Analog Isolators

Bullzi

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Jun 2012
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Colorado
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Hi Everyone,

I have a RFQ where the customer is asking for Analog and Digital Isolators to be put on both ends of the cables.

Have you ever seen a request like that? Of course to me a "Digital Isolator" would just be a relay but I have never used Analog Isolators before. So other than "Isolation" what is the purpose of them? What advantages do they bring to a system? Any idea why they would want them on both ends?

Thanks for any info you can provide.
 
Analog signal isolators will help protect the expensive PLC analog I/O points, and can alleviate issues with non-isolated PLC inputs. They can also be useful in cases where you need to put one signal through two or more devices. At both ends seems extreme. I think some of the engineers that write these specs realize that the more difficult and expensive it turns out, the more they get paid.
 
Perhaps the engineer is confusing surge suppressors with isolators? I've never seen isolators on both ends of a loop, and a digital isolator would indeed be a relay.
 
Could a digital isolator be for RS-485? (digital communications)?

I routinely install a 485 isolator to protect the relatively expensive card with the RS-485 port and when doing multidrop networks, I put a 485 isolator half way down the line, just in case a fault (read that lightning strike) hits, one only loses half the network.

Technically, an isolator on both ends protects both the signal source and the analog input, but I've never seen that done in 35 years of analog work.

Surge suppressors on the field end are not uncommon for industrial pressure and temperature transmitters.
 
I've never seen one on both ends either. I guess technically it protects the transmitter too instead of just the PLC, but doesn't really make sense to me.

Sounds like generic corporate specs written by someone who doesn't actually do real work.
 
I think that they mean over voltage / surge protection. That makes much more sense for each end of the line, for protecting against lightning induced damage.

I'd ask them what equipment they would prefer, with model numbers and work back from there.
 
Thanks for all the info everyone. I think you are right in that they are talking about surge suppression. What surge suppressors have you guys used in the past? I checked with Phoenix and they are very expensive ($112 Each and I need about 48 of them to put 1 on each end). Any other options?

Also this process has got me thinking about how I build panels. How many of you guys use analog isolators or surge suppressors? Up to now the only time I have used any kind of isolation was for Intrinsically Safe applications. In my normal designs I only use fuses to protect the Analog I/O. I have never had an analog point fail for any reason. Have I been lucky or is the Analog I/O pretty robust and Isolation/Surge Suppression only necessary in extreme applications? Give me your opinions.

Thanks!!
 
https://m.erico.com/Category.html?category=R1417&country=us&language=en&applications=,

We use these universal transient barriers as our standard. Reasonably slim and may be slightly more affordable than $112 but I'm not sure what they'd cost over there.

We work in water and wastewater, and these are a must for us for any equipment which is either in a sump or well or installed in contact with a pipeline. However I tend to only put one each side if it's a long cable run over exposed terrain. That's where a near lightning strike has the potential to cause... well, potential difference! If the transient is being conducted by the pipeline or process fluid there's nothing much I can do to save the instrument.

Make sure the earth to each surge diverter is suitably sized and individually run back to and earth bar. using fine multistrand flexible copper appliance wire is also good practice for avoiding skin effect impedance issues (lightning has a surprisingly large high frequency component).

There's no guarantee that surge protection will save the analog. But there's a very good chance that it will stop the entire PLC letting the black smoke out if a signal cable ends up carrying a direct strike back into the panel.
 
Well I ended up getting this job and they "lightened" up on the Isolation on each end thing.

This is a water pipeline job and there will be underground vaults that have transmitters in them. Also there will be a Mag Meters in the vaults that I am going to communicate with via RS-485. All the wiring will be in conduit buried underground. The PLC will be in a Climate Controlled room. The max cable length is going to be about 150' long.

I do want to make sure that I do my best to protect the PLC from spikes as much as possible. So I am open to suggestions. Do I just put suppressors in the Vaults? Or do I put them in the PLC panel? The Transmitters are not high dollar units. The protection devices cost almost as much as the transmitters. So I am not worried about protecting them as much as the PLC and Meter as they are the big ticket items.

Thanks for any suggestions you can provide.
 
Underground cable is still prone to over voltage conditions from nearby lightning strikes (Resistive coupling). If such atmospheric conditions are common then I'd still strongly recommend surge protection at each device.

If that's problematic due to cost, then on the RS485 line as it enters your PLC cabinet. Or even better, in its own small enclosure as the cable enters the building, with a 6mm2 fine strand copper earth wire back to the main earth.

I would ask the client about what happens if those flowmeters die. Whilst the equipment might be cheap, if it causes the automatic shutdown of a few city blocks because someone thinks there's a pipe burst, they might want to reconsider!

I'm also surprised people are still building underground vaults for equipment. Here those would require a confined space entry, 3 man team with rescue plan etc. I'm working with our clients to bring equipment up to levels where CSE is no longer required. Obviously a bit out of the scope of the Electrical guy in your scenario, but I'd be making sure I wasn't jumping into any pit more than 1.5m deep or one that's fully enclosed. Even innocent looking empty chambers can be killers from lack of oxygen or toxic gases from unexpected sources, like solvents or paint etc. <OT safety rant over>
 
I'm also surprised people are still building underground vaults for equipment. Here those would require a confined space entry, 3 man team with rescue plan etc. I'm working with our clients to bring equipment up to levels where CSE is no longer required. Obviously a bit out of the scope of the Electrical guy in your scenario, but I'd be making sure I wasn't jumping into any pit more than 1.5m deep or one that's fully enclosed. Even innocent looking empty chambers can be killers from lack of oxygen or toxic gases from unexpected sources, like solvents or paint etc. <OT safety rant over>

Sorry to sound like your typical HSE (or EHS) fundamentalist, but safety is never off-topic. You'd be surprised how much and many people are unaware of what a confined space may be.
 
I had a job with RS-485 running about 750' underground to a pump building for VFD comms. A lightning strike toasted the PLC serial port a few years ago. I installed one of these when I replaced the PLC:

https://www.commfront.com/collectio...solator-repeater-converter?variant=9208600963

For 4-20mA signals, for short runs I just fuse them with these

For longer runs, signal isolators are probably a good idea and especially if they were in the original spec. I have just ordered one of the Prosense signal splitters for a different job to see if I like them. This model might be good for cases like yours:

https://www.automationdirect.com/ad...ers/high_density_signal_conditioners/sc6-4102
 
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