# noisy 4-20mA inputs



## ibew415 (Mar 23, 2014)

In addition, I put my mA clamp on the input and found it to be pretty stable.

The devices make up in 4sq metal boxes, could EMF be coming through the open mounting holes?

OR, could the 4-20 input being series'd through 3 different SCADA systems be an issue? I have been told the other 2 systems are not erratic. only the new "Mission Control" system. Mission being the brand name.


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## Cow (Jan 16, 2008)

Can you look at the program and see if they have an analog filter on the 4-20mA input? They need to have something or else you will see the reading jumping all around.

Have you verified the drain wire/shield is only grounded at one point? Usually the PLC cabinet.


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## drsparky (Nov 13, 2008)

Cow said:


> Can you look at the program and see if they have an analog filter on the 4-20mA input? They need to have something or else you will see the reading jumping all around.
> 
> Have you verified the drain wire/shield is only grounded at one point? Usually the PLC cabinet.


That would be were I would look.


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## ibew415 (Mar 23, 2014)

Cow said:


> Can you look at the program and see if they have an analog filter on the 4-20mA input? They need to have something or else you will see the reading jumping all around.
> 
> Have you verified the drain wire/shield is only grounded at one point? Usually the PLC cabinet.



Its a new system to me....its not a PLC. 

http://www.123mc.com/m110.asp

Each unit can accept 2 analog inputs. I'm not sure about the filter. Would the filter be an aftermarket device, or in the program?

the first thing i checked was the sheilding, and it is only terminated on one end.


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## glen1971 (Oct 10, 2012)

When you checked your shields, did you unhook them and meter to ground? They should be open... 
Is your controller ground sized adequately? Is it connected as close to possible to your rod(s)? Or did they tag off somewhere that may be adding to the issue?
Is your VFD properly bonded?
Does your VFD speed signals have isolators on them? It may be worth installing them.
Are your power conductors routed for a long distance adjacent to your analog wires? Are they in the same wireway, conduit system or cabinet (other than where required)?

Just a couple of things I'd look at...


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## ibew415 (Mar 23, 2014)

I think I will have a look at the controller ground. 

I ran a separate conduit between the PLC and the Mission controllers. The only analog in the Mission system is input. There are no VFD speed output cables. I did find a nice isolator, I'm not sure if it will solve all my problems and end world hunger. Any suggestions on brand? 

https://www.asi-ez.com/member/~ASI4...jSvwncVtTskE-1FATDxwzisGm-TZbKMWQ_BoColzw_wcB


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## glen1971 (Oct 10, 2012)

I mainly use Weidmuller ones and haven't had any issues with them...

http://canada.newark.com/weidmuller/9908800000/output-isolator-18vdc-to-30vdc/dp/11B1489

Not sure if others have better ones.. I haven't worked with the ones you have the link for.. The 2 channels in one unit might work better for you application...


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## Rollie73 (Sep 19, 2010)

I'm not familiar with that particular unit but what does it have for a power supply?

I ask because I've had some issues with low voltage ungrounded power supplies causing very erratic readings.


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## ibew415 (Mar 23, 2014)

It has a 120volt to 12vac power supply in a separate cabinet. The neutral of the secondary side is not bonded to ground. However, the frame of the power supply (transformer) is grounded. 

The analog loop power comes from the PLC as MY system only reads the analog signal. The 2 other scada systems seem to have normal readings.




Rollie73 said:


> I'm not familiar with that particular unit but what does it have for a power supply?
> 
> I ask because I've had some issues with low voltage ungrounded power supplies causing very erratic readings.


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## pjholguin (May 16, 2014)

Are both of your signal channels erratic? Do you have the means to put a meter(with recording capabilities) on one of the signals to verify the signal integrity? This way you can determine if it's field wiring or the new system.


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## JRaef (Mar 23, 2009)

A common issue I see with VFD installations is that people don't read the instructions saying that the output wires MUST be EITHER in grounded steel conduit, or shielded VFD output cable, with BOTH ends of that shield grounded. Without that, the output of a VFD is a relatively powerful local FM radio transmitter that has a tendency to bleed all over anything without the most robust of _metallic _enclosures or shielding. people like to use PVC conduit in pump stations to avoid corrosion, but don't realize that not using shielded cable lets the RF out.

I recently had that sort of issue with some IFM programmable pressure transducers like the ones in the picture below. Although they looked like they were all metal enclosed, it apparently wasn't shielded very well around the keypad membranes and the transducers picked up a LOT of RF when the VFDs were running. I determined that was it because if I covered them with foil, the signal stabilized (but of course, you couldn't see them any more. They had run shielded VFD cable, but only grounded one end.


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## Magoo5150 (Mar 1, 2007)

JRaef said:


> A common issue I see with VFD installations is that people don't read the instructions saying that the output wires MUST be EITHER in grounded steel conduit, or shielded VFD output cable, with BOTH ends of that shield grounded. Without that, the output of a VFD is a relatively powerful local FM radio transmitter that has a tendency to bleed all over anything without the most robust of _metallic _enclosures or shielding. people like to use PVC conduit in pump stations to avoid corrosion, but don't realize that not using shielded cable lets the RF out.
> 
> I recently had that sort of issue with some IFM programmable pressure transducers like the ones in the picture below. Although they looked like they were all metal enclosed, it apparently wasn't shielded very well around the keypad membranes and the transducers picked up a LOT of RF when the VFDs were running. I determined that was it because if I covered them with foil, the signal stabilized (but of course, you couldn't see them any more. They had run shielded VFD cable, but only grounded one end.


On many installations I have found steel conduit is not adequate. Now I always run high quality VFD cable regardless.


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## pjholguin (May 16, 2014)

How about the enclosure of the Mission Control Systems is it properly shielded? I have seen some system PCBs with separate shields placed over the analog circuitry.


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## Jlarson (Jun 28, 2009)

Gotta love a good cluster of control equipment, no water system is complete without. You almost always have to end up putting in isolators and/or splitters in those situations where you have multiple controllers and separate power supply.


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## pjholguin (May 16, 2014)

Jlarson said:


> Gotta love a good cluster of control equipment, no water system is complete without. You almost always have to end up putting in isolators and/or splitters in those situations where you have multiple controllers and separate power supply.


Been there and experienced that!


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## ibew415 (Mar 23, 2014)

pjholguin said:


> Are both of your signal channels erratic? Do you have the means to put a meter(with recording capabilities) on one of the signals to verify the signal integrity? This way you can determine if it's field wiring or the new system.


I do not had a recording meter like a fluke 289. But, i did monitor the signal with a mA clamp for about an hour and found nothing erratic. I wish I could watch it over the course of a few hours or a day.


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## ibew415 (Mar 23, 2014)

pjholguin said:


> How about the enclosure of the Mission Control Systems is it properly shielded? I have seen some system PCBs with separate shields placed over the analog circuitry.


They seem to be in a nice aluminum 12x12x4 with a metal cover


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## ibew415 (Mar 23, 2014)

Jlarson said:


> Gotta love a good cluster of control equipment, no water system is complete without. You almost always have to end up putting in isolators and/or splitters in those situations where you have multiple controllers and separate power supply.


I'm thinking an isolator may be the route we go. I haven't been back to the site for a while to look into how everything is grounded yet.


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## Jlarson (Jun 28, 2009)

Absolute Process Instruments makes a lot of great isolators and splitters.


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## oliquir (Jan 13, 2011)

had similar issue, had to put a separate power supply for sensors and it had to be old linear type (with transformer, no switching ones) and no more issue


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## Glock23gp (Mar 10, 2014)

I didn't see your repose to th question if your shield is ONLY grounded at the source and that it's taped off or isolated at the other end creating a loop?


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## Fordge48 (Apr 22, 2015)

If you are using a Class 2 24vdc power supply try tying your shield to the 24Vdc negative. If you have a bad ground at the system panel the shield may not be working. Class 2 power supplies will arrest any noise on a shield within its circuits without affecting the signal.


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## ibew415 (Mar 23, 2014)

I installed a 2 channel loop powered analog isolator, and there seems to be a voltage drop across the filter. So the tank level went from 89% (before the filter install) to 60% tank level after the install. After discussion with the SCADA contractor, we are going to give our new SCADA system dedicated transducers, with their own power supply. Our customer does not want to stop using his 2 other existing SCADA systems until he trusts the new one.


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## ibew415 (Mar 23, 2014)

https://www.dropbox.com/s/a6tz1pc1n5955va/2015-06-26%2008.47.14.jpg?dl=0

I was programming a Delta VFD and I saw this home made analog filter. I was wondering if anybody has used this set-up, and if so what size of capacitor do you install?


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