What are OSSD safety outputs?
This is a very common question in the control/automation world from many different sources. The Acronym OSSD stands for Output Signal Switching Device. This says a lot but does not really tell us what we are switching or why.
Can a relay contact be considered an OSSD? How about a Programmable Logic Controller (PLC) outputs? What about a pressure switch with a PNP output? From a logic standpoint just based on the words, any output that switches the signal (high/low, one or zero, open or close) would seem to satisfy the acronym.
Looking more closely at the term as I understand it in the safety world, OSSD simply refers to a safe output. Typically a pair of semiconductor outputs that are safety rated (an OSSD pair). The outputs are commonly 24vdc but they could be other voltages or relay contact-based as well.
The 24vdc outputs utilize pulsed signals and are tested by a safety evaluation unit (i.e. safety relay or safety controller/PLC) to determine if the output pair is on/off or malfunctioning (i.e. shorted). A standard non-pulsed 24vdc output does not have this type of diagnostic capability.
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Most higher-end safety products such as RFID interlocks, safety laser scanners, and light barriers utilize OSSD safe outputs due to the need for higher diagnostic coverage rather than contact closure-based switching. Higher diagnostic coverage typically equals greater safety performance and more cost.
Many contact-based safety interlocks and devices are rated at a maximum Safety Category 2 PL d (in a properly designed safety circuit). The same devices with OSSD solid-state safe outputs are rated for Safety Category 4 PLd or e (again, if utilized in a properly designed safety circuit). This is due to the diagnostics and redundancy built into the device by the manufacturer.
Many applications for machine safety may be able to utilize the more basic non-OSSD safety output interlocks and products rather than the more costly and advanced OSSD products. This is typically determined by a Risk Assessment of the machine or application.
When in doubt please speak to a well-qualified Functional Safety Engineer for further clarification. Safety means LIFE!
Equipment Service Technician
1 年Also i believe these can be wired in series with each other and standard E-Stops
Equipment Service Technician
1 年Yes. Excellent information. Was looking for this information for ages on internet.
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2 年i have connected dsp laser with an AL dc module (nuova eletronica) but iam not receiving any signal on OSSD1/OSSD2.what can be the problem?,also can i connect the out put signal direct to the contactor?i will really appreciate if my request is attended to as soon as possible.
PLC & Robot Engineer
2 年Hi Rod, I believe this explanation is incorrect: "The 24vdc outputs utilize pulsed signals and are tested by a safety evaluation unit (i.e. safety relay or safety controller/PLC)", because device does the diagnostics and do the evaluation, not the safety controller.