Content
# Zenon Train 1 and 2 analog issues
Personnel -
Kevin and Seth
(describe the problem)
The analog signals with most noticeable effect:
LIT3426_1
LIT3426_2
LIT7526
There are 5 analog devices that use external 24v power supply. the three above are included. The devices are divided between two analog input cards.
After concluding that the analog signals having the issue were all external loop powered devices, we replaced the power supply. The issue persisted.
It is worth mentioning that this issue becomes worse (starts causing problems) when we have a large rain event. This is the second time this issue has occurred.
We observed the analog input from LIT3426_1 with a clamp milliamp meter. The signal was bouncing a little but the process is dynamic.
We disconnected individual cable shield drains, and individual analog signals while observing the scaled value on the HMI. There was no improvement.
We connected a milliamp simulator using "source" mode (internal power supply) to the analog input at the terminal block inside the plc cabinet in place of the LIT3426_1 level transmitter and observed the scaled value on the HMI. The value was steady.
We then connected the milliamp simulator using "simulate" mode (external power supply) to use the instrument power supply in the plc cabinet, again in place of the LIT3426_1 level transmitter and observed the scaled value on the HMI. The value began to oscillate 1 to 2 inches. The simulated value was steady at 8ma. It was concluded that the issue is present when the internal DC power supply is used as the source.
It should be noted here that the instrument cable shield drains are bussed and connected to an isolated grounding bar. This grounding bar is not earthed and is isolated from the backpanel.
I had the thought to try connecting the DC supply negative to the shield drain ground bus. Once we made that connection the analog signal on the LIT3426_1 input still in "simulate" mode was now steady. We disconnected the ground wire and observed the analog signal began to oscillate again. We reconnected the ground wire and made it permanent.