Dual 4-20mA signal but need a single 4-20mA signal output

[SaraXelon]

Big picture question on instrumentaion logic. My existing setup is a single level transmitter (4-20mA) feeding circuit via a controller. To avoid a failure of the circuit, we would like to add a second level transmitter (so that one LT fails, the other will continue to work and feed the controller and its circuit). However, the controller circuit is too complicated, we would like to do the following:

Take 2 LT inputs (2 x 4-20mA), run it through a black box. Out from the black box, we need a single 4-20mA signal for the controller and the downstream circuit. The black box should be also able to know a failed LT (fail low or fail high or power supply failure) and switch to the working transmitter automatically.

Is such black box device possible? Is anything commercially (readily?) available? If so, please provide the detail.

I appreciate all inputs and responses.

(PS: Controller failure is also similar to the LT failure; but at this point we want to make LT fail proof and worry about controller later)

 

[IRstuff]

Seems to me that your concept reduces reliability, overall, since it's likely your "black box" is at least as unreliable as your LTs, and you've got a single point of failure now, coupled with a single point of failure in the single cable.

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[danw2]

Moore Industries STA safety module takes a single 4-20mA input and alarms on its status, so if the xmtr faults to 21.5mA, it would detect the failure and alarm. The illustrated spec sheet shows 1oo2 voting using an STA for each of the source transmitters, but includes the undefined "logic solver" to pick which of the two is used.

Link to STA spec sheet: Link

app note on web page: Link

 

[SaraXelon]

Thanks danw2. I looked at the STA. Isn't that STA works with single sensor and provides 2 process alarm points + system fault? I'm in need of ONE black box (or multiple black boxes) that takes 2 level transmitters and provide me with ONE 4-20mA signal output.

 

[IRstuff]

You need to do the fault/reliability analysis to determine how much risk you can tolerate, where that risk is, and whether your solution does anything to the risk at all. Your solution contains two single failure paths, and if they are more likely than a failure in the transmitters, then you've not gained an reliability.

TTFN (ta ta for now)
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[SaraXelon]

Per vendor, cables meet the highest standard (certified to IEEE 383 - tested and proven to environmental effects) for any non-nuclear use. Also, they are exposed to mild temperatures only. So cable failure is out of the question and hence, two single failure path is not an issue, except for that level transmitter. I clearly understand there are other single point of failures with controller and its circuit but that is something we are also looking for. But the priority is the level transmitter for additional reliability.

 

[danw2]

I have never seen a commercial, solid state, stand-along, 4-20mA signal selector module.

Probably because the alternative of PLC availability.

There is the path of using a DPDT relay with gold plated contacts. But you need some discrete signal to drive the coil with.

 

[SaraXelon]

Okay. Thanks for your inputs