Safety optic fiber connection for digital outputs?

jesusvasquez · Sep 8, 2014

Hello everybody,

I'm designing a safety system for a HV power supply. The idea is to use a series of 2 contactors to supply power to the system controlled by a safety PLC. So, it is a typical application.

The problem is that the power supply and the contactors are are installed on HV platform, isolated from ground, where the PLC is installed. I think the only possibility is to use an optic fiber link.

Does any body know is there is any way to connect a contactor to a PLC output using an optic fiber link, without compromise the cyclical checking performed by the safety module (as I understand, the outputs are continuously check in order to detect wire breaking).

The alternative that I have is to use a distributed IO system installed on the platform, connected with a Ethernet Optic Fiber to the PLC: for example using a simens S7-400 PCL with a IM467FO modules (on ground) and a ET200M system with a IM153-2FO module on the platform. But I wonder if there is something more simple and cheaper.

Thanks!

mk42 · Sep 8, 2014

jesusvasquez said:
Hello everybody,

I'm designing a safety system for a HV power supply. The idea is to use a series of 2 contactors to supply power to the system controlled by a safety PLC. So, it is a typical application.

The problem is that the power supply and the contactors are are installed on HV platform, isolated from ground, where the PLC is installed. I think the only possibility is to use an optic fiber link.

Does any body know is there is any way to connect a contactor to a PLC output using an optic fiber link, without compromise the cyclical checking performed by the safety module (as I understand, the outputs are continuously check in order to detect wire breaking).

The alternative that I have is to use a distributed IO system installed on the platform, connected with a Ethernet Optic Fiber to the PLC: for example using a simens S7-400 PCL with a IM467FO modules (on ground) and a ET200M system with a IM153-2FO module on the platform. But I wonder if there is something more simple and cheaper.

Thanks!

The Siemens safety IO do have wire break checks and other diagnostics, but those aren't affected by the cyclic bit pattern check i think you were talking about. The bit pattern test is only there to prove to the IO module that it is capable of changing the signal state, not anything related to the field. You should be monitoring the actual feedback of the contactors to ensure that any commanded shutdowns actually occur.

Depending on the module used, it may be possible to turn off the diagnostics like wirebreak, but that could also affect the safety rating of the system.

I've never tried using fiber for IO control, so I'm not sure what options exist there. I've only seen fiber used for the network connection. Instead of the parts you listed, you could use Optical Link Modules to convert the copper profibus to fiber and back, but I don't know how much savings that would offer over the alternative you mentioned.

jesusvasquez · Sep 10, 2014

Thank you mk42.

I still don't have much experience, but if I understood correctly some sort pulse train should be transmitted on each output channel and read back by the same channel (not the feedback contact) in order to check wirebreaking and/or shorts to mass, 24V, or other channels. Is that correct?

The solution that I was thinking was to connect the PLC output to the contactor input command using a digital optic link (an another digital optic link to connect a contactor auxiliary contact to a PLC input as feedback channel). But in this case, if me previous explanation of the diagnostic was correct, the module will only check the connection until the optical link and not to the physical input of the contactor. So, I am not sure how the system rating is affected by this.

mk42 · Sep 10, 2014

jesusvasquez said:
Thank you mk42.

I still don't have much experience, but if I understood correctly some sort pulse train should be transmitted on each output channel and read back by the same channel (not the feedback contact) in order to check wirebreaking and/or shorts to mass, 24V, or other channels. Is that correct?

The solution that I was thinking was to connect the PLC output to the contactor input command using a digital optic link (an another digital optic link to connect a contactor auxiliary contact to a PLC input as feedback channel). But in this case, if me previous explanation of the diagnostic was correct, the module will only check the connection until the optical link and not to the physical input of the contactor. So, I am not sure how the system rating is affected by this.

Sorry, what I spoke about was for the outputs, the inputs work as you describe.

The wirebreak/short circuit testing you mention is generally optional, and in some cases is not even possible. However, then you must make take extra care with your wiring and take other precautions to keep the same level of diagnostics. Theoretically, if you included an optical pathway for both the safety supply pulse and signal return, that might allow you to keep wire break detection on.

However, I have no idea if that would be a legal safety circuit. I've never heard of using optical pathways for signal wiring, only for networks. With safety, there is a big different between what will WORK, and what is actually ALLOWED.

Safety optic fiber connection for digital outputs?

jesusvasquez

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mk42

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jesusvasquez

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mk42

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