A legacy loop I am working on has a P+F active I.S barrier that has one 4/20 ma hazardous area input and a split into two outputs for feeding into two separate receivers. However the input actually coming in is from a Non I.S transmitter. So the barrier is just used as a signal splitter.

I have a slight concern that active I.S isolators may not be suitable for use with non I.S transmitters because of their sensitivity to the loop's inductances and capacitances especially on loops with fast process variable changes - like those in flow loops. Thus signal quality may be affected even if slightly.
Any informed opinions out there?

Rgds
B. Wills

 

Can't answer the question about resposne for fast process variable, but if you have a concern, buy a signal isolator. They are cheap and remove the barrier. use the barrier as a spare for other IS loops.

Or

remove the barioier altogether and place the two seperate recievers in the current loop.

From what I know (understand), the inducance and capacitance of an IS loop is considered because of the potential energy they can store and as such are kept doen to levels that do not create a potential for an explosion in the hazardous area. I have never had to consider the effect of capacitance on the loop. I have not looked at a barrier certificate for a while, but do they actually have capacitance and inductance or is it only the transmitter
(which you cant do anything about anyway unless you buy another one).

Sorry i could not help much.

Mick

 

B. Wills,

I have seen this work and not work. The best solution is to ask the manufacture of the IS barrier and the transmitter. If you have not observed any problems with the loop, I would not be overly concerned, otherwise take the device out and replace it with a suitable splitter/re-transmitter.

Your going to have to look into the output characteristics of the transmitter and the input characteristics of the IS barrier and the fastest way to do this is ask the two manufactures. P+F will probably be able to tell you if there is a problem before the transmitter manufacture can.

MJH

 

The inductance and capacitance associated with I.S. specifications pertain to limiting energy storage and release in the hazardous area. They have nothing to do with the reaction time of the loop and should be negligible with respect to signal reaction time.

The limiting contributor to reaction time will most likely be the transmitter. Most have some kind of low-pass filter to reduce noise. Some have fixed response filtering; others have an adjustable filter. Whether the transmitter is I.S. or not is not the determining factor for filtering.

You might also check for some kind of filtering in the barrier itself.

 

One question you need to ask is - "Why is the barrier there in the first place?". Is the transmitter installed in an area that may contain an explosive atmosphere?

It is far more likely that some-one has replaced an IS transmitter with a non-IS one than that someone has added a barrier to a circuit for an application that doesn't need one. One of the common misconceptions with IS systems is that the barrier provides total protection and anything can be added downstream of it without posing a hazard.