I have an IS circuit with the following components.
The signal from the transmitter comes to a surge protector, passes on through an IS barrier and on to the DCS Input module.

How do i calculate what is the impedance of the loop and whether my module supports the same?

I do have a figure of the input impedance that my module supports, but what information do i need to look for in the IS Barrier and if the surge protector adds on?

Thanks

 

From your description, the only external item in the DCS input module's loop is the output stage of the isolator. If your input module has an active input you just need to check the isolator output is loop powered and the voltage range it needs matches the supply range from the input module.

If your input module has a passive input, then the isolator output should be a source, and not loop powered. It's worth checking that the input module resistance matches what the isolator output can drive into.

If in doubt contact your isolator supplier before you switch on.

 

Before you blow up the plant you are in there is a couple of things about IS barriers you need to learn.

IS barriers are energy limiting devices. The aim of them is to limit the amount of energy that is present in a circuit that is in a hazardous area. The aim is that if there is a fault there is not enough energy present ignite what you have. This why you need what you might ignite, what devices you have in the area, what your barriers are capable of and what the cable characteristics are.

ALL IS barriers have limis as to what they can protect. The certificates list 2 very important bits of information the maximum inductance and maximum capacitance. These relate to the complete circut fed by the barrier. You have to add the inductances and capacitances for the cable(s) and devcie in the field.

All the devices you can connect to a barrier are either simple like limit switches or complex like NAMUR sensors and 4-40mA transmitters. The differance is that simple devices have no inductance or capacitance themselves. Certified hazardous area devices all have the devices inductance and capacitance on the certificate.

All cables have as part of their basic data values for inductance per meter and capactitance per meter, usually mH/m and uF/m. A basic sensor cable is 2 bits of metal separated by an insulator and twisted together. All multicore cables have inductance and capacitance. The data sheet tells you how much per meter you have of each.

Either

You know how much cable you have and calculate the total inductance and capacitance and verify it is less than the maximum allowed for the barrier.

Or

You subtract the values for the device from the barrier and use the cabel data to work out the maximum amount of cable you can have.

These calculations go into a report call an IS dossier which should contain (at least), your calculations, the data sheets for the cables and the certificates for the barriers and devices.

Dont panic more than a few people have not done calculations and later found out the cables were too long for the barriers used.

Pepperl & Fuchs is one of the major suppliers of barriers check out their web site it has lost of information and a lot of "how to" information.