PLC for safety related systems

[leitmotif]

I have seen quite a few comments on this site and others that PLCs can do almost anything. On the flip side I have seen recommendations against using them on what I am going to call "safety related" systems including
boiler combustion control and safety circuits
elevator controls
aerial trams
punch press controls
traffic lites even

In some cases ie traffic lites the hardware is so developed and there are so many installations that mass production cuts the cost.

It seems to me that we control lots of things using computers (NASA space ships, missles, automatic aircraft systems, PAYROLL, and the list goes on). Takes good hardware and software AND sensors of course.

OK at last THE QUESTION
Why not use PLCs in some "safety related" equipment??

Some obvious answers
1. no one else has put out the bucks for listing from some approval agency ie UL CSA etc etc.

2. LIABILITY I put it in to avoid further discussion. IF the Wright Brothers had to deal with this we'd still be walking.

I would like to hear your thoughts on this.

Dan Bentler

 

[Eric Nelson]

Why not use PLCs in some "safety related" equipment??

You sort of answered that question with your "PLCs can do almost anything" statement. "Anything" can include failing to operate properly when an unsafe situation occurs. Beyond hardware failures, software (the program) is probably the biggest culprit. Not all programmers are up to the task of writing a fail-safe, bug free program. I don't want to relay on the ability of some 'programmer wannabe' when it comes to safety.

For example:

|   Button 1    Button 2        Cycle

|-----] [---------] [-----------(  )

May function just fine for many, many years as a two-hand control, until the day someone loses a hand...

beerchug

-Eric

 

[leitmotif]

Eric,

Completely agree - the simple circuit you showed would not suffice for any two hand control on a punch press I have seen. I tried to cover this issue by saying good programming.

Dan Bentler

 

[Doug_Adam]

Letmotif,

There are some PLCs that are purpose built for safety and have the safety cirtification with some agencies. Two that come to mind are the Pilz safety PLC and the Siemens S7, although not all the PLCs from each supplier are safety rated.
Also, some other manufacturers have safety rated PLC cards, although these are probably just ordinary safety relays with feedbacks directly wired into them.

As for boiler control, Pilz claims their PLC is up to the job, however not all local authorities believe them just yet.

Hope this helps,

Doug

 

[CaseyK]

Ah yes, press controls. So many considerations. Make them fool proof AND idiot proof. Figure out how to make them so they can't rest their elbow on one button, pushing the other button with their hand, while shoving their other arm in to the press. At least that is what the way the safety timing requirements were explained to me. I looked at the timing and sequence charts of what was required and decided that since stuph was available off the shelf, that was the way to go. One supplier had brought me a flyer for some safety curtains and press controls, and wound up with a nice sized order shortly thereafter. The company had been plauged with injuries, the help just wouldn't keep there hands out of the equipment. The operators were constantly asking for one of the palm buttons to be bypassed.

You can only make people as safe as they want to be!

Press safety and PLC's are covered by many areas. They realy like fail-safe. Here is a "LINK" to a PLC designed for press operation. Entertron in conjunction with KTI Technologies from NJ introduced a redundant control system designed specifically for press manufacturers. No need for two PLC's and two Safety relays, pretty slick!

Safety and liability came be scary words, when in the vicinity of that lawyer word.

regards.....casey

 

[Ken Moore]

I think the fact that a program can be changed is the major fault for PLC's when it comes to safety. You can be the best programmer that every drew a breath, and have some idiot change your code in the middle of the night. End of safety.

Ken

Yes you can use passwords etc, but I wouldn't want to bet my life on them.

 

[cpmiller]

I just installed a S7 315F Safety PLC and after setting it up, I am quite sure it will handle any problems. It is redundant and self checking. As for a passwords one is required before the PLC will even run in Safety Mode so no changing the code unless I give the password out, in that case I am to blame.

Also since it has been installed I have not gotten one call in the middle of the night with someone saying " The Estops will not reset"
since they now have an idiot screen to tell them the exact state of all the safety switchs, access gates, and Estops.

 

[elevmike]

leitmotif,

This is really a question of how the PLC is used on a machine, rather then if it can or should be used. The statement that PLCs should'nt or cant be used on elevators, or other types of equipment that require carefull safety considerations, is totally bogus. Safety is more a matter of system design. The idea is to design your field circuit so that even if the PLC has faild and lock on all the outputs, the overall circuit design will not allow the equipment to run in an unsafe manner.

 

[Hundikoer]

Like elevmike said,
most important is design. You can use high-end safetyPLC (SIL3) or ordinary PLC with cert. (SIL2), but if you install it unproperly or program it "ordinary" way, then it will not be as reliable as it must be.
Using safetyPLC's you can use distributed IO's over Ethernet. That can make life easier.

 

[JohnW]

I have used a PILZ Multi, which is a programmable safety relay somewhere between a "bog standard" dumb safety relay and a safety PLC such as the Pilz PSS. The multi becomes cost effective if you need a system with more that about 4 safety relays, in this case I used it on a 2-roll open rubber mill instead of about 8 safety relays. The program is password protected on the PC and it is stored on a SIMM card and once it has been "closed" it cannot be altered at all. Because of the hazardous nature of the machine I had Pilz validate the program for me.

Look Here

 

[shoeman]

PLC+Safety = Me+Willies

I have done several systems for SIL1 SIL2 and SIL3 applications. When you are looking at this issue, there is no one rule that says YOU MUST USE XYZ EQUIPMENT. Personally, anyone wnating to use a PLC, especially one from AB, would be nuts. AB has that lovelable feature of dumping the program on a memory fault. You cannot have that happen at all. I am a process guy and a programmer. I want to know how, where and why instead of nothing about a fault like that. Safety systems should sit there and never be used in a perfect world. On the chance they need to act, you had better hope they do.

In designing a systems, Paul Gruhn said it best several years ago. "Be able to show you used good engineering practices and be able to defend yourself in court."

Siemens, ABB, Invensys all offer safety systems rated for Different SILS. THis is not an area to save money on hardware. The area to save money is to make sure the right questions are asked and not to paint yourself into a safety corner. You had better make damn sure it is really a SIL3 and not an overblown 2.

 

[leitmotif]

Safety thanks for info

Thanks for the input

What does SIL (1, 2 or 3) stand for? I assume it is a standard for performance of electrical equip for safety applications - whose ?

After 20 years is safety what you have all said is nothing new redundancy, fail safe design etc.
1. Design it fool proof and they design a better fool.
2. You can do the best design with the best equipment but when you hand it to the operator it is all up to him.

Saw a respirator where a guy drilled a hole in it so he could smoke with respirator on - kind of ingeneious in a way - but it also defeated the purpose of the respirator -- they eventually fired him.

Dan Bentler

 

[JohnW]

SIL stands for Safety Integrity Level

Here is a good explanation here

 

[MarkTTU]

Never had a safety related problem using the AD PLC's... Been using them for years, 205 now 06, and they're wonderful, BUT its all about system design.

 

[chamo_plc]

Communication between TP270 & S7-200 CPU226XM

Hi there.

I’m new in communication between HMI Siemens and PLC Simatic S7-200.
I need to communicate a CPU 226XM with a HMI TP270 and I don’t know how to do this.
May anybody help me please?.
I need MPI cable pin-out too.
Thanks a lot for your help.

Chamo PLC