Input line monitoring in DCS

  • Thread starter Sengupta, Subhro
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Thread Starter

Sengupta, Subhro

Hi !

I want to connect alarm switches (Fire alarm switches) to DCS and monitor short circuit and open circuit of the line in the DCS. Can anyone help how to do it. Also in case of PLC, what do I do ?

Thanks and Regards

Subhro Sengupta,
Control Systems,
Bechtel, New Delhi,
Ph. 0091-124-6343107 Ext. 2410,
E-mail - [email protected]

Paul Gruhn, P.E. Moore Process Automatio

It's a function of the input module, which not all systems can do. Fire inputs are normally de-energized open circuits, so an end of line device is required in order to set up a trickle current to monitor for line breaks. Discuss it with your hardware vendor. There are systems designed
specifically to do this.

Paul Gruhn, P.E.
Safety System Specialist
Siemens Moore Process Automation, Inc.
Houston, TX
[email protected]
713-666-7686 (phone)
713-666-8421 (fax)

Ralph G. McDonald, P.E.

To monitor a line with a PLC just use an analog input. As an example use a 24 VDC power supply for loop power. Add a 600 Ohm resistor to the +24 VDC line to the switch, at the switch attach a 4800 Ohm resistor in line with the switch, and another 4800 Ohm resistor around the switch and resistor. When the line returns to the analog input install another 600 Ohm resistor. Then a jumper from the other analog input to the common of the power supply.

Now under normal conditions, ( assuming low lead resistance ) when the switch is open the total loop resistance is 600 + 4800 + 600 or 6000 Ohm and the current is 4 mA. When the switch closes the two 4800 Ohm resistors are in parallel which gives a loop resistance of 3600 Ohm and the current is 6.67 mA. If the wire is shorted in the field the loop resistance is 1200 Ohm and current is 20 mA. If the wire is open the current is zero.

You may pick any values for the resistors that give you resonable ability to dicriminate between conditions. You may even monitor several switches on the same loop with carefull choices of values for each resistor.

I hope this helps:

It is my recommendation to check if this beautifully catalogued feature of line monitoring really working or not. If it does not work
how many ways it confuses the end user. You may understand that this I am speaking out of real life experience where you also had been in the design of the system.

Best Regards
Nilanjan M
Haldia Petrochemicals, India
Ralph, neglecting the argument about expense, (yeah, I know about the most shouted phrase, SAFETY FIRST) the very scheme you suggest is what
leads to the VERY COMMON practice of bypassing fire/gas alarm inputs.

What about grounded inputs? Furthermore, what happens if the ground path is resistive?

Phil Corso, PE
Trip-A-Larm Corp