Load Shedding Control Mode - nvent 910 Series Installation, Operating And Maintenance Instructions

7.3 LOAD SHEDDING CONTROL MODE

Load shedding is a control mode that may be programmed and initiated only by an external
communicating device or by the Model 780/GCC-9000 Group Communications Controller,
which overrides temperature control and forces the output of the controller OFF until reset
by the 780/GCC-9000. When using a GCC, load shedding is initiated by a contact closure (or
opening) on one of the four contact inputs on the GCC. Each contact input initiates a load
shedding command for the group of 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
command. If no command is present, the controller resumes normal operation. If a load
shedding command is present, the controller will continue to hold its output OFF, until one of
three conditions occurs:
1. The GCC contact input or zone definition bits of an external communicating device that
initiated 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
communications ceases, the controller will return to normal operation.
3. Communications between the controllers and the external communicating device go offline
for approximately two minutes (as occurs when the 760 Hand Held Programmer is used to
communicate with the controller).
IMPORTANT: The controller will return to normal operation if communications between the
GCC or external communicating device and the controller is disrupted in any way. This will
return temperature 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 may be set up for load shedding control.
There are three parameters that must be set up in the controller to completely 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 and its alarm temperature must be less than
the CONTROL SETPOINT temperature; otherwise, 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 may 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 manual for details on how to set up these options. Note that the Operator Console may be
used to enable or disable the load shedding feature but not set any of the other load shedding
parameters.
IMPORTANT:
• Fail safe mode is always disabled if the SWITCH CONTROL MODE is set to either of the two
proportional ambient control modes, or the TS CONTROL MODE = EXT. INPUT, FAIL OFF/ON.
• If the TS CONTROL MODE uses both TS 1 and TS 2 to calculate the control temperature, the
HTC will turn on its output if the following conditions are met:
– load shedding is active
– fail safe mode is enabled
– the control temperature falls below either of the LOW TS ALARM settings
– 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.
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