Shorting an analog output on purpose?

[Bob O 8]

I am going to assume the AO is 4-20 mA and I know the hardware is a Flex 1794-OE4. Has anyone ever been asked to short the analog out [Example terminals A-0 to A-1] of the card under a certain set of safety conditions or any condition? The terminals would be shorted using a set of contacts from a relay. Thanks

[Mickey 71]

I have seen "open" it but not short it. Why short it? For what purpose?

[Bob O 8]

Mickey, I am told they want to do this to force the signal to 0 and in turn force the valve to the closed position.

[TConnolly 13]

I saw it once on a vacuum furnace - the over-temperature alarm relay shorted the 4-20mA command coming from the temperature controller. If you have a form C relay then you could use one contact to short across the input of the field device and the other contact to open the 4-20mA loop, that way you don't have to worry about the load on the analog output.

[TConnolly 13]

Depending on the input impedance of the the valve and the output impedance of your PLC AO you could throw in a small resistor as long as the overall impedance stayed lower than the max impedance of the AO. In theory the 4-20mA AO should be able to deal with a short but I wouldn't bank on all manufacturers building their product that way. The command shorting relay on the vacuum furnace I mentioned was shorting across a very large rack mount Honeywell controller with a voracious power appetite and a power supply to match.

[Bob O 8]

Alaric, Here is what they are proposing. Analog_Output.bmp

[Mickey 71]

Yes, that's the reason I have done this. But Alaric post number 4 show a good method. I didn't short the field device though. Just open the circuit.

[TConnolly 13]

If that is what the customer is proposing then give them that. Their design. Their responsibility. As I said, I've seen it before. As a precaution however I would configure the output for 0-20mA and program it to output 0 mA when ever CR06330 is off and operate in the 4-mA range whenever CR06330 is on.

[Bob O 8]

Mickey and Alaric, Thank you for your help. Bob