How to protect an inductor (coil) of a Pressure/current converter by inserting a device in series with the loop 4-20ma?

hello guys
i have pressure/current converter SRI983 where its main electronic component is the coil that changes current to pressure or the opposite .and this transmitter is 24V and 4-20ma .
but i'm facing a big issue of damaged coils for a years and now someone suggested a solution to install a fuse in the loop ( 20 ma ) to protect the transmitter from high current >20ma.
that will be convenient if the current is the cause of this problem.
but it can be voltage , and maybe noise , and maybe something else you know dear guys.
I searched the internet and they said maybe a zener barrier will be my perfect solution because it limits voltage , current , energy as they said .
now i came here to get the final solution , what do you propose guys?
if you propose a zener barrier provide me the right one for my circuit by the current and voltage i provided?
is it loop powered or requires external power ?i heard about earth or ground what should i take into consideration ?
The SRI983 is a Foxboro/Invensys/Schneider electropneumatic positioner. From the description, the I/P coil has failed.

What's the failure mode? shorted windings from failed wire insulation? Open circuit?

I find it hard to believe that an active 4-20mA analog output can damage a DC force coil designed for that purpose.

There's no chance that someone connected a 24Vdc power supply directly across the positioner input? That'll fry a force coil in short order.

Close lightning strikes can create surges but a surge is likely to take out active components in the analog output, as well. Here's a link to a 4-20mA loop lightning/surge protector (but I'm dubious):

And you have to consider that through the years of multiple acquistion/transitions, Foxboro>Invensys>Schneider, that the integrity of the insulation specs for coil wire might have been not been maintained like they were back in days when Foxboro was Foxboro. There's nothing you can't buy cheaper, but there's a reason it's cheaper . . . . might just be poor coil construction/material that no 'protection' is going to make up for.
Correct, nothing between the 4-20mA output and positioner's input.

Any electro-pneumatic positioner (or I/P) is designed to be powered only by the voltage/current provided by direct connection of an 'active' 4-20mA analog output. An active' 4-20mA output is one powered by the output device (PLC/DCS/RTU/Loop controller).

The 4-20mA analog output will regulate its voltage to provide the necessary current output to drive DC current through the coil.

A separate power supply is NOT connected in the AO/positioner loop.

A separate DC power supply is NEVER to be connected across the positioner's input terminals. The DC voltage left connected across a coil will draw enough current to overheat the coil and burn it out in short order.

PLC/DCS/RTU/Loop controller 4-20mA analog outputs are designed to drive at least a 500 ohm load and this particular positioner has only a 200 ohm nominal load, well within the specs of any commercial 4-20mA AO around nowadays.
Now i understand what you mean and i agree with you, in fact i consider the 24 v and 4-20ma coming from the PLC as a voltage and current sources.
Now since we are in the same page, can you suggest me a device or component that can help me to protect my I/P converters
Just to inform you sir, we used many brands (foxboro, rosemount... Etc) since 1995, but we have more than 20 damaged ones.
Now when they recruted me, I'm trying to install a fuse, or zener barrier or that kind of circuits.
But before all that i must make my diagnostics and find the cause that burning out the coils!!
I am not an industrial forensic analyst. these are my thoughts on the situation.

1. Is this quantity of failed positioners a "real" problem, as opposed to ordinary "wear and tear"?
What is the total population of positioners vs the number of failures?
Could this be just ordinary failure rates given the service conditions for the population of positioners?

2. Where do the failures occur and at what rate do failures occur?
What is the distribution of failures? Is this a case of failures at 19 different valves out of a total of 20 failures or have 2 or 3 valves had continued, repeated positioner failures?

3. Do the failed positioners all use the same technology (some smart positioners do not use a force coil)?
Are there associated analog output (driving positioner) failures ?

4. What were the true/real failure modes?
Who has determined the stated failure mode(s) and are those assessments reliable?
Can it be said with any degree of certainty that the failures are electrical vs mechanical vs plain old dirty air problems?


1. Nothing lasts forever. We all know that service life depends on service conditions. Extreme heat or cold exposure, humidity, vibration and dirty electrical service (surges/noise/common mode) greatly affect service life.

What service conditions could contribute to the failures of these positioners?

For instance, the dirty little non-secret behind the vast majority of positioner failures is dirty supply air - water, oil and dirt in the supply air. Every manufacturer states positioners need instrument air meeting the ISA 7 spec. I've walked into instrument/electrical shops and seen piles of 'bad' positioners stacked in the corner of the shop awaiting a slow day when someone will be assigned to disassemble, clean them and restore them to performance. They always ask, "do you have a positioner that will work with dirty/wet air?" No, nobody does. Is it possible that in the past X years before you were recruited that positioners failed due to dirty air rather than coil failures?

2. With regard to reported failure modes, the USA's Nuclear Regulatory Commission published a report on Aging and Service Wear of Air-Operated Valves Used in Safety-Related Systems at Nuclear Power Plants" (1995). You have to understand nuclear has to be the most regulated industry out there; far more than even pharmaceutical regulation; by an order of magnitude. With all the bureaucratic paperwork involved, the report mentions a number of undetermined or vague types and causes of valve failures. Even with all that regulation, the documented part can be vague and unclear (and these are the folks who take failures seriously). So I'm asking myself, how reliable is a scribbled notation "bad coil" on a service report hurriedly written by a tech who's late to his (fill in the blank) activity at the end of his shift when he knows that whatever he writes is a just a bureaucratic formality - nobody ever checks it or bothers with it?

3. I have seen a case where vibration broke the coil leads in a I/P, so it might be possible in an electropneumatic positioner as well. The I/P vendor analyzed one of their general purpose 4-20mA/3-15psi I/P's for failure after the 3rd failure in 2 years on the same valve and the cause was a broken coil lead. There was considerable vibration where the I/P was mounted. Are you sure the coil failure is not mechanical related to vibration, and not electrical related to surges/noise?

4. 4-20mA active outputs (internal power supply) are generally not fused because they don't have to be; the AO regulates the current like it should and it can regulate the output of direct short circuit. However, loop powered 4-20mA analog inputs with an external DC power supply are sometimes fused to protect against a short circuit in the loop that puts the 24Vdc across the AI input and burns up the dropping resistor.

5. A passive I/S barrier can act to prevent arcing-sparking electrical faults from reaching the positioner. There are powered I/S Isolators that perform that same function. A local vendor should be able to help you select one if you want to try one.

6. If you look at low amperage fuses, be sure to look at the spec for the time it takes for low amperage fuses to open at amperage near the rated value.
Thank you david from the bottom of my heart.
When i was reading your answer, i thought you work in my site (gas compression station ).
I thought air service (gas in our case) isn’t a problem at all, but i assure you natural gas contain small black particles accumulate with time surely will be so important !
This 8 I/P positionners (2 installed in each machine) each year at least 1 is damaged and now all of them replaced and now they keep damaging.
Even though the machines starts for one month a year and about 11 months OFF , considering the high vibration of the pipe, the Turbine is the source of vibration will affect all the circuit of surge.
Just to inform you sir david that the other 11 months the coils keep energized (24v and 4ma) but we cut off the air service (gas) since it is easy for the operator in the aim of protecting the transmitter.
Me too I don't consider the source of our issue to be electrical... We have our energy source and very stable and no perturbation at all, i can assure you that. And fuses won't be a solution because a coil will take what it requires, i mean when the coil is damaged and the problem happened it comes the role of the fuse.
Also since 1995 many I/P Were damaged and many brands and constructors (foxboro, rosemount etc) which means the problem aint coming from low quality transmitters.
So as a conclusion i assure you the pressence of :
-important vibration during that month or two of service.
- since we use gas as a service, it contain black molecules we found everywhere in our installation.
-the coils are energized even there is no need (i mean no air service), i dont know if that is appropiate.
-high summer temperature above 50.
Thanks in advance.
Abdouzita... thanks for schematic.
1) How about the environment, cold, hot, steamy?
2) Refinery, Chem plant, other hazardous process?
3) Are prevailing wind from another process facility?
4) Are failures appearing in same area of plant?
5) Are failures occurring in same Instrument enclosure, cubicle, panel, or rack-room?
6) Are the failures in a local panel cooled with environmental air? Instrument air? Other?
7) What has mfg revealed about failure-mode?
Regards, Phil Corso