Since I do understand that barriers and isolators limit the current and voltage to the instruments installed at field hazardous area, my question is:

1. will I still be getting 24VDC at the transmitter side when connected in barriers or isolators?

2. will I still be receiving an output of 4-20ma when the supply voltage is less than 24 volts let say 16-18VDC?

 

The intrinsic safety barrier has a current limiting resistor that imposes significant resistance to the 4-20ma loop. I found an excellent discussion in this on-line article:

<b>www.omega.com/temperature/Z/pdf/z131-148.pdf</b>

You will probably not have 24v appear at the transmitter; only that the voltage drop across the transmitter, barrier resistor, conversion resistor, and wire resistance will add up to a sum of 24volts. The current, however, is always controlled by the transmitter to the 4-20ma range acting as a variable resistor. As a rule of thumb, the whole circuit should have a total resistance of about 600 ohms, but I am not sure that this applies to a 12 or 24 volt power supply.

 

The answers are below are as numbered:

1. The answer to this question depends on the type of barrier used. If your use a grounded barrier, you will have a drop across the barrier, which may be significant if the voltage burden of the transmitter is large (>12 volts). If you use an isolated repeater type of barrier, the current output loop current signal and supply voltage of the repeater will duplicate the input current signal and supply voltage (hence no voltage drop) or in some cases the output supply voltage can be higher.

2. The main question is what is the required voltage burden of the transmitter, e.g. how much voltage does the transmitter require to provide a 4-20mA output? This is typically in the 12-13 V range for a process transmitter. If you cannot supply this when considering all the resistances in the loop (Kirchhoff's circuit law), your transmitter will only go so high and stop, e.g. you might get 15 mA or 16 mA. The transmitter's manufacturer may supply a voltage/resistance limit or they will provide a loop resistance limit chart (usually in the form of a chart with power supply voltage vs. ohms). See what your resistance limit is and compare this to your total loop resistance and that will tell you if your loop will provide 4-20 mA.

Regards,
Bill

William (Bill) L. Mostia, Jr. PE
ISA Fellow, SIS-TECH Fellow,
FS Eng. (TUV Rheinland)

"No trees were killed to send this message, but a large number of electrons were terribly inconvenienced." Neil deGrasse Tyson

Any information is provided on a Caveat Emptor basis.