All That Wire & Is It Safer? Comment: Simpler is Better
The possible answer to this question totally changed in 2002 with the new issue of NFPA 79 allowing safety PLC’s and safety busses. Previously, NFPA 79 clearly required anything “safety” must be hard wired. Now, seven years later, hundreds of safety PLC’s are reported to be in use in the U.S. and the answer to this question is not so clear. The answer could be technology based. Or, is there a broader way to address this question looking at a company’s policies, procedures, best practices, training, and management philosophies - to mention a few?
With all these concepts in mind you might want to ask some of these questions:
1. Is the particular safety function easy to tamper with?
2. Is the particular safety function easy to by-pass?
3. Is the particular safety function monitored with diagnostics?
4. Is your operator trained, able, and authorized to open control cabinet doors to find and reset a relay with a fault diagnostic displayed?
5. Does your control system send a report every time a change is made to the control system?
6. For CAT 3 or 4 hazards are you using redundant circuits and components for your safety functions?
7. Per your risk assessment have you mitigated all hazards to tolerable risk levels?
So, the answer is - it’s not so simple!
What additional questions can you add in trying to answer this question?
Posted by J.B. Titus on August 1, 2009
September 2, 2009
In response to: All That Wire & Is It Safer?tanstaafl commented:
i have only been working with industrial equipment for ten years or so, but i’ve been a programmer and worked with semiconductors of all sorts for over thirty-five years. in that time, i have seen a gazillion ways (usually unexpected) for computers and automated equipment to fail. i don’t advocate doing away with safety PLCs by any means, but depending upon your safety budget, it might be a good idea to have more than one safety system, each being implemented in a completey different technology. regardless of how it’s implemented, of course, the simpler it is, the better, and it doesn’t get much simpler than a hardwire stop circuit.