D8 Series User’s Guide
Chapter 7: Troubleshooting and Reconfiguring
143
4. Verify the sensors:
• For thermocouples, remove the thermocouple leads and use a digital voltmeter to
measure the resistance between the positive and negative thermocouple leads. A value
of 2 to 20Ω is normal. Readings in excess of 20Ω indicate a problem with the sensor.
• For RTDs, measure between the IN+ and IN- terminals of TB1. RTD inputs should read
between 20 and 250Ω.
5. To verify that the controller hardware is working correctly, check any input (except an RTD)
as follows:
a. Disconnect the sensor wiring.
b. In the
Input
menu, set the
Input type
parameter to
J thermocouple
.
c. Place a short across the input. On the loop that you are testing, the controller should
indicate the ambient temperature.
Earth Grounding
If you suspect a problem with the ac ground or a ground loop:
• Measure for ac voltage between ac neutral and panel chassis ground. If ac voltage is above
2VAC, then there may be a problem with the ac power wiring. This should be corrected per
local electrical codes.
• With ac power on, measure for ac voltage that may be present between control panels’
chassis grounds. Any ac voltage above 2VAC may indicate problems with the ac ground
circuit.
• With the heater power on, check for ac voltage on thermocouples. A control output providing
power to the heaters will increase the ac voltage if there is heater leakage and an improper
grounding circuit. Measure from either positive or negative thermocouple lead to ac ground.
AC voltage above 2VAC may indicate the ground lead is not connected to the D8 TB2/J2
ground terminal.
If the above tests indicate proper ac grounding but the controller is indicating incorrect temperatures
or process readings:
• Verify which type of sensor is installed and that the
Input type
parameter in the
Input
menu is
set accordingly.
• For an RTD or process input, check that the correct input scaling resistors are installed
(see Scaling Resistors on page 148) and check the input scaling parameter settings
(see Setting Up a Process Input on page 69).
• If readings are erratic, look for sources of electrical noise. See Noise Suppression on page
• Eliminate possible ground loops. See Ground Loops on page 27.
• Contact your supplier for further troubleshooting guidance.
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