from the Instrument department...
Line Monitoring
Continuous process industries, DCS questions. topic
Posted by Ramesh on 4 March, 2013 - 2:25 pm
I saw the First time of line Monitoring in digital Input in the ESD system. I know it will detect the four possibility cases

1 Open contact
2 Open circuit ( wire Break )
3 Close contact
4 Short circuit ( Wire Short circuited )

but I didn't know how it was working Practically. Can Anybody send the Circuit Diagram of that one With Details to me


Posted by Mihir Ramkrishna on 5 March, 2013 - 6:02 am
>1 Open contact
>2 Open circuit ( wire Break )
>3 Close contact
>4 Short circuit ( Wire Short circuited

You will be installing a precision resistor, practically on the field contacts. All four conditions u have mentioned will have different current values.

Posted by Dick Caro on 5 March, 2013 - 11:13 am
The Digital Input card for this type of fault-detecting contact input is really a low accuracy A/D converter. It supplies typically 5-12volts DC power to the contact. There is a known resistor placed across the field contact switch, and another across the terminals of the DI card. The current flow in the loop is measured to determine if the contact is open or closed. Zero current flow means an Open Circuit (broken wire or failed power supply.) When the contact is open, the current flow will be at a minimum. When the contact is closed, it shorts out the resistor across the contact to make the current flow at a high value. If the two wires to the contact are shorted to each other, the current flow will be at a maximum, but it is not really possible to detect the difference between a short circuit and an open contact since the only difference is the resistance of the lead wire.

Dick Caro

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