IEC 61508 - software & firmware interlocks

[megloff11x]

NFPA 79, 9.4.3(3) and 11.3.4 note that a device used for software or firmware safety functions must meet the requirements of IEC 61508.

Are there any other listings or requirements to be met when looking to specify such a control system?

I have a system that requires a category 1 controlled E-stop due to possible hazard if an uncontrolled category 0 E-stop is employed. Since this is a process shutdown, it will have to be software/firmware controlled, and power removed after it is rendered safe & stopped.

Can anyone recommend PLC or controller models that comply with this and other needed standards?

Also, for belt & suspenders, if the system doesn't respond, I would like to provide backup in the form of a time delay relay to pilot the various contactors. The time delay would be long enough for the controlled stop to complete plus a safety margin. If it didn't stop itself because of a runaway program, a category 0 stop would soon follow.

Is this allowed by code and what listing(s) would such a time delay relay require?

I once ran a CNC milling machine that ignored its E-stop mushroom button. I won't mention the make and model, but I had to run around chasing the conduit path to find it's main disconnect switch while it mindlessly carved a channel in its own worktable. It's amazing how much self-inflicted damage can occur in less than five seconds.

Matt

 

[petersonra]

Re: IEC 61508 - software & firmware interlocks

Sometimes we used to have what was called tool relief. The estop would be triggered and whatever had to happen to relieve pressure on the tool happened, and then the power dropped. Usually, the length of time from estop to power off was measured in a fraction of a second. For instance, after an estop was pushed, maybe a hydraulic cylinder on a drilling slide would start to retract and after 1/10 second power would drop completely. That 1/10 of a second was enough to relieve the forward pressure on the tooling. The only thing I have ever seen used for this purpose is an off delay pneumatic timer. I have not done any machine tools like this in a while so I am not quite sure how it is handled these days.

There are plenty of safety PLCs these days where you can implement safety functions in software. AB has them as do other manufacturers. I seem to recall there are some requirements you take a class before you can "legally" use them. They seem most useful for implementing complicated estop and guarding systems.

Don't quote me on this, but I believe that the enable input on VFDs and similar devices is covered by the firmware requirements.