We have a pressure transmitter connected to a analog input card through a 3 way DC/DC isolator. HMI reading and local transmitter reading differs by 0.2mA only when plant running. During shutdown if you loop check it has no current drop. i replaced isolator and find the same result. I tried to by pass 3 way DC/DC isolator then I found no current drop. But other analog input loops having isolator has no issues even it has isolator. I don't know why only this loop has issues with isolator.Can anyone advice why current drops only here?

Note: transmitter, PLC analog input card, and isolator receive common 24 VDC POWER SUPPLY. 24 VDC power supply negative grounded.

 

I have experienced a similar situation before and found that the problem was due to 'too many' loads being connected in a 4-20mA loop (like Panel Indicator, Recorder, Logic Input Card and the DCS Input Card all connected together in series with the transmitter like what you have mentioned).

It is a norm that 2-3 loads are connected to a transmitter 4-20mA loop in existing electronic systems. And then when we try to take this signal to a newly DCS System (AI Card), then the AI Card (as it is an additional load) burdens this loop further to an extent which could cause lower mA.

If you can shed some light into how many loads connected in your 4-20mA loop and what is the source of power supply to your loop (is it the DCS Card or an external 24VDC) it could direct us to the root of this problem

What convinces me to think in this angle is because you have mentioned this doesn't happen in all loops (may be because other loops do not have many loads connected).

Additionally, you have simulated 4-20mA and found xmtr and DCS to show same reading (hence same mA) but i doubt if you performed this simulation by inadvertantly isolating the other loads in that loop in which case the problem became 'hidden' when you tried to diagnose.

 

2 wire, loop powered heavy duty industrial pressure transmitters are always isolated at the transmitter and are typically not the devices that need a loop isolator. 3 wire and 4 wire devices frequently need an isolator in the loop to break a ground loop.

However, a junction box with water or moisture inside can produce a ground loop from open terminals to ground.

Or the moisture in a junction box can create a relatively high resistance path in parallel with the transmitter which bleeds current.

PIE's 532 loop calibrator detects the loop current when it is used as a floating simulator source supplying the loop voltage, as shown on their single page app note, here:

http://www.piecal.com/products/technotes/532_TechNote_201.pdf

Is there a junction box between the transmitter and the isolator, or between the isolator and analog input?

> I tried to by pass 3 way DC/DC isolator then I found no current drop.

It sounds like bypassing the isolator while the plant was running produced no current difference, hence, solved the current difference problem.

If bypassing the isolator produced good results, why keep the isolator in the loop?

 

I'm with David, why do you need a signal isolator?

Check your loop with a simulator, does it differ by the same 1.25% all across the range? If it's only at the 20 mA end you might be running out of power supply Voltage.

I have seen where the instrument fuses were indicating type with an LED, if the fuse blows the loop still looks normal up to 50% then maxes out limited by the LED draw current.

 

Hi.

Thanks for your response.

A separate 24VDC 20A supply positive wire goes through a fuse to a PRESSURE txt positive and negative side of pressure txr connected to a positive of an isolator input side. Isolator input side (Weidmuller 8594810000) negative is connected to negative of 24Vdc power supply. Also isolator gets a separate power supply from the same 24VDC supply. Isolator output side positive goes to GE FANCU ALG 223 card input channel and negative of isolator output goes to GE FANUC ALG 223 card common negative.

Isolator has input side output side and power supply. Totally 6 terminal points. hope it helps to help me out.

 

Hi

thanks for your response.

mA drop observed only during plant running. when plant shut down then if I loop check, I can see no drop in mA. we replaced a new isolator, new pressure transmitter, still mA drops when plant running. WE have two VFD driven Motors, but all the signal cable trays and power cables trays are separate.

if no isolator, then no current drops, then V could remove isolator but I'm curious why the same setup works for other loops and why not only this loop.

Kindly reply how to avoid this.

 

Hi thanks for your response.

When plant shutdown, I did a loop check and found no drop in mA amps.
When plant running, the loop was reading around 5.625% in pressure txr and 4.375% in HMI so difference is 1.25%.

Kindly reply if you get any ideas.

 

Did you compare the transmitter local display 5.625% with the mA output Should be 4.35 mA. I'm wondering if when running the power supply Voltage is less and not enough to drive the signal properly.
Another thought, do you have a ground somewhere in the field so you get a DC offset (zero shift).

 

I'm wondering if the earth grounded negative of the power supply creates a ground loop with the (-) side of the AI card due to a change in ground potential when plant is running, effectively creating an offset in spite of the loop isolator.

If the power supply earth ground connection to the power supply (-) is temporarily lifted, does the offset disappear?

 

David and I came to the same conclusion, next time you get a shutdown lift the ground point off the power supply negative and it should be isolated from ground, or at least 1,000 Ohms from ground.

Also check the shield wiring is not grounded in the field by lifting the ground of those, again 1,000 or more Ohms to ground.