Corrective Action (continued)
Step 2. When a splice is found, break it and repeat the procedures in steps 1, 2 and 3 above. Step 3. Continue to work back toward the instrument. Repeat the checks until all junctions or splices have been checked and reconnected in proper polarity.
Check all terminal screws to insure that they are tight. Check wires for corrosion or any varnish and clean where necessary.
Short in Thermocouple or Leadwire
When a thermocouple or its leadwire are shorted, this becomes the new hot junction, and the instrument will record the temperature at this point.
If the terminal connections are correct, use the following test:
1. Trace thermocouple leadwire backward toward instrument. Check at junction boxes for splices. Leadwire reversals which cause low readings always exist in pairs. If one reversal connection is found, look for a second.
A short in the thermocouple is usually caused by broken ceramic insulator beads, metal in the protection tube and by broken thermocouple connection heads.
A short in the thermocouple leadwire may be caused by pinched wires where they are unprotected. Frayed insulation on leadwire may be an indication that a short is present.
A partially shorted thermocouple leadwire may be caused by moisture soaked leadwire insulation. Readings will always be low. Trouble is easily identified by disconnecting the instrument and the thermocouple from the leadwire and checking for leakage between the leadwires and the conductors with an ohmmeter. If leakage is found, the wire in the conduit in which it is installed must be checked to determine the cause. Moisture inside the conduit is one possibility when non-moisture proof leadwire insulation is used. Moisture in conduit may be present in applications where high humidity exists or where conduit is run underground. If this is the case, the leadwire should be replaced with wire having moisture resistant insulation, such as polyvinyl.
Two Grounds in a Thermocouple Circuit
A thermocouple may be grounded in one place. However, when a thermocouple is grounded at two different locations, the instrument will indicate an average temperature of these two locations.
A visual inspection should be made for bad insulation, broken terminal blocks or pinched wires. Make a continuity check after ungrounding the thermocouple to see if further grounds exist. If so, remove ground or replace defective wire or thermocouple.
Wired to Wrong Terminals
Instruments with thermocouple break protection will read upscale and can never be made to coincide with the setpoint. This is also a symptom of an open in the thermocouple circuit.
A check of the wiring in the thermocouple circuit or a continuity check should determine where the open circuit exists.
Wired to Wrong Instrument
In new installations, the thermocouple wire may be wired to the wrong instrument. In this case, the control may be erratic or the control of one instrument may be overheating while another instrument on the same application will be considerably different and that zone may be cold.
The wiring should be traced or continuity checked to insure that the wiring is correct.
Barber-Colman Company, Loves Park, IL.