Isolating 2 wire analog devices with Point I/O

[trueninjalo]

Hey folks,
Hardware: *=Must use these devices.
Allen Bradley 1734-AENTR (Dual Ethernet Adapter)*
Allen Bradley 1734-IE8C (8Ch Analog Input Module)*
SINEAX TI 816 (Passive DC signal isolator)*
Omega LD630A-20 (2 wire Linear Position Sensor 4-20mA)
Omega FMG470-03-D6I (4 wire DC Powered Magmeter 4-20mA)
Omega FMG490-03-A5I (4 wire AC Powered Magmeter 4-20mA)

I am having trouble planning an analog device expansion. There is going to be about 12-16 devices added to an area. It is a mix of 24VDC 2 wire and 24VDC 4 wire and 120VAC 4 wire devices (Temp, Psi and Flow sensors). All output 4-20mA signals. I am a bit confused about wiring and the documentation for the Analog Module and the Isolation Module are vague.

Attached is how I think they are supposed to be wired with and without the isolation modules. I am fairy certain about the 4 wire devices but the 2 wire is what has me questioning. the layout. Can someone review this and let me know your thoughts?

Respectfully submitted,
AJ

 

[danw]

The way I read the Sineax spec, it is a passive device deriving its operating power from the 4-20mA input circuit.


https://www.camillebauer.ch/src/download/PM-TI816_5_SIN-DB__02-000708.pdf

The 4 wire devices will provide the power that the Sineax needs to operate and those appear to be wired OK.

The 2 wire field device has no power therefore no output, therefore the Sineax will not operate and there will be no 4-20mA at the input card.

The problem is that if you use the Sineax with the 2 wire device, the input side of the field device needs to connect to the power supply (-) to get its power to operate. That connection to the power supply (-) is common with the (-) output of the Sineax [(+) connects to IN-0, like the 4 wire devices], which bypasses the concept of isolation, it's like, why bother with an isolator?

There are loop isolators that are powered by 24Vdc that provide DC power to a 2-wire loop powered field device on the input side, but provide full isolation between the input, power and output side. That's the kind of isolator that's needed.

I think that there's a high likelihood that the Sineax will work with the 2-wire, given that every 2 wire loop powered device I've run into has an isolated 4-20mA output, so there is no local ground to have a potential different that the ground potential at the power supply (-). But it's a big world and if that particular 2 wire device does not have an isolated output, then the Sineax will not provide isolation, even if its wired in the circuit because of the common connection to the power supply (-).

To use the Sienax with the 2 wire sensor,
1. power supply /fuse (+) to sensor (+) terminal.
2. sensor negative terminal to Sineax input (+) terminal
3. Sineax input (-) terminal to power supply (-) terminal
4. Sineax output (+) terminal to 1734 IN-0 terminal
5. Sineax output (-) terminal to power supply (-) terminal
Note that 3 and 5 have the same connection point

 

[trueninjalo]

Thanks Danw