PLC Control and Stop, not E-Stop, buttons

[PeterW]

Well here in the UK that would be considered as poor practice. Any safety devices that have been operated should not automatically restart, when reset, There should be a separate reset!! I.E through a pilz relay or the plc.

He's not talking E-Stops.

Over here I've never seen a Pilz or equivalent, not saying they are not used, just every place I have been they do not use them, they don't even use positively guided contacts, just bog standard relays.

The first job I worked on, at an airport in Canada, they regularly pressed E-Stops to walk on conveyors to unjam the system, when getting off the conveyors restarted when pulling the E-Stop out, not even a warning sound, whoom, started.

 

[jude]

Re PeterW's comment. At our plant in Canada we had a piece of equipment imported from the USA 2 years ago inspected and passed as conforming to code for safety and electrical, earlier this year we had an accident and when the Ministry came in to investigate they issued an order that we had to comply to the current regulations which required a lot of work and cost

 

[Stephen3776]

Okay just to answer the E-Stop comments NFPA 79 clearly states resetting the E-Stop cannot restart the machine.
On the issue of my question a little investigation reveals there is no other stop button for the machine, thus it fails by not having at least on catagory (0) or catagory (1) stop. Thus, I will know tomorrow if I can change the logic to incorporate the aux. contact as a latch for the motor and make the HMI pushbuttom momentary.

 

[tracer]

Dunno if this is relevant to the description, but I have several older machines that the original estops were maintained when pushed, but on one, someone had replaced the maintained e-stop with a springloaded estop in one spot and a red push button in another, in either case, the machine would stop when either button was pressed, but it wouldn't stay in e-stop condition if either PB was released, luckily nothing on that machine would resart automatically,

 

[gadge]

With an S7 system are software E-Stops allowed (in the UK)? Im trying to find a reg to confirm this?

 

[L D[AR2P#0.0]]

Yes, look at the IM/315/7/9F CPUs + associated fail-safe I/O

 

[Jeff23spl]

can it be a poor pratice where a maintained push-pull button retain the stop function were replaced by a spring return one ?
Or sometimes when 2-3 remote device control the same equipement with improper mix of button and control philosophy, it gives bad results ?

 

[daba]

Time and time again I come across situations where regulations are, shall we say, "interpreted" in different ways depending on different circumstances.

I'll admit to being unfamiliar with the regs named in this thread, but it is my considered opinion that if they can be "interpreted", then they must have been written incorrectly, or badly !

Now I don't believe these organisations would go to print with "woolly" documents, I can imagine the man-hours spent in the creation, checking, and verification of the material. I may be way off the mark here, but perhaps there's a tendency towards cost-saving "interpretation", rather than "adherance".

OkiePC said "CYA", and here in the UK that's prevalent. Legislation here puts the onus of responsibility on the individual, not the company. In severe cases, an individual may be brought to trial, to explain his actions, even though he may have been "following orders".

PeterW's comment "...you would not have to change.." is contrary to the nature of "safety in the workplace". Just because a machine or process met regulations when it was built, does it make it safe?.

An analogy - Vintage and Veteran cars in the UK still have to go through an annual test for "roadworthiness", even though they were built before the tests were introduced, they are not "exempt" from being tested. Admittedly, some of the tests are excluded, but the remaining ones ensure the vehicle will be "safe" on the road.

A factory manufacturing foodstuffs has to comply with current regulations, at all times, to ensure the safety of the consumers, why shouldn't all manufacturing environments have to comply with current regulations to ensure the safety of their workforces ?

IMHO - To CYA, if you have safety concerns, you must raise them with superiors, in writing, ensure you get acknowledgment of receipt, again in writing....

Safety is everyone's responsibility, not just the people who sign the pay-cheques (checks to you guys across the pond), or who pay for equipment.

Apologies if anyone finds this view controversial.... just my 2c

 

[muttleydoo]

Would you consider a stop button as a safety device? After all a safety device must not be controlled by the plc logic but must be hard wired only using appropriate devices. As the stop button or pause button whatever you may call it is logic controlled, then as long as the machine is correctly guarded as it should be then I see no safety regulation has been breached. The operator simply needs to know by means of training what the machine control functions are and how they behave.

If the machine safety has been correctly configured them there should be no movement possible with the guards open.

 

[daba]

It all depends primarily on what that particular button "looks like" and less importantly, how it is labeled.

Absolutely right about the guarding and such, but if "safety" relies on the operator holding the button in, then its a no-no

 

[OkiePC]

All machines have stop buttons, and this machine has an unsafe one. If this machinery is capable of causing injury, then its wrong. You can go to court to defend your non-action, or you can fix it as soon as possible. Send your boss an email outlining the violations to CYA, then it's on them, but for the sake of the actual fingertips involved, hang your freaking padlock on the machine and tell it's going to be down for an hour, and when you power it back up, explain how the new reset button works.

What are they going to do, make you put it back? Refuse.

I have been in this situation, and kept a machine down over 24 hours. It didn't harm my career at all.

 

[daba]

Well said OkiePC - you got my vote, now come to the UK and shake-up our government, lol

See, i can do long posts, and short posts....hehe

 

[Stephen3776]

I am making progress, got the need to hold the stop button in to maintain a stop removed from teh logic. The basic problem lies in teh PLC "Run" command being a maintained contact in the stop string and not a momentary contact in the start string. IMHO both contacts are needed.
So a partial victory on this on, unfortunatley, new contrustion using HOA switched in lieu of pushbuttons and the HOA are complete PLC inputs to a ControlLogix L63. No hardwired safety in the motor starter circuit except for the disconnect on the bucket. Tomorrow I am replying with an alternate wiring diagram.

 

[PeterW]

PeterW's comment "...you would not have to change.." is contrary to the nature of "safety in the workplace". Just because a machine or process met regulations when it was built, does it make it safe?.

Does it make it 'unsafe'?

If something is unsafe then it should be changed ss a matter of course, but just because something does not comply with existing standards does not automatically make it unsafe.

As mentioned, unless its changed, in the UK you do not have to change your plant everytime the regs change, you don't have to re-wire your house either.

I was involved in a couple of projects that moved production lines from London to the North of England, in both cases the equipment was put in plants that had equipment of the same age and complied to the same standards. The moved equipment was updated, the rest was not.

One of thess plants was the largest food factory in Europe, it would have cost millions to upgrade the entire plant.

But if something was unsafe, you bet it was changed.

 

[Stephen3776]

I am not sure how anything completley dependent upon a PLC to start and stop can be considered safe. The input module or the output module fails and there is no stopping the machine without going to the MCC and throwing the disconnect.