Hardwired safety

Can someone please point me to some U.S. law or code which dictates that machine safety functions (e-stops, guards, etc) cannot be run through a standard PLC, and must be either hardwired or attached to a safety-rated controller?

Yes I know it's common sense but some of my co-workers think it's just extra work and expense. "Why not just wire the guards into the PLC?" they say. I have a very large project coming up that will require zoned safety, and I have to be able to justify the extra bucks for a safety PLC or something similar.

NFPA 79 would be a good place to start. ANSI B11 (not sure which subsections) also have information on safety and emergency stopping. I believe OSHA has requirements as well but again it's been too long since I've sifted through it to point out the correct sections you would need.
Basically you can't use a standard plc as a safety device because that is not it's intended purpose. It becomes too easy to program around a safety device. ANSI B11 TR6 is developing a standards on safety controllers to be implemented hopefully next year. My boss sits on this subcommittee.

All I can find in NFPA 79 is that emergency stop operators must have "direct opening operation" (10.7.2.2), which I take to mean that they must physically break the circuit. No mention of guard interlocks.

USA Industrial Automation Safety

Hi,

A good place to start is BANNER ENGINEERING Safety.

They have got a lot of documents online that can help you
in your job.

See also:

http://www.bannerengineering.com/en-US/products/6/Machine-Safety