Watlow CPC400 - Manual (Page 162)

Chapter 6: Troubleshooting and Reconfiguring CPC400 Series User’s Guide
148 Watlow Anafaze Doc. 0600-2900-2000
Checking Analog Inputs
Follow these steps to troubleshoot problems with the ana-
log inputs to the CPC400 controller:
WARNING! Shorts between heaters and sensors or wiring er-
rors can cause potentially lethal voltages to be
present in the CPC400 and associated wiring and
devices. Only qualified personnel taking appro-
priate precautions should attempt to troubleshoot
or service equipment.
1. If the process variable displayed in interface software
does not agree with the process variable on the control-
ler display, verify that the controller is communicating.
2. If the process variable indicated on the controller dis-
play is incorrect:
a) Verify that you have selected the correct input
type for the affected loops.
b) Verify that sensors are properly connected.
3. If the sensors are correctly connected, with power on to
the heaters check for high common mode voltage:
a) Set a voltmeter to measure volts ac.
b) Connect the negative lead to a good earth ground.
c) Check each sensor input for ac voltage by connect-
ing the one lead from the voltmeter to the sensor’s
positive input connection and the other lead from
the voltmeter to the sensor’s negative input con-
nection.
A voltage greater than 530V Å (ac) on one or more sen-
sor connections indicates a heater leakage or a wiring
problem. Correct this problem.
4. Check for voltage differences between sensors. A volt-
age difference between any two sensors in excess of
280V
Å (ac) indicates a wiring problem or short.
a) Connect the negative lead to the first sensor con-
nection.
b) With the positive lead measure the sensor-to-
sensor potential at each of the other sensor con-
nections.
c) Move the negative lead to the next sensor.
d) Repeat steps b and c to measure the voltage be-
tween each pair of sensors.
e) Correct any problems indicated by excessive
voltage readings.