Hi. In Control panel i am using 2 wire configuration for analog wiring. on one terminal we provide +24Vdc and other terminal we connect the cable from AI Channel. But i cant understand why there is an earth cabel connected to the third terminal? which commonly connected to Power supply earth. Is it required or is it a precaution? Pls reason it.

You will need to check the documentation for the I/O card you are using. If the loop is working then presumably the earth of the power supply and the I/O card are already connected (common earth for multiple input channels perhaps ?)

Thanks Steve.
From the module AI1 channel cable comes to one terminal of TB (terminal block). For single channels 3 terminals are provided in TB. One is 24VDC+ which is shorted to every terminal for each channel. And third terminal is earth cable shorted to each terminal for every channel. Now in general i have not seen wiring like this. I have not seen earth cable shorted in TB for each channel. If it is Instrument ground it should be grounded at the Instrument. Panel earth is different which is already connected to the body. Interesting thing is these earth cables were connected to panel earth which i removed because panel earth and instrument earth should not be connected. But why we are taking earth of instruments to the panel. This is the first time i am seeing this. I have been answered it is connected to the shield of the transmitter. And it is not grounded anywhere but just looped. So the earthing cables shorted in the TB are not exactly earth cables i guess. It is just to increase the confusion i guess.
Thanks Steve for showing interest...

Can you please refrase or give us a bit more information?
There can be different kinds of Analog Inputs - will show you some:
a) Current Closed Loop 4-20mA
b) Voltage Loop - 0-10VDC
c) Current Closed Termistor 10kOhms
d) Voltage / Current Loop based on 3 or 4 wires (commonly for PTCs)

Also, you can have AI with INTERNAL POWERED CIRCUITs and AI with EXTERNAL POWERED CIRCUITs.

So, please, can you give more information about your exactly need?
What are you pretending to wire into or from?
How many cables do you have? 2, 3 or 4 wires connection?
What kind of PLC are you using?
Do you know if your PLC will handle such kind of different "sensors" automatically?

Please let me help you, but get me some more needed info.
Bye

I have seen this type of wiring wherein for every analog input channel, three TB's are allocated. TB1 is usually for +, TB2 for -, and TB3 for Drain+Shield. Connection to the controller, however, is only 2-wires: + and -. Usually, the drain wire and shield at the controller side is left "floating". Doing so ensures proper earth grounding of the cable.

If this is the case, then earth grounding of the cable helps reduce the effects of environmental noise on analog signals. Hope this helps.

Theoretically speaking, as Paulo Said, this could be for Drain/Earth connection.

However, if thats the case, DO NOT forget that for noise control, you can not connect the grounding at both ends, which means, connect only at one end. Otherwise, you may occur in a very common error: cable loop conditions, and this is not good.

Hope this also helps you.

Thanks Luis and Paul. Yes i am sure it is for shielding purpose to reduce noise. And it must be earthed on a single side to prevent close loop for current.

it is required for surge suppression and noise reduction