Understanding the Instrument Transmitter TEST Connection.pptx

PPT presentation explaining the operation of the TEST connection in a typical 4-20mA loop instrumentation transmitter. Discusses methods and options for measuring loop current and the mistakes and advantages of using the TEST connection method vs serial ammeter method. Also discusses some troubleshooting tips for instrument transmitters and 4-20mA instrument transmitter loops.
Helpful for engineers and instrument technicians.

1. Understanding the Instrument Transmitter TEST Connection Short Explanation by Mike Glass Orion Technical Solutions, LLC

2. Most industrial instrumentation 4-20mA transmitters include a TEST connection terminal along with the (+) and (-) terminals. What is this connection for, and how does it work? Note: The connection diagrams and detailed functionality vary in different equipment. Always consult your specific equipment references to apply these concepts.

3. The TEST connection is intended to provide a way of measuring 4-20mA LOOP CURRENT without disconnecting the signal leads. This is done for several reasons: Minimizes wear and tear on terminal connections. Minimizes chances of shorting out the 24v power supply and blowing fuses. 3) Avoids causing a power cycle (temporary shutoff of transmitter) when disconnecting the lead to insert an inline ammeter.

4. The (TEST) terminal: The transmitter has an extra terminal designated as (T) or (TEST). The TEST terminal is not connected to the normal loop, but is used only for testing. Let’s see how it works.

5. There is a DIODE inside the transmitter placed in the 4-20mA current loop path between the (-) terminal and the (TEST) terminal. Note – the variable resistor represents the transmitter internal circuitry regulating the 4-20mA current flow

6. There is a DIODE inside the transmitter placed in the 4-20mA current loop path between the (-) terminal and the (TEST) terminal. Note: The variable resistor represents the transmitter internal circuitry regulating the 4-20mA current flow

7. Equivalent circuit diagram for a typical 4-20mA transmitter loop hookup with TEST connection: Transmitter Connections Full Loop Hookup

8. This is one way to measure loop current: We insert the ammeter in series (typically between the (-) terminal and field wiring, to minimize chances of a short that blows the loop fuse).

9. But, to insert the ammeter, you have to temporarily disconnect the field wiring on one leg of the transmitter. When you break the loop path, the transmitter will power down. This causes a power cycle, that could cause problems.

10. Using the TEST connection: When the ammeter is connected across the diode, the flow simply goes through the ammeter instead of the diode, because the low impedance of the ammeter results in a very low voltage drop across the diode, which causes it so lose forward bias (or shut off).

11. Best of all, the transmitter loop current never changes – So there is NO power off cycle, etc.. *Powering off a transmitter resets many (most) of the diagnostic alerts… So you may not become aware of transmitter problems or issues.

12. Notes: Since diodes conduct a tiny amount of current even when not forward biased, there may be a portion of the current going through diode that would not be measured by the ammeter. This is dependent upon the ‘burden voltage’ of the ammeter. Typically field calibration & test equipment has a low enough burden voltage that this does not have any noticeable effect – especially for typical industrial instrumentation specification tolerances of 0.5 to 1.0 % of span . But for instances where extreme tolerances are required and/or if you do not know the effect your ammeter will have on this method, you should do some investigation to be sure this method will not introduce small errors that could matter in some instances. Transmitters vary in how the TEST connections are setup internally – The sample diagrams in this presentation are for reference only. Consult your equipment reference manuals for details and specifics for your equipment.

13. Be cautious NOT to connect the Test to (-) terminal and other lead to (+) terminal. This can cause 24v power to bypass the internal regulating circuitry of the transmitter, and likely blow a fuse (in ammeter or the loop power fuse) or may damage the diode. When/if the ‘test’ diode fails open, the only way to get the circuit to work is by running the (-) lead to the (TEST) terminal. To my knowledge the diode cannot be field repaired for any major transmitter brands. If you see this odd connection setup on some transmitters your plant, it is because someone (or lots of ‘someone’s) are improperly connecting when testing or installation, and are damaging the internal diodes – and worse yet, they are then ‘fiddling’ with the wiring until they see it appear to work right… Sadly, this happens fairly often in the field (especially during commissioning if the techs do not adequately understand how the test connection, or 4-20mA loops in general actually work).

14. The solution to these problems is TRAINING. Techs must be taught more than just the basic procedures. They must understand how things work. Orion Technical Solutions covers this and many other important topics that cause lots of problems in the I&C field in our various Instrumentation, Calibration, and Controls courses. Visit our website at www.orion-technical.com for more information.