Over-temperature protection using PLC to control temperature in a vessal
Over-temperature protection using PLC to control temperature in a vessal
(OP)
I am using a PLC to control the temperature in a vessal. I am looking for opinions on using the same PLC for controlling the temperature, as well as, monitoring and protecting against over-temperature.
I have seen this done 2 ways.
1. Use the PLC to control the heater contactor, and use a seperate over-temp module to control a seperate contactor to protect against over-temperature fault conditions.
2. Use a PLC with 2 thermocouple inputs, one for the process temperature, and one for the over-temperature. The same PLC will then control 2 outputs, one for the heater contactor, and one for the over-temperature contactor.
Option 2 is obviously less expensive, as it does not require a seperate module. However, is option 2 legitimate? Is it safe, or can it be made safe? Is it code compliant?
Thanks, in advance, for your feedback.
I have seen this done 2 ways.
1. Use the PLC to control the heater contactor, and use a seperate over-temp module to control a seperate contactor to protect against over-temperature fault conditions.
2. Use a PLC with 2 thermocouple inputs, one for the process temperature, and one for the over-temperature. The same PLC will then control 2 outputs, one for the heater contactor, and one for the over-temperature contactor.
Option 2 is obviously less expensive, as it does not require a seperate module. However, is option 2 legitimate? Is it safe, or can it be made safe? Is it code compliant?
Thanks, in advance, for your feedback.





RE: Over-temperature protection using PLC to control temperature in a vessal
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Over-temperature protection using PLC to control temperature in a vessal
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RE: Over-temperature protection using PLC to control temperature in a vessal
If you are self insured get a write-off from your boss whoever that is and file it.
If you are not insured, put in series two over-temp devices.
RE: Over-temperature protection using PLC to control temperature in a vessal
Keith Cress
kcress - http://www.flaminsystems.com
RE: Over-temperature protection using PLC to control temperature in a vessal
- separate sensor
- separate logic solver/controller
- latching output that interlocks the energy/fuel supply, latches when tripped, requiring manual reset
NFPA calls it "excess temperature limit controller". The market calls them high limit controllers and they're submitted for FM approval (CYA).
What are you going to say when the prosecuting attorney has you read this, out loud in court,
"8.16.10 The operating temperature controller and its temperature-sensing element shall not be used as the excess temperature limit controller".
RE: Over-temperature protection using PLC to control temperature in a vessal
That was a better answer.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Over-temperature protection using PLC to control temperature in a vessal
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