# Maybe someone will learn from this...



## MechanicalDVR (Dec 29, 2007)

Thanks!

That was a good read and a learning experience for sure.


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## micromind (Aug 11, 2007)

I've never had any trouble with 4-20 next to 480 either. This is a new one for me too. 

I wonder if a capacitor on the 4-20 would help?


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## emtnut (Mar 1, 2015)

Should be using Bristol pressure Tx's :biggrin:

OK, that was 25 yrs ago, they are probably made in asia now too !


Unrelated, but still on the 'i've never seen this before'
We had a Modicon RS-485 loop in a plant that was operational 95% of the time. Every now and then it would go down.
I sent every electrician and instrumentation guy out, but they never found anything wrong.
Finally, one day it died. Wouldn't come back online.
We found that the 6 pair Cat5, had 1 pair connected, the other was not connected. It was connected to the 3rd pair. The system had been communicating almost flawlessly by inducing the comms onto the unused pair.:surprise:

Back to you OP ... I guess the CMMR in the new transmitter is not as good a the original, eh :wink:

Are Bristol Tx's still the cats Azz ? :biggrin:


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## MDShunk (Jan 7, 2007)

I've use currently, and have used in the past, 4-20 instruments from every manufacturer under the sun and have never experienced what you posted about. Very often even in the same cable tray as a lot of 480. Strange. I always considered 4-20 devices immune to almost everything you can throw at it, thus their popularity.


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## splatz (May 23, 2015)

MDShunk said:


> I've use currently, and have used in the past, 4-20 instruments from every manufacturer under the sun and have never experienced what you posted about. Very often even in the same cable tray as a lot of 480. Strange. I always considered 4-20 devices immune to almost everything you can throw at it, thus their popularity.


That's what I was thinking, one of the selling points of the 4-20ma sensors is supposed to be immunity to this kind of thing. 

Early thought was that the shield was acting as an antenna and introducing problems - but if the shield was totally disconnected when you did your tests with THHN on the ground, that's out. 

Just out of curiosity, I'd see if it makes a difference if you put that thhn directly over the conduit, but twist it up kind of like bell wire. 

Also just out of curiosity I'd test the current on the 4-20 loop with an analog meter at various points, and I'd look for AC current on the loop as well as DC. 

I rig up two 9V batteries in series and a few resistors with an analog meter to test 4-20. 

> - 9V + > - 9V + > resistor > + meter lead > analog meter > - meter lead > wire 1 > sensor > wire 2 > back to -9V

I'd test the loop isolated from the power source to see if something's happening on the loop itself with this sensor. 

I am not exactly sure what I'd be expecting to see or what the next step would be, just casting around at this point.


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## oliquir (Jan 13, 2011)

ive seen that with prox switch that were not working right because cabling was run in parallel with vfd cables (no other design choices). we had to change model of prox for a better one made for electrical noisy environment.


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## MDShunk (Jan 7, 2007)

This is really bugging me, because I can't really think of how it's possible. What's the part number of the pressure transmitter you're using? I want to order one and try to duplicate this. Maybe on a scope. Is it 2-wire or 4-wire? Where's the power supply for the loop? I've seen situations where transmitters would do funny things when two power sources were introduced in the system. Like a flowmeter output that makes its own loop power has to be set to "passive" because the PLC analog input card already is powering the loop (for instance).


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## just the cowboy (Sep 4, 2013)

Cow
I have not seen it on 4-20 either but you just proved it can happen.

Since you work with 4-20 loops I have a 4-20 simulator / meter I am looking to give away to someone that will use it. PM me if you want it for free, I have not used it for years hope I took the batteries out.

Cowboy


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## Rora (Jan 31, 2017)

Assuming you're changing the span for the feedback, you'd expect the gain to decrease with 0-200 psi vs. the previous 0-232 psi, yielding better resistance to noise.

The fact that both of the older transducers worked leads me to think the new ones use different circuitry with weaker current regulation.


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## JRaef (Mar 23, 2009)

Most likely, your old transducer was isolated, your new one is not, which is the trend, because they WANT you to now order a separate isolator module or spend a lot more for the isolated version. What that means is that the old isolated transducers were running with no reference to ground, they were “floating” so although the signals may have picked up radiated noise, it was the same on both sides of the signal and the transducer was simply giving you the differential. In the newer unisolated versions, one side is grounded so any radiated or common mode noise from the VFD is changing that differential value because it is only affecting one side. 

For the European mfrs like Schneider, often the only way they indicate this is via symbols on the packaging or data sheets; no words, because words must be translated and that costs money. Unfortunately for us in North America, we don’t experience those symbols enough to know what they mean, we rely on words, like “isolated” or “not isolated”. Siemens did the same thing years ago When I worked there, it caused a lot of headaches and they spent a bunch of money trying to educate people on what these international pictographs meant, but who has time for that? Siemens eventually gave up and just sold off their entire line of sensors to Pepperel + Fuchs.


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## just the cowboy (Sep 4, 2013)

*No posting when on vacation*



JRaef said:


> Most likely, your old transducer was isolated,.


STOP no posting from Tahiti, your on vacation:smile:


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## paulengr (Oct 8, 2017)

I’ve seen this before but it’s not usually the transmitter or shielding, it’s the receiver, especially Allen Bradley.

First things first. There should be a dedicated cable between the VFD ground and the motor frame, preferably with no splices, wire nuts, etc. if you don’t the drive will usually work but spew common mode noise everywhere.

Second, the cable shield if any should be grounded at ONE end. If you ground both ends you create a loop between the shield and ground which is an antenna.

Third, if it’s your typical DIN mount brick for an excitation source and you’re using 2 or 3 wire, ground the negative side at the power supply.

Moving on to the usual problem, an ideal 4-20 mA receiver is magnetically or better yet optically isolated both from itself and all other inputs, and there is enough of a “blanking” interval to ignore other input problems. That’s not always the case, particularly with almost everything sold by Allen Bradley but others are just as cheap on the inputs. Make sure you ground ALL unused inputs. If you have say 4 inputs and only 2 transmitters, put them on every other input and ground the unused channels.

Now the next part you have to watch how your power distribution is done but put a meter between returns and/or returns and common/ground at the drive measuring AC. If it’s not almost zero sometimes and you need to be aware of your power sources, short the return sides between channels so you remove channel to channel common mode interference. Also with the transmitter disconnected measure the resistance across the receiver(s). Now hook it back up and measure the voltage, AC and DC. Knowing ohms law (V=IR) you can easily convert the 4-20 mA signal yourself. That’s Ohms law...the thing nobody ever uses on another thread. This is how I troubleshoot 4-20 mA problems. Between that and a $20 box that runs on a battery and gives me 4, 12, and 20 mA of simulated current so I can isolate wiring issues from the transmitter. You can buy the ridiculously overpriced Fluke clamp on process meter but it doesn’t give you AC readings and since 99% of the receivers out there use either a 250 or 500 ohm precision resistor, it doesn’t really serve any useful diagnostic purpose.

Once in a while it turns out the power supply (excitation) is poor or broken so you get rectified AC out instead of DC so check voltage at the source terminals too (AC and DC).

Sometimes you can block some or most of the AC with ferrite donuts wrapped around the lines. That’s the IEC solution but it’s limited.

If it’s really bad you can get external optical isolators which is an LED cemented to a phototransistor so you have 100% isolation.

As I said...99% of the time loop problems like this are simply crappy receivers or basic wiring issues. Instrument wiring is NOT like power wiring. The rules are different.


Sent from my iPhone using Tapatalk


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## JRaef (Mar 23, 2009)

just the cowboy said:


> STOP no posting from Tahiti, your on vacation:smile:


The wifey is reading a book, I didn’t bring one. But God help me if she finds out...


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