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