D8 Series User’s Guide
Chapter 1: System Overview
19
Design your system to be safe even if the controller sends a 0 percent or 100 percent output power
signal at any time. Install independent, external safety devices such as the Watlow TLM-8 that will
shut down the system if a failure occurs.
Typically, a shutdown device consists of an agency-approved high/low process limit controller
that operates a shutdown device such as an mechanical contactor. The limit controller monitors
for a hazardous condition such as an under-temperature or over-temperature fault. If a hazardous
condition is detected, the limit controller sends a signal to open the contactor.
The safety shutdown device (limit controller and contactor) must be independent from the process
control equipment.
WARNING!
The controller may fail in a 0 percent or 100 percent output power state.
To prevent death, personal injury, equipment damage or property damage, install
external safety shutdown devices that operate independently from the process
control equipment.
With proper approval and installation, thermal fuses may be used in some processes.
Power-Fail Protection
In the occurrence of a sudden loss of power, the D8 controller can be programmed to reset the
control outputs to off (this is the default). The controller can also be configured to restart to data
stored in memory.
A memory-based restart might create an unsafe process conditionfor some installations. Use a
memory-based restart only if you are certain your system will safely restart. See Power Up Loop
Mode on page 103.
When using the controller with a computer or other master device,you can program the software to
automatically reload desired operating constants or process values on powerup.These convenience
features do
not
eliminate the need for independent safety devices.
Contact Watlow immediately if you have any questions about system safety or system operation.
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