# any reason to have 4-20 isolator on PLC Input



## Andy32821 (Jan 1, 2019)

If they current to current isolators and you have no grounded current loops getting rid of them is probably a good idea. If the field instruments are outdoors and the isolators are for electrical fault or lighting protection you probably would be better off replacing it with a modern version surge protector.


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## bill39 (Sep 4, 2009)

Almost all of the recent fresh and waste water plants projects I worked on required isolators AND surge suppressors on every analog input and analog output PLC point. This was in the consulting engineers’s specification.

The reasoning is to prevent a spike or whatever from taking out an entire PLC module or worse.


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## glen1971 (Oct 10, 2012)

Every thermocouple had one on a PLC5 that we just upgraded to a Controllogix. At best guess they were a left over from the original installation, two plc's ago and pre-declassification of the compressor building. With all of the changes, and recent failures of a couple of them, we removed them and replaced them with the appropriate terminal blocks. No issues and haven't lost a minute's worth of sleep over it.


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## gpop (May 14, 2018)

With out lightning protection (surge protection) we would toast a few cards (or input points) a year on well heads that are out in the fields. surge protector is cheap protection. 

We probably have over 1000 analog input points and we have probably 12 with protection so its a case of they are important in the right places.


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## varmit (Apr 19, 2009)

On a lot of the older instrumentation equipment, the transmitter would have it's own loop power supply and the PLC would also supply loop power. The isolators were needed to avoid connecting two separate power supplies together. In harsh environments, the isolators also provided a safety factor in case of a loop short or contact with a higher voltage. (Think of someone somehow putting 480 on an instrument loop. Destroying one isolator is better than taking out the entire process PLC.)


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## mburtis (Sep 1, 2018)

All of our stuff is inside the building so I'm not worried about lightning etc. If i keep the contractor electricians away from stuff high voltage shouldn't get hooked up wrong. Recently we had four different electricians with the same company in four different cabinets hook 120 into a 24 volt line going to 4 valve actuators, probably cost them between 5-10 k in replacing cards in the actuators. Im betting the engineer mislabeled something. 

I'm betting all of these are left over from when they originally (over engineered) and built the plant. Like we have explosion proof everything when only one room is marginally a danger. The original loops all had half a dozen componets that are no longer used (recorders, indicators, alarm annunciators etc) but the wiring still runs through them. Ill have to look into the specific instruments as far as the power supplies and grounding. Like i said there are a lot of newer instruments that run straight to the plc.


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## paulengr (Oct 8, 2017)

Allen Bradley input cards are frequently NOT isolated. That’s a big advantage of using the Spectrum and the Acromag cards or even Beckhoff or Siemens IO. . Check the AB specs carefully. If the card number ends in “I”’ it’s isolated. Otherwise it’s not.


Sent from my iPhone using Tapatalk


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## mburtis (Sep 1, 2018)

Good to know i will be sure to check the card part numbers and look them up.


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## oliquir (Jan 13, 2011)

most plc analog input cards are not isolated, they used a common 0v between them, unless they specify that all channels are isolated


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## scotch (Oct 17, 2013)

Often intrinsic isolators were used in oil-gas installations for safety purposes....


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## telsa (May 22, 2015)

To the OP:

What you're really doing is re-engineering the circuits.

That's a very risky move. 

No-one -- but no-one -- will praise your work.

However, if should something go south -- you are the goat.

You're certain to be standing before the Man -- like Ralph Kramden:

Hamma...hamma...hamma.

&&&&&

You can't win.

No-one is paying you to re-engineer ANYTHING.

You don't even know ALL of the considerations that went into the build.

You are GUESSING.

Prayers to ET will not avail you. Everything is so conditions specific.

If you think the old gear is too old... REPLACE IT.

Then, no-one will fault you.

You are simply not in a position to even know all of the factors that led to the original design. 

As for your personal and professional opinion -- who is paying for it?

That's right: nobody.

EEs that think they are electricians come off as IDIOTS.

The versa is vice.

Electricians that think they are EEs come off as UNPAID idiots.

Break the habit.

&&&&&&

You'd be shocked at the geniuses I've known FIRED because they couldn't stick to their craft. 

Yeah, they were RIGHT. And they were FIRED.

The 'system' does not want -- nor will tolerate -- being 'corrected.'

&&&&

OP: your role is to be the miracle man -- not to be the guy that sleeps in. :devil3:


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## mburtis (Sep 1, 2018)

Well the problem is that the plant has been updated a lot through the years with minimal engineering review of the older systems. And being a municipality we aint going to pay someone just to look at it. In fact we recently had one of the original engineers who built the place here as part of an update project. He opened the cabinet and said "holy xxxx those things are still hooked up". In the end i might just start replacing them with newer ones and have an extra on hand so i don't have to worry about it one way or the other.


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## bill39 (Sep 4, 2009)

mburtis said:


> Well the problem is that the plant has been updated a lot through the years with minimal engineering review of the older systems. And being a municipality we aint going to pay someone just to look at it. In fact we recently had one of the original engineers who built the place here as part of an update project. He opened the cabinet and said "holy xxxx those things are still hooked up". In the end i might just start replacing them with newer ones and have an extra on hand so i don't have to worry about it one way or the other.


Be very cautious when moving the isolators. You will need to be sure which instruments are 2-wire and which are 4-wire. It may not be as simple as simply replacing the isolator with a terminal block.


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## cmdr_suds (Jul 29, 2016)

You have to review the specific analog input card specs to know if it is isolated or not.
In my experience, most loop isolators (not to be confused with intrinsic safety barriers/isolators) are not really needed. They are often there because of generic, boiler plate, specification requirements. I have seen them useful when having common mode noise issues and sinking/sourcing loop issue. Unless it was put there to solve a specific problem, then it is waste of money and another potential failure point. Surge suppression is far more cost effective at protecting the input card then a loop isolator.


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## cmdr_suds (Jul 29, 2016)

bill39 said:


> Be very cautious when moving the isolators. You will need to be sure which instruments are 2-wire and which are 4-wire. It may not be as simple as simply replacing the isolator with a terminal block.


True. You need to understand the devices you are connecting to/from. 2/3/4 wire device, current sinking/sourcing, single ended input vs isolated input. Excitation voltage source, and intemeadiate devices in the loop like a chart recorder or process meter.


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## cmdr_suds (Jul 29, 2016)

You have to review the specific analog input card specs to know if it is isolated or not.
In my experience, most loop isolators (not to be confused with intrinsic safety barriers/isolators) are not really needed. They are often there because of generic, boiler plate, specification requirements. I have seen them useful when having common mode noise issues and sinking/sourcing loop issue. Unless it was put there to solve a specific problem, then it is waste of money and another potential failure point. Surge suppression is far more cost effective at protecting the input card then a loop isolator.


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## mburtis (Sep 1, 2018)

Yes I fully plan to understand every detail of the application before i go ripping stuff apart. Part of the curse of being a recovering engineer. Luckily my job allows me all the time I need to figure stuff like this out. Some interesting points have been brought up for me to investigate.


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## splatz (May 23, 2015)

Would anyone else agree that isolators are more reliable than surge protection? IME surge protection helps, but isolators are almost never beat, if something goes underground or between buildings, I have always tried to bump up to isolation rather than just MOV surge protection.


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## oliquir (Jan 13, 2011)

yes isolation is better for surge but isolation may alter the signal a little depending of quality of isolator (some signals offset,...)


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