Load Shedding Control Mode - nvent RAYCHEM T2000 Installation, Operating And Maintenance Manual

5.3 LOAD SHEDDING CONTROL MODE

IMPORTANT: The load shedding fail safe mode is not supported when using proportional
ambient SSR control, since ambient temperature is being monitored rather than pipe
temperature.
5.2.5 PROPORTIONAL AMBIENT CONTACTOR CONTROL
(FOR CM 2000+ MODULES USING CONTACTORS)
When a CM 2000+ control module using a contactor controls its output using the ambient
temperature, this control mode should be used.
A CM 2000+ control module implements proportional ambient contactor control as follows:
• The output may not be switched on/off rapidly when using a contactor, so proportional
temperature control is implemented by applying the required duty cycle over the selected
CYCLE TIME.
• The output is fully on for a portion of the CYCLE TIME as determined by the calculated duty
cycle, and it will be completely off for the remainder of the CYCLE TIME.
• The duty cycle is calculated each time the output toggles, based on the ambient temperature,
PROPORTIONAL BAND setting and the CONTROL SETPOINT temperature setting.
• The control module monitors ambient temperature rather than pipe temperature and
compares it to the CONTROL SETPOINT temperature as in the proportional ambient SSR
control mode.
• If the temperature sensed by the control sensor is at or below the CONTROL SETPOINT
temperature minus the PROPORTIONAL BAND setting, then power is applied to the heating
circuit with a duty cycle of 100%. The control module output will be fully on for 1/30th of the
CYCLE TIME setting before the duty cycle is calculated again.
• If the temperature sensed by the control sensor is equal to or greater than the
CONTROL SETPOINT temperature, then the output will have a duty cycle of 0%. The control
module output will be off for 1/30th of the CYCLE TIME setting before the duty cycle is
calculated again.
Proportional Ambient Contactor Control Temperature Band For CM 2000+ Control Modules
Control Sensor Temperature
Š Setpoint
= Setpoint-proportional band/2
£ Setpoint-proportional band
IMPORTANT: The load shedding fail safe mode is not supported when using proportional
ambient contactor control, since ambient temperature is being monitored rather than pipe
temperature.
Control modules may be put into a load shedding control mode only after receiving a load
shedding command issued from a GCC (Model 780/GCC-9000 or AC 2000+). This mode overrides
temperature control and forces the output of the control module off until reset by the GCC.
If the load shedding feature is enabled in the GCC, the GCC will issue a load shedding
command to the control modules at least once every minute. The status of this load shedding
command is determined by how the GCC is configured. The Model 780/GCC-9000 GCC can be
configured to generate its load shedding commands from the status of its four local contact
inputs or from four remote contact inputs. The AC 2000+ GCC may only be configured to
generate its load shedding commands from the status of four remote contact inputs.
Each control module may be programmed to respond (i.e.: enter the load shedding mode) to
any combination of the four GCC contact inputs. Sections 3.4.18 to 3.4.20 of this manual give
details on setting up the load shedding parameters of the control module.
When power is applied to the control module, it determines if the load shedding mode has
been enabled. If enabled, the control module immediately enters the load shedding operation
(holding its output off) and waits to see if the GCC has issued a load shedding command.
The three-character display will flash "PrI" when the load current would normally be displayed.
If no load shedding command is received from the GCC, the control module resumes normal
operation. If a load shedding command is received, the control module will continue to hold its
output off, until one of the following four conditions occurs:
1. The GCC contact input that initiated load shedding deactivates and the GCC issues a load
shedding command that will terminate load shedding mode.
Duty Cycle
0%
50%
100%
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