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4- 20 mA Transmitter Signal Greater Than 20 mA

10/11/2018 11:39 AM

Helo,

I am seeking for some clarification, when we have a short circuit in a cable connecting the sensor to the controller or a short circuit at the sensor input due to moisture or oxidation, etc, what will be the expected output or reading at the controller? I once experienced a scenario when a cable from an RTD had a short circuit and the display on the controller was way far from the expected reading. what i don't recall is whether it was a positive value or a negative value e.g -3000 C (too cold) or +3000 C (too hot) but the value was in thousands. Also under what practical circumstances do we have a > 20mA reading other than a short or open circuit in the transmitter on board electronics and or sensor saturation. Does this apply to all other transmitters like loadcells, position or displacement, pressure, level etc? that output 4-20mA

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#1

Re: 4- 20 mA Transmitter Signal Greater Than 20 mA

10/11/2018 1:14 PM

Here's some troubleshooting tips that might help...

http://www.devarinc.com/pdf/Troubleshooting%20a%204-20mA%20loop.pdf

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#2

Re: 4- 20 mA Transmitter Signal Greater Than 20 mA

10/11/2018 5:33 PM

A 4-20mA output from a transmitter will always be in this range unless either the loop resistance it is connected into is too high to sustain the top end, or there is a fault of some kind in the transmitter.

The reason for a "live zero" at 4mA is simply to enable fault detection, the most common of which would be a transmission loop going open-circuit for whatever reason. However, an inadvertent short circuit in the loop part-way between transmitter and receiver will have the same effect in the receiver as any other reading of less than 4mA.

<...-3000 C (too cold) or +3000 C (too hot)...>

Such indications are clearly nonsense, as

  1. the Celsius scale does not go below -273.15, and
  2. most common materials are either molten, vapour or the products of combustion at the upper figure.

Although humans know this from everyday experience and from training, the equipment can detect and take action very quickly if nonsensical readings are received, provided it has been correctly programmed by those humans.

<...what will be the expected output or reading at the controller? ...>

The receiving equipment will be programmed for a defined input scale to match the range of the sending scale on the transmitter. It is commonplace these days to program the analog inputs of the receiving equipment to make it display an out-of-range fault alarm of some sort when the current received is either less than 4mA or greater than 20mA by a little amount, and to program further to invalidate the received signal within the receiving equipment so as to mitigate the effects of this loop fault. Examples:

  • Say the range of the 4-20mA were 0-100 units. An input of 22mA could give "100 and over-range fault" in the receiving equipment, rendering the 100 invalid, and an input of 3mA could give "0 and under-range fault" rendering the 0 invalid using this set-up.
  • An invalid signal can also be programmed to freeze the last valid reading as the display or the control measurement for other functions within the receiving equipment, determination of which depends entirely upon the application.
  • Further, the inputs of several sensors can be compared in "two-out-of-three" or even "three-out-of four" voting scenarios in uncommon and extremely critical cases where the failure of one measurement loop must not affect the operation of the process while an alarm is raised for attendance to fix the fault.

<...Does this apply to all other transmitters...?...> Each application is different, however the principles outlined above will suffice for most applications.

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#3

Re: 4- 20 mA Transmitter Signal Greater Than 20 mA

10/11/2018 9:17 PM

https://www.predig.com/indicatorpage/back-basics-fundamentals-4-20-ma-current-loops

A 4-20ma current loop generally consists of a sensor, transmitter, power source, loop, and receiver. The transmitter at the sensor end of the loop uses the sensor output to control the amount of current in the loop, converting the extreme values of sensor output to the range of 4-20 ma. The Receiver measures this current and converts it into units that correspond to the sensor measurement. So the Sensor, Transmitter, and Receiver are matched together to give the correct reading.

If there is a short circuit at the sensor end, the transmitter loses control and the current will be higher and will depend on the loop resistance and power source voltage. A broken loop or supply failure will result in less than 4 ma. The Receiver may or may not be designed to detect these fault conditions.

If the Receiver does not detect fault conditions, the reading on the Receiver end will depend on how the transmitter encodes the sensor output to current control. The indication at the receiver will be beyond the encoded extreme value of the 20 ma loop current.

https://www.electronicdesign.com/power/detect-4-20-ma-loop-faults

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#4

Re: 4- 20 mA Transmitter Signal Greater Than 20 mA

10/12/2018 10:22 AM

Thanks gentlemen, This has been helpful to me. Regards.

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#5

Re: 4- 20 mA Transmitter Signal Greater Than 20 mA

10/12/2018 4:30 PM

Temperature transmitters always have an external sensor and always have a sensor fault condition, which is either upscale offscale or downscale offscale.
The European NAMUR association has a published standard, NAMU 43, for mA fault levels (see below).


Most industrial pressure and temperature transmitters allow one to select either manufacturer or NAMUR fault levels. The table below shows manufacturer fault levels for some pressure transmitters.


It takes some configuration, as others have pointed out, to use the fault levels on the receiving end (PLC/DCS/HMI) to annunciate a fault. It's my anecdotal impression that most people/plants just live with a very high or very low value.
A mA low fault condition infers that there is NOT a wiring issue, no cut cable/conductor/wire (a zero mA condition). The circuit has to be complete to signal the low mA condition.
Short circuits are another story - short between what and what else? Shorting the transmitter out of the loop where 24Vdc power is directly across the load resistance (no current regulation by the transmitter) is more than likely to burn out the load resistance.

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#6

Re: 4- 20 mA Transmitter Signal Greater Than 20 mA

10/14/2018 1:25 PM

Thanks Carl E.

The shared values are to help me decide the upper and lower values. Am planing to configure the limits in the PLC code using a comparator that shall compare both the upper and lower limits with the predefined value then it can flag a sensor failure to the HMI after a preset time delay.

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