PLC's and Automation Safety

I'm a college student doing research on PLC's. I'm trying to find out if PLC's have improved safety standards under hazardous working conditions. From the research I have done so far, I see the only advantage PLC's have over relay logic is the ease of reprogramming processes instead of rewiring relays. What was plant safety like before PLC's?

It's going to be tough to get a definitive answer on that. The implementation of automation, including PLCs, has been concurrent with the implementation of OSHA and similar safety regulations, increased safety education, more strident union activity, reduced staffing, and similar trends that affect safety.

Anecdotally I would feel confident in saying that PLCs have improved safety. They eliminate operator entry to hazardous locations in many cases, they make the incorporation of additional safety precautions economical, and they have reduced the number of boring repetitive jobs that lead to operator inattention.

PLCs are not, and should never be considered safety devices. It is important that you design your system with that in mind.

I don't think PLCs by themselves actually make the workplace safer. I have seen some shocking places fully PLC automated. On the other hand, one of our plants is well guarded, correctly isolatable, and has all the E-stops correctly wired, but is controlled with drum controllers and relays.
As Tom stated, I think that machine safety has developed in parallel with the developing of the PLC.

From my point of view, I think that the primary driver of machine safety is attitude. In the past, factorys were accepted as dangerous places to work. That was just a part of life, and if you didn't like it you could find a job else where. Some pioneer somewhere must have questioned the assuption, so the idea developed as follows:
-Factories are dangerous places to work, accept it.
-Factories don't have to be dangeous places to work, how can we improve our safety?
-Factories should be safe places to work.
-Factories must be safe places to work, and it is your obligation to make it safe.

During the development of these ideas, technology has kept up. PLCs have often been a part of the safety system, even if only monitoring dedicated safety relays. However, some PLCs are now rated as safety devices, for example, look here: www.pilz.com

Other manufacturers of safety PLCs are:
Siemens
Allen Bradley
Omron (may not yet be released)

Hopefully this helps, if not then refine your question and post again.

Hey, check out the "X-Scream", controlled by AB PLC.

http://www.manufacturing.net/ctl/article/CA6262677#Interactive

Let's just hope the are not using redundant CLGX PLCs with 17 slot racks using version 4.56 of the CNB module firmware for the X-Scream. And then let's hope the designer and engineer selected the correct fail state of I/O due to CNB module failure. Of course if you upgrade to version 5.50 you only solve the problem for 7,10 and 13 slot I/O racks. 5.51 then fixes the issue with 17 slot racks. Backplane noise may not cause the CNB to lock up in the first few weeks of operation, but sooner or later may cause a problem. Enough with horror stories....

The safety part of this thread is important in that as said earlier a PLC is not a safety device. The provider of the X-Scream system (hopefully) reviewed all failure modes - loss of power, loss of a single component, loss of any other source such as hydraulic fluid/pressure, etc... and made sure the equipment failed to a "safe" state.

The old days used to have "e-stops" as a recommended practice from the manufacturers that simply turned on/off a MCR relay and deenergized power to the outputs. Today there are safety relays that are programmable and generally require the system be re-started from known safe states before allowing a PLC to take control.

Safety standards are becoming a focus for many manufacturers (looking to expand markets and make things better). They have been around for awhile with more emphasis on standards recently from what I am seeing. Take a look at the IEC 61508 standards.

http://www.google.com/search?hl=en&lr=&q=IEC+61508

Nismo_4 said:

What was plant safety like before PLC's?

Click to expand...

What plant safety?

Some machines that I saw in the 80's had one, sometimes two or three e-stops.

I don't remember any in the 70's. Just start and stop.

But then, I am in a somewhat remote area. While I was in Peoria, bigger facilities like Catterpillar may have well had better safety setups.

regards.....kc

I consider the subject of your research to be completely invalid. PLC's, even 'safety' PLC's are useless blocks of plastic, semiconductors, and metal.

The improved safety in industrial (and non-industrial) enviornments has been driven by insurance companies and governments.

The means of achieving a safer working enviornment is accomplished primarily by keeping operators away from moving parts and pinch points, via guards, or limit switches, or two-hand operators. Then of course, there is proper training. And proper equipment (safety glasses, shoes, dust/gas masks).

The only real 'relationship' between safety and PLC's is that PLC's have become the predominant means of control at the same time as laws reguarding workplace safety compliance have become extremely stringent.

How 'bout this:

The advent of PLCs and HMIs has greatly streamlined the process of developing a control structure and interface. With the increased efficiency has come additional time and resources to focus on machinery safety.

Of course, the truth is as stated above: Plant safety is the direct result of: (1) Fear of OSHA, (2) Fear of lawsuits, and (3) Fear of bad publicity. PLCs really have nothing to do with it.

After 20 years in the safety business, and as a control electrician you would think I could come up with a quick and easy answer. I can't. All of the previous comments are correct.

I think what happened is the simultaneous effect of two or three trends.
1. PLCs were originally "invented" ?? by a GM contractor who was tasked with finding a way to replace hard wired controls. It was expensive to change production lines. VOILA the first PLC in 1970 or so??
2. OSHA came about in 1970.
3. Industry got smarter and labor got smarter as time went on.

SO I think as things evolved in making production less expensive with more automation, and management and workers got smarter regarding production and safety (yes they DO go hand in hand) we have seen huge improvements in both production and safety.

Maybe another way of saying it is what was first the chicken or the egg?
OR maybe it is evolution -- a very complicated interinvolved mutual support and synergistic process.

Dan Bentler

Slightly off topic

leitmotif said:

1. PLCs were originally "invented" ?? by a GM contractor who was tasked with finding a way to replace hard wired controls. It was expensive to change production lines. VOILA the first PLC in 1970 or so?

Click to expand...

Actually in the mid 60s and there was a GM advanced mfg engineering team at the Ypsilanti Transmission plant that worked with Morley and IIRC DEC. I know two of the engineers (now retired) that were apart of that team. I got to see some pictures of the projects they were working on. I got the sense that they felt Morley falsely took credit for some of their work but who's to know now.

With all of this, you may wish to change your topic of paper to "Risk Management". All of the above posts identifying regulations are true. If you look at the various regulations (work place, environmental, others...OSHA, RMP, SPCC, even SARA) in industry, the end users implement these requirements now in a form of risk management which translates into cost avoidance ($$$$). Shop floor safety, operational safety, management safety, business management all are managed based on Risk. Risk of fines, risk of injury (and resulting litigation and cost) etc...

PLCs ands HMIs can be made to be safer

When I started as a plant electrician we were just installing the first PLCs in the plant. All the mobile equipment was controlled with GE static logic and hardwired controls and annunciator lights. As we replaced these with PLCs and HMI it became obvious that we now had the flexibility to improve not only the control (and safety) of the machine but most importantly to display the conditons in a way that made the operator understand without poking around. For instance I have been asked to add 'maintenance' modes on equipment for specific maintenance tasks. I use the HMI to explain the changes that the maintenance mode has on the operation and what specific equipment needs to be locked out before proceeding. The HMI monitors the power status of the starters and gives positive feedback when all the proper equipment has had power removed (and hopefully locked out).
All this would be impractical if not impossible using relay logic and indicator lights.

So Yes PLCs (and HMIs) have the potential to greatly increase industrial plant safety.

Brian

After reading some of the posts and re-reading the origionl post I have some additional comments.

Nismo_4 said:

I'm a college student doing research on PLC's. I'm trying to find out if PLC's have improved safety standards under hazardous working conditions.

Click to expand...

Yes, but..Not as a safety device. PLCs have actually enhanced workplace safety, including but not necessarly limmited to in the following manners:

1) In the sense of automating processes, and removing the human operator from a potentially dangerous operation.

2) PLCs tend to remove/releave the need for a worker from repetitive tasks, wich often results in strain injuries over time.

2) Reliability. PLCs are far more reliable then the large number electro-mechanical components they often replace. Again not as a device to insure safety, however in any case enhanced reliability always = enhanced safety.

Look at it this way: We use PLCs for many of our elevator controls. The control systems are designed to assure safety by not useing the PLC as a safety device. However the PLC is much more reliable then a typical hard-wired relay logic controller. In an application such as a medical facility, that enhanced reliability provided by the PLC would reduce the chance of a component failure while transporting a critical-care patient.

Interesting read:

http://www.ab.com/safety/safety_now/april01/back_school/

Quote

" PLCs are far more reliable then the large number electro-mechanical components "

Mike,
I disagree, I base this on 27 years of experience as an Instrument Tech
working with both systems. Why are e-stops hardwired with electromachanical components? IMHO