Input signal gives a temperature value within normal range even though field measurement shows it is out of range
- S100 I/O module DSAI 155 Thermocouple burn-out detection
The following problems have been reported from some installation with grounded thermocouples and must be observed by all users:
Q. In case of a break in the thermocouple circuit due to burn-out or otherwise it is still possible that the input signal gives a temperature value within normal range and consequently the fault is not detected.
R. If the recommendations for grounding are not followed the noise immunity may be substantially reduced.
a) If acceptable for other reasons do not ground the thermocouples. A break in the thermocouple circuit will then result in an out-of-range value which indicates a fault.
b) If the thermocouples must be grounded follow the installation rules in order to maintain noise immunity, i.e.
- Thermocouple ground point, connection header and the DSTA 155 ground must have the same ground potential. Install DSTA 155 as close to the thermocouple as possible in order to avoid long compensation cables, which may impact the accuracy of the measured value.
c) If the thermocouples must be grounded consider some of the following alternatives for detecting break of the thermocouple circuit:
- If the thermocouples must be grounded consider some of the following alternatives for detecting break of the thermocouple circuit: Use high/low limit indication and set a narrow 'normal range' value if possible. Use application software to check acceptable change rate of single temperature value or difference between temperature values. Use double thermocouples for critical temperatures.
Medium and Long Term Solution
a) For new installations consider using S800-I/O. The thermocouple input module AI835 has eight channels with reliable indication of break of the thermocouple circuit both with grounded and non-grounded thermocouple.
- Thermocouple input not working properly