Electrical Isolation Between DCS and PLC

If all else fails, read the instructions.

Hi Saranya,

Electrical isolation is to prevent any loss in signal due to high voltage interference. The signal that is to be transmitted to DCS / PLC should be noise less.

The isolator can be of opto coupling type.

Hi SG, Good answer from Craig. You can use P&F's following device: http://www.pepperlfuchs.com/products/product.jsp?product_id=18381 MODEL : KFD2-STC4-EX1.2O This will act as an isolator for field transmitter and provides two isolated 4-20mA outputs for DCS and PLC. We are using this device at our fertilizer plant. This type of device is also available in product range of MTL.

This is a guess, but it could be that the DCS and PLC are both trying to power the loop, therefore you need an isolator between the two. When you say isolator, you are talking about an analog signal (right?) and in my experience they are fairly simple to wire up. It is only 2 inputs (+ and -) and 2 outputs (+ and -). The isolator breaks the electrical current between the two circuits, in a optic isolator by turning the signal into light and then back into electrical current.

Keep in mind that all my electrical knowledge comes from beating my head against the wall at 2 AM in a plant trying to get something running so it should be taken with a grain of salt.

Hi Saranya

If I understand the question correctly, you are trying to ask why would we require to use an isolator the DCS or PLC from the signal transmitted to it from the field. In this i do believe you are talking about the analouge signals though. In any automated plant with ACS or DCS systems we require to keep these signals clean when we provide the input to the DCS or PLC as any interferebce or noise on these signals which it can pickup invariably due to various reasonsa and some sesitive field devices could prove very problamatic in either analyzing the data for you will see more than what is actually happening and you wont understand why the machine or equipment is behaving in a certain way. and hence these are required.Then there are always the isolations that you would do to insure that the picking up of a mchine that would parallaley create some other ripples in some signals elsewhere. Due to poor grouding standards ..

I Hope i have understood the question right though,

Hi all,

In this case he is refering to a galvanic/opto isolator. This is used for intrinsic safety. You can also use a zener barrier. The determination of which to use depends on your earthing as well the length of the loop.

The other type of "isolator" (this is not the correct term in my opinion) is the signal isolator, better known as the signal converter. This is used where the card powers the loop and the transmitter powers the loop. If you do not use a signal converter here - you will find your channels being pulled to ground.

Regards,

Craig

Ahh, that is my confusion on this. I hope the OP writes back...