Safety and PLC???

[kamenges]

I think Ron's point (one I agree with, by the way) is that the parts don't make the system. You can throw all the redundant, self checking e-stop relays and force activated safety switches you want at a project. If they aren't applied correctly and based on a sound risk assessment and fault analysis you still have an unsafe system. Actually you have WORSE than an unsafe system. You have a system that fosters the PERCEPTION of safety while being unsafe in reality. Now THAT will get someone hurt.
So the major point in my mind is you need to fight the urge to think:
'There. I put in a safety device. Now everything is cool.'
This is not necessarily true.

Keith

 

[panic mode]

kamengas,

I absolutelly agree :nodi:

And yet all guys I know that are doing PHSR have mechanical
background and know nothing about how the circuit works.
They are going to let someone get killed or seriously hurt
one day. They simply trust too much to electrical guys
(yeah, I'm electrical too so don't kill me yet... ).
It's amazing how many people cannot copy and apply simple
Cat4 E-Stop circuit from the book.

 

[BobB]

Interesting topic.

Here in Ozz it is illegal to implement safety lockouts in the PLC. E/stop circuits etc have to be hard wired.

On a crane job I am doing at the moment I have opted for an Omron CS1 PLC with an IEC compliant type 4 safety relay mounted on the PLC rack. The PLC has no control over the safety relay but has the ability to monitor the status of the device and raise alarms etc on a touch screen.

Type 4 IEC compliant involves the use of 2 contactors in the power circuit to the crane and 2 input circuits. If one input circuit fails, both contactors open. If 1 contactor fails to open the safety relay locks out.

My 2 cents worth. It is the most convenient, and inexpensive, method of safety relay implementation I have used. The tremendous advantage is that the device is mounted to the PLC rack, thus taking up a lot less space, costing a lot less and having the advantage that the PLC can monitor the device and alarm to a screen or SCADA system.

Another advantage is that the device has some extra inputs that can be used as standard inputs. Highly recommeneded.
beerchug

 

[rsdoran]

I would respectfully suggest that you pull out some of your old textbooks on the structure and design of transistors, opto-isolators etc.. Also review the wiring diagram of a basic 8 pt input unit and you will see that the opto unit is isolated from the TTL or transister that send the signal to the CPU.

Solid state components often fail in the on/closed position. In the case of an input point, wheither or not there is power on the led side ot he opto isolator has nothing to do with the failure, or shorting of the photo-transister that it's lighting up.

Re-read my post, thats what I said. THE original poster on this forum at this time stated inputs failed with no power going to it...that I have never seen happen, no power no input EXCEPT in a case of reverse feedback..ie not a plc problem

As far as age goes and what I have seen I think I have been around a good while myself...even tho I aint the sharpest knife in the drawer, I have a fair understanding of many things.

MY POINT was not to use a PLC as or regard it as a "safety device", rather that it is JUST A DEVICE. The plc is used to assist in automating a procee...ie replace relays and other devices...IT CAN NOT BE SAFE OR UNSAFE...it is just a component in a system.

Safety is the designer/engineer/programmer's job to assess and determine what is needed. Its not a PLC's fault whether the system was created properly, and just because a safety PLC is used doesnt necessarily mean that the system is safe.

I played the Devil's advocate because the examples I saw didnt show the plc at fault per se, more of an example of what can happen when X device fails.

 

[elevmike]

rsdoran,

I will admit that I may have misunderstood the intent of your post.

The origional poster quoted me from another forum. The examples are real.

The input in question was on due to and internal failure on the PLC input module. The terminal strip was removed and the input stayed on, no reverse feed. This may seem strange but it can happen.

You are absolutly correct. Its not the fault of even faulty equipment, that a machine is allowed to operate in an unsafe manner. It's the implemintation and design of how the components are used. That is why I made the origionl post on the other forum.

If the programmer can make a machine unsafe by his program alone, then somebody has to go back and look at the over-all design.

In the cases I sited the designers used the PLC as a safety device, or a device to assure safety. This was an improper use of the PLC.

I really think were all on the same page.

Regards, Mike.

 

[Eric Nelson]

In case anyone is not convinced...

Here's proof that some people are just plain STUPID!...
[attachment]
beerchug

-Eric

 

[elevmike]

Now thats innovative.

You gota hand it to a bunch of guys who are going to get the job done...event if it kills them.

 

[BobB]

Here are a few more safety at work ones for you

 

[SAK-CO TECH]

Too Cool!!

Eric, this would be safer if 5 guys were hanging off the back of the bottom forklift for a little extra counterbalance! Been there, done that!

Bob, that was simply a great slide show. This one will be saved!

Regards,
Chris

 

[Rick Densing]

Not to take away from Bob B's other pictures, but the one of the hazmat guys and the man in the shorts is a fake.

 

[Rick Densing]

This guy has some good pictures.

http://www.driedger.ca/dp/DP.html

 

[PLucas]

6 die in a Tragic accident!

Although this photo has nothing to do with 'safety at work', it is still a tragic accident that should be avoided at all costs!


Paul

 

[rsdoran]

I dont blame that guy, I would be crying in my beer too. He should try the Homer Simpson method to clean up.

 

[elevmike]

It could have been much worse.

The victims could have been John Labbat "Extra Stock"! A fine 8.5 Canadain Brew that must be transported manually from across the river under the cover of darkness via speed boat.

I've never done that, by the way.

 

[Sleepy Wombat]

Iv'e always liked this one also...