I completely disagree with Bruce Durdle's suggestion: <i>make the Estop a lockout so they have to walk to a central location and get the key to restart.</i>

Don't ever do ANYTHING to discourage use of the E-Stop. If hitting E-Stop is a big deal, they won't hit it. They may even start to circumvent automatic E-Stop devices like mats, light curtains, or gate switches just to avoid the hassle. If someone sneezes and it startles you, your reaction should be HIT THE E-STOP FIRST, ASK QUESTIONS LATER.

Granted, there are some processes where hitting E-Stop really is a big deal, and recovery is a nightmare. In that case, you've ALREADY got the disincentive, and there's no reason to add the hassle of the key switch.

-James Ingraham
Sage Automation, Inc.

 

James -

Fair comment - but I've seen some strange things in my time - would you believe an electrical supervisor who preferred to open a 3.3 kV MCC cubicle and use the trip button on the breaker housing rather than use the "open" button on the panel? Took us an age to retrain operators that that wasn't the right procedure once he left.

I think, like anything else, E-stop locations and procedures depend on the exact situation. My background is petrochemical/process control and it was common practice on at least one site to use a lockout/key reset. But an e-stop was a big deal - at least 24 hours down-time and a few thousand hours off the life of equipment because of thermal stresses. Any e-stop action (or other plant trip) usually resulted in a bit of an inquest. Key-switch was very useful in locating which of the many switches had activated - as much as insurance for fault-finding as anything else. It also confirmed that there wasn't a transient process condition that had escaped the relatively crude SEM system we had.

The best solution is to make it very easy to do the right thing in the circumstances - like locating a fire alarm call-button right alongside the exit door rather than in the middle of the stack of flammable material. The same sort of mindset as making sure that any essential routine maintenance on running plant can be done without using overrides or jumpers to get around protection. This means getting alongside operators and trying to look at the real problems they have to deal with and sorting out those issues, rather than just assuming they are all prats without a clue. Unfortunately, a lot of designers focus on the hardware and software and totally overlook the wet-ware.

Cheers,
Bruce

 

I worked on a system where the operators would hit the e-stop as they knew it would take 45 mins to get the line running again meaning they would get an extra tea-break. They installed lock-out switches but the operators discovered that if they gently pressed the push-button it would activate the e-stop but wouldn't lock-out the device so it didn't prevent the problem. In the end we replaced all the e-stop cabling with additional cores to send a digital input back to the PLC identifying which button had been activated and shift reports would state who was working in that area at that time. Funnily enough, it stopped the problem immediately.

Also, I have worked on many systems where problems were caused from shutting down via E-Stop instead of system stop. In many cases some improvements could be made by simple software/hardware changes but sometimes on larger existing systems this wasn't always feasible. Also I have to agree that in my opinion the e-stop should not be used unless it is an emergency condition.