Grounded Thermocouples


Thread Starter

George Robertson

I understand that some DCS do not like grounded thermocouples. What experiences has the list had?

How about the Delta V mv temperature input card? It appears to be optically isolated so I don't see why grounded TCs should bother anything.

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George G. Robertson, P.E.
Manager of Engineering
Saulsbury Engineering and Construction
One way to overcome this problem is to utilize isolating modules. Many companies, such as Phoenix, make such programmable thermocouple fully
input/output/power supply isolated modules (mV input and 4-20 mA output). These isolators also greatly reduce inherent noise pickup problems inherently associated with long thermocouple leads since they can be positioned close to the thermocouple junction and the isolated 4-20 mA output loop is relatively noise insensitive.



Malvern Jones

I have experienced such problems on a Honeywell TDC3000 system. It must be said that the oil refinery had a local "floating" intrinsically safe earth. This was not bound to mains earth. As a result, it was possible to have potential difference between the two earth systems. using isolators doesn't always overcome the problem as it is difficult to isolate and provide cold junction compensation in the same unit. My advise would be to spend the money removing the problem, instead of compensating for it. Non grounded T/c's are much cheaper than isolators!
I had a heck of a time getting some to work using Allen Bradley Flex-I/O. We had to ground the - side of the DC power supply. Worked perfectly. Make sure you KNOW which type od T/C you have installed. I prefer ungrounded if at all possible.