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EHELP: STA Analog Output Protection

EHELP: STA Analog Output Protection

Q:  In specifying the Moore STA / HLPRG / 3PRG / U / -AO [DIN] our end user has a concern about electrical protection for the analog output. The desire is to use 4-20mA Output Loop Voltage sourced by DCS for the AO; however, this voltage may fault at up to 132 VAC. Should our design add fuse protection in the external voltage circuit upstream of the STA to protect it from these surges? Or does the STA include features that protect it from these faults?

A: A fuse will limit current but not excessive voltage. If the DCS cannot be configured for a sinking input, then I recommend using a “sacrificial” isolator. Connect the sourced output of the STA into the input of an output loop powered isolator. The output of the isolator goes to the DCS’s sourcing input. If there is a 132V fault voltage from the DCS, it will risk damaging the isolator and not the STA.

An example of such an isolator that would work is an ECT/4-20MA/4-20MA/12-42DC [DIN].

If this analog output from the STA is part of a safety path, then consider using the SSX/4-20MA/4-20MA/12-42DC [DIN] as it is a Functional Safety isolator designed for safety instrumented systems.

Download the ECT-DIN data sheet for more information.

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Published: October 4, 2016