Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

A thermocouple wire is brought back to a thermocouple input card in the PLC. The thermocouple cable is 10 feet long. The thermocouple is measuring a fluid flowing at 125°C. Due to gouging of the cable insulation, the two wires in the cable have been shorted together 5 feet from the PLC input card. The result of this short on the temperature observed at the PLC input is:

A. There will be no issue; the input will read 125°C at the PLC.
B. The short in the cable will cause damage to the input card channel and a fault will be displayed.
C. The input will read a temperature closer to the ambient temperature.
D. The temperature at the PLC input card will read 62.5°C since the short is halfway between the measurement point and the PLC.

CCST Answer

The correct answer is C, "The input will read a temperature closer to the ambient temperature." When a thermocouple is shorted at another point other than the intended junction used for measurement, another junction, meaning another thermocouple, is formed. The reading at the programmable logic controller (PLC) will be most influenced by the “new” thermocouple that is formed by the short circuit, which is between the intended measurement point and the PLC.

ISA12 is a group of standards relating to electrical equipment in hazardous (classified) locations. NFPA: 70-84 is the National Electrical Code (NEC). CENELEC is the European Community for Electrotechnical Standardization electrical standard.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, Second Edition, ISA, 2005.

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