# PLC 5 Analog Input Issue



## glen1971 (Oct 10, 2012)

So I'm faced with a bit of a weird one on a PLC5.. There are 2 analog inputs (pressure transmitters) on the same card that are reading a bit erratic. They bounce around by about 200 kpa (29 psi). Through troubleshooting by others it was determined it is caused by ground loop and noise on the shield.

Here's what I've done to no avail...
- Checked the DC input voltages to the panel. They appear good at around 26 vdc (24 vdc UPS).
- Isolated all the shields and metered to the ground in the panel. I found about 20 that showed continuity to ground. I fixed over half of them, and have left the ones I can't fix yet unhooked to a ground terminal. Neither of the two transmitters had an issue on their shield.
- Checking the display on the transmitter and it appears to be solid.
- Checking the current input at the PLC terminal strip, they are both steady.
- I followed out the conduit and there is no junction box in between, so they run straight back to the cabinet.
- If you leave the shield unhooked it fluctuates quite a bit. If you connect it to ground it gets a lot worse.
- There is no current (ac or dc) going through the shield that I could measure (Fluke 87)
- There is no voltage (ac or dc) on the shield that I could measure (Fluke 87).
- The PLC Power Supply, backplate, power cable and Analog input card was all replaced while troubleshooting another issue on site.

Today I was going to verify that I haven't lost the negative to the card and see if I can verify that the voltages are correct. Other than that I'm running out of what to check. Any ideas??

They are saying that it started occurring after I removed some old wiring in the same panel. None of the wires were in service (fuses all pulled, negatives were connected to the negative strip, AI wires were connected to an empty terminal). Looking at the alarm history there has been lots of analogs coming in and out of alarm with no rhyme or reason as to why... By lots I mean 2 pages worth typically every other day..


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## emtnut (Mar 1, 2015)

glen1971 said:


> Any ideas??
> 
> They are saying that it started occurring after I removed some old wiring in the same panel. None of the wires were in service (fuses all pulled, negatives were connected to the negative strip, AI wires were connected to an empty terminal). Looking at the alarm history there has been *lots of analogs coming in and out of alarm with no rhyme or reason* as to why... By lots I mean 2 pages worth typically every other day..


I'd start with the ground for the PLC panel itself back to the source.
And bonding within the panel itself.


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## cuba_pete (Dec 8, 2011)

When you say "it" gets a lot worse, I am assuming you are talking about the signal fluctuations and not the noise on the shield.

Depending on the signal frequencies the bonding of the shield will require different methods. Low frequencies below 30 kHz will generally require bonding at the source end only. Frequencies between 30 and 300 kHz can benefit from source or both source and terminal end bonding, but is dependent on the length of the conductors (sympathetic or mutual interference from standing waves). You may also require multiple point bonding, but that is extreme and sounds like an unlikely cure in your case (since wiring removal started the issue).

Leaving the shield floating is not an option with a reliable reference. In that case you are just playing with the antenna you have created and balancing the signal to noise ratio at an acceptable level. This will likely change over time and circumstance.

Also, since you apparently have a "known" cause in the modification and removal of wiring (someone touched it) you may as well start out fresh with a known method and make it right.

As emtnut stated, making sure your EGC is correct goes a long way, too. The reference for the equipment is likely the same once you get into the control circuitry for the equipment (i.e., they are all using the same ground reference). If your shield is not using the same ground reference, then the noise level on the shield could be elevated and then capacitively coupled to the conductors it is supposed to be "protecting". Everything should be using the same reference...bonded at some point.

A lot of the methods used in communications facilities parlay well in the PLC world since that is just another type of communication, albeit internal.


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## glen1971 (Oct 10, 2012)

Thanks for the replies! At least I've got a couple more ideas to check..

Cuba - Sorry 'bout that but "a lot" was referring to the signal fluctuations on the HMI.. 

I did have a bit of another thought that I was gonna try, if I can swing it.. The terminals at the PLC are the original ones from 1978 and are the SAK version with the black pull type disconnect with the pin. The pin looks to be silver/galvanized color.. I was wondering about a loose connection at that point and I was going to try and replace them with either a SAKR (disconnect) or a SAK4 terminal. Just another idea that came up in my sleep...


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## cuba_pete (Dec 8, 2011)

Yeah, well according to Weidmüller, their SAK's are _"synonymous with reliable connections..."_

But you have found otherwise...

Good connections are a huge part of proper shielding and interference reduction.:thumbsup:
.


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## glen1971 (Oct 10, 2012)

It's worth a shot.. A smoking gun would be nice to find... Rather than just grasping at straws and fixing shields that have been grounded on both ends for 38 years and working fine.. lol


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## emtnut (Mar 1, 2015)

glen1971 said:


> They bounce around by about 200 kpa (29 psi).
> 
> None of the wires were in service (fuses all pulled, *negatives were connected to the negative strip*, AI wires were connected to an empty terminal). Looking at the alarm history there has been lots of analogs coming in and out of alarm with no rhyme or reason as to why... By lots I mean 2 pages worth typically every other day..


Hmmm ... were any of those ground wires ? You may want to run a temp #6 back to the panel (if it's do-able) to see if that helps.

200kpa .... what % of full scale ?


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## glen1971 (Oct 10, 2012)

emtnut said:


> Hmmm ... were any of those ground wires ? You may want to run a temp #6 back to the panel (if it's do-able) to see if that helps.
> 
> 200kpa .... what % of full scale ?


The negative strip had no ground wires on it. It is fed from another DC Distribution Panel, a panel ahead of that and then the UPS. The panel is tied to other panels with rigid conduit and double locknuts. It is tied to ground with a #2 to the main grid. All grounds in the panel to the shield terminals are #14 to a common ground bar, with the #2 connected.
If I remember right one of the TX's is 0-5,000 kpa and the other 0-10,000 kpa...

Today:
- Measured voltage between negative and ground throughout and got 0.02 vdc.
- Measured voltage from card negatives to + and got around 26 vdc
- Measured voltage from card negative to ground and got 0.6 vdc on all negatives.
- Used a mA clamp on meter and watched the current at the terminal on the AI card and it only fluctuated 0.01 mA, while the HMI was jumping 200 kpa.
- Measured continuity between the panels and ground connection and it all looks good.
- Measured ground current at the terminal block for the shields both connected and isolated with no noticeable increase in current.
- Measured voltage difference across both + and signal on each transmitter and got 0 vdc.

Next step I'm guessing will be to throw a scope on it. And maybe try to add a set of batteries on the 24 vdc System. It is supplied from a 120 vac UPS circuit and hasn't been an issue since it was installed in 2010.


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## emtnut (Mar 1, 2015)

glen1971 said:


> The negative strip had no ground wires on it. It is fed from another DC Distribution Panel, a panel ahead of that and then the UPS. The panel is tied to other panels with rigid conduit and double locknuts. It is tied to ground with a #2 to the main grid. All grounds in the panel to the shield terminals are #14 to a common ground bar, with the #2 connected.
> If I remember right one of the TX's is 0-5,000 kpa and the other 0-10,000 kpa...
> 
> Today:
> ...


If you have a 4-20mA source, that would be good to try (assuming current loop)

Also, thinking about the old wiring removed ... If you have unused AI's, they need to be shorted out... that can cause noise in the AI card itself.

In the PLC program, I'd make sure that AI filtering is set the same for all the AIs as well.

Keep in mind .... 0-10K kpa , 200kpa fluctuation is 2%
That roughly translates to 0.3mA fluctuation !


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## glen1971 (Oct 10, 2012)

emtnut said:


> If you have a 4-20mA source, that would be good to try (assuming current loop)
> 
> Also, thinking about the old wiring removed ... If you have unused AI's, they need to be shorted out... that can cause noise in the AI card itself.
> 
> ...


Thanks for the help!
I've sourced to both of the channels that are giving the most grief and they were fine. Measured current with a clamp on at the card and it only fluctuated by about 0.01 ma. Meter in series showed the same thing.

When I took the old wiring out, the fuses were pulled alread. The wiring to the AI card had been relocated to another terminal strip, so there were no wires on the other side of what I was taking out..


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## joelowrider (Jul 2, 2011)

Do you have a calibrator so you could source the 4-20. If you do drive the input with a calibrator to verify that the card is not the issue.

Also what is the scale (4-20 = 0 to ?)


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## splatz (May 23, 2015)

A trick that sometimes works with oddball RF on phone lines is to ground all the unused pairs. When you removed the old wiring, did you remove the entire length of it, or just get it out of the panel? 

Did you try the temporary ground wire?


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## glen1971 (Oct 10, 2012)

joelowrider said:


> Do you have a calibrator so you could source the 4-20. If you do drive the input with a calibrator to verify that the card is not the issue.
> 
> Also what is the scale (4-20 = 0 to ?)


Read above.. Fluke 744 was used...
0 to 5,000 kpa and 0 to 10,000 kpa


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## glen1971 (Oct 10, 2012)

splatz said:


> A trick that sometimes works with oddball RF on phone lines is to ground all the unused pairs. When you removed the old wiring, did you remove the entire length of it, or just get it out of the panel?
> 
> Did you try the temporary ground wire?


I unhooked all the pairs from that conduit, and taped them as a bundle and tyrapped them off to one side. I'm not sure if there are any spare pairs in the conduit with the pairs that is giving me grief now...
No I didn't do the temporary ground wire yet... I won't be back down on that site til next week, so I'll do some thinking and see if I can think of anything that I've missed...
Thanks everyone for the help!!


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## jmoon (Dec 5, 2013)

I've ran into a similar issue before with a long run to a level transmitter that would read goofy intermittently. 

Is your signal cable by any chance running in a cable tray / in the same conduit as any of your power cables (motor leads, welding disconnects, 480 lighting etc)? Is your signal cable shielded? Induced voltage from a nearby run to a 480v motor was causing unwanted noise in my case.

Good luck!


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## glen1971 (Oct 10, 2012)

jmoon said:


> I've ran into a similar issue before with a long run to a level transmitter that would read goofy intermittently.
> 
> Is your signal cable by any chance running in a cable tray / in the same conduit as any of your power cables (motor leads, welding disconnects, 480 lighting etc)? Is your signal cable shielded? Induced voltage from a nearby run to a 480v motor was causing unwanted noise in my case.
> 
> Good luck!


It is in a dedicated conduit with only analog signals in it.. All wiring is 1 Pair 16 shielded.. 85% of the wiring is in rigid aluminum conduit and was initially installed in 1977..


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## DriveGuru (Jul 29, 2012)

Potentially ignorant question but are you sure the shield is only grounded at the Plc and not anywhere else? Also the plc's ground is solid?


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## MWayne (Nov 8, 2010)

glen1971 said:


> It is in a dedicated conduit with only analog signals in it.. All wiring is 1 Pair 16 shielded.. 85% of the wiring is in rigid aluminum conduit and was initially installed in 1977..


We have had problems with noise getting through aluminum conduit to our 4-20mA signals before, usually temporary because of inrush from starting a large motor. Just a thought.


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## glen1971 (Oct 10, 2012)

DriveGuru said:


> Potentially ignorant question but are you sure the shield is only grounded at the Plc and not anywhere else? Also the plc's ground is solid?


Not an ignorant question at all.. A separate ground bar was added to a painted backplate, so I thought there could have been an issue there. So I unhooked all the shields from it and metered it to the existing ground bar, and it was at 0.1 ohms... There is a 2/0 lugged onto the original ground bar that goes out of the cabinet and ties onto the main ground grid for the site, which ties into structural steel and a rod within 20' of the lug in the PLC..


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## glen1971 (Oct 10, 2012)

MWayne said:


> We have had problems with noise getting through aluminum conduit to our 4-20mA signals before, usually temporary because of inrush from starting a large motor. Just a thought.


This seems to be happening all the time.. Whether the station is down, idling or online.. The largest electric motor on site would be a 30 hp, or 4 - 25 hp (as they are started together)... There are several VFD's (ALB 700's) on site..


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