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Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/12/2014 3:25 PM

Hi all

I have to remove a two wire 4-20 mA loop powered transmitter. When I remove it, PLC logic causes a shut-down, which I am trying to avoid. Is there a resistor that I can connect in the place of the removed transmitter? I do not care about the measuring value, anything in the 4-20 range, to avoid the shut-down.

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#1

Re: Resistor in the place of a removed 4-20mA loop powered instrument

08/12/2014 3:48 PM

1. You may not "care about the measuring value", but apparently the PLC does. There is probably a good reason for the interlock - usually equipment or personnel SAFETY!

2. You must not have knowledge/access to the program, or else you would bypass the interlock in the PLC. But I'm guessing there is controls person there who does. Talk to them.

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#2

Re: Resistor in the place of a removed 4-20mA loop powered instrument

08/12/2014 3:51 PM

You could always bridge-out the logic, Boss. Indeed, if your logic were sufficiently robust, you would have a table of simulation values that you could flip using a set of Engineers' toggle bits already in the code. Failing that, insert a resistor, making the resistor before breaking the transmitter.

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#3
In reply to #2

Re: Resistor in the place of a removed 4-20mA loop powered instrument

08/12/2014 4:17 PM

You'll have to do your own arithmetic to work out the value, because I don't have your numbers.

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#4

Re: Resistor in the place of a removed 4-20mA loop powered instrument

08/12/2014 5:37 PM

If you say that you have to, then you have to; put a 1500 Ohm resistor in parallel to your old transmiter and then cut the transmiter out.

Make sure it's a safe move, you don't want anybody injuried.

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#5

Re: Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/12/2014 8:08 PM

You are asking how to fake a valid process variable in order to prevent the controller's control logic from properly going into a fault mode based on an invalid process variable. Controllers are supposed to fault when the process variable is invalid, or missing. That's one of the beauties of the 4-20mA current signal, with its 'live zero' where a 0.0mA signal means "TROUBLE, open circuit" which signals an alarm state to the controller that the input is invalid.Thankfully, adding neither a resistor, diode, transistor or capacitor is going to 'fake' a process signal (in the absence of a real process signal) so that the controller continues as normal. Only substituting another 4-20mA signal can fake a 4-20.

The concept of auto/manual modes, also called hand/auto, is that a control output signal (to a final control element, like a valve) can be manually manipulated. Most process controllers have an auto/manual switch/button to toggle between manual and auto modes or a panel might have a hand/off/auto switch to do the same thing.

On the other hand, sometimes a controller's auto/manual switch is deliberately disabled (or hidden) when a risk assessment determines that manual mode is not sufficiently low-risk.

The resistor you're thinking of is used when two (receiver) devices, like a controller and an indicator, are in series in the output circuit of a field transmitter. This is done because an indication of the process variable is needed for deciding how to manually control a process. In this situation, it is common to find the input shunt resistor placed across the case housing terminals (or removeable terminal plug) of the 4-20mA analog input (on both the controller and the indicator). That way, either device may be removed from its housing while leaving the 4-20mA loop wiring intact. With the resistors in place, the loop doesn't go 'open circuit' if one of the devices is removed; it remains a complete circuit so the remaining device continues to display the process variable.

But this is done with a working transmitter whose signal is a valid process variable, not to fake a process variable signal.Another control scheme allows the control logic to 'switch' analog values to the input of control loop, frequently with interlocks that need to be overridden or a time-out on the switched state, but stuff like that has be programmed in the control logic.

Edit - I just realized that I assumed that no one would short the field wiring of a removed transmitter, but there's always the guy who doesn't know. I'll post a 2nd post to address that.

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#11
In reply to #5

Re: Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/13/2014 3:35 PM

You are correct if the transmitter is a current "source" device however if the transmitter is a current "sink" device it can easily be bypassed by using the proper impedance to fool the PLC/DCS system as long as the impedance is within the I/O range of operation.

Not only do I agree with you on the liability and risk surrounding any technician defeating a process signal or other control permissive but I also have great concern because there are source devices that if open circuited will fail catastrophically.

Another bad situation would be if the transmitter is part of a "loop-tuned" circuit and the loss or change of signal caused the loop calibration to be off. (If the process is making medicine or controlling any explossive process the result could be fatal.)

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#6

Re: Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/12/2014 8:42 PM

Silly me, I'm thinking if the transmitter is removed, there's no current in the loop.

But, even if the transmitter is removed, the power supply is still there and likely still powered.

Someone will short the remaining wiring, putting the power supply's 24Vdc directly across the analog input shunt resistor, pumping ~ 95mA through the typical 1/8 or 1/4 watt 250 ohm resistor, dissipating about 2.3 watts, which will burn out the resistor.

If it's an external resistor, it's just a replacement, but many AI cards use internal dropping resistors, some even lower resistances (50 or 100 ohms) than a typical DCS 250 ohms which will smoke and provide that fragrant burnt smell we all know and love.

Putting 1500 ohms in parallel to a 250 ohm resistor brings the total resistance down to 214 ohms, but it's still a burn-out situation with the 250 drawing 95mA. Yahlasit is not suggesting putting 1500 ohms in parallel with the analog input, he's suggesting putting it in place of the field transmitter.

Putting 1500 ohms in series with the 250 ohms, drops 85% of the 24V across the 1500 ohms and limits the loop current to 13.7mA.

I concede, that's a fake input signal. So, yes, there is a resistor you can use to fake a signal because the power supply is still there and pumping electrons when the wires are shorted, but be warned that there's hazards in not knowing what the process really doing and that's the case with a fake input signal.

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#7

Re: Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/13/2014 12:09 AM

This is a common safety system in industrial equipment. The idea is to sense if a current loop has gone open circuit and take some kind of action if it has.

In your case you are not using the sender device any more so substituting a resistor is the logical solution. I'd use about a 470 Ohm.

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#8

Re: Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/13/2014 5:47 AM

Why do yo have to remove the transmitter?

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#9

Re: Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/13/2014 7:10 AM

I have to remove it for calibration. This is a dew-point instrument monitoring the water vapout content in a compressed air adsorption dryer outlet. There are no low-low or high-high limits in the logic, and the process time variables are not effected by the dew-point measured, so it is only for indication to the operator. Only in the case that instrument is missing, logic causes a shut down of the drying process.

I have connected a 1700 Ohm resistor, and the measuring value is about 40%. It works ok.

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#10
In reply to #9

Re: Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/13/2014 8:26 AM

One could alter the logic so that the absence of the signal causes only an alarm.

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#12

Re: Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/14/2014 12:36 AM

This is the advantage in PLC. The PLC will receive either '1' or '0' from the field instrument. Normally '1' means healthy and'0' means unhealthy(means no signal causes shutdown). so when you want to do something in the field instrument you can substitute the required/safest value in the PLC through your Engineering station and you can do anything in the field so that no shutdown will happen. PLC will not take any action as long as it gets the satisfied value.

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#13

Re: Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/28/2024 4:41 AM

The other thing one might do is to program a one-minute delay timer on any instrument failure so that shutdown only occurs after that timer has elapsed. That gives a period of one minute to disconnect the transmitter and to insert the resistor in the terminals, which seems reasonable. The process will then continue to operate with the resistor in place of the transmitter.

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#14

Re: Resistor in the Place of a Removed 4-20mA Loop Powered Instrument

08/28/2024 4:45 AM

A third possibility is to program a fake variable and toggle between the real and the fake using an engineering bit in the code. When the bit is set, the process toggles to the fake variable and continues, leaving ample time for the disconnection of the transmitter in question and the connection of its replacement.

The fake variable could be derived in software from the real variable, enabling bump-less transfer at the time of the toggle.

The toggled engineering bit ideally brings up an alarm so that its presence cannot be forgotten.

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