E-Stop Circuit Wiring

[corkers]

We use a system of production stops as well as emegency stops. Production stops stop the facility they relate to eg the conveyor and where an estop would stop all conveyors in sight.

The production stops are push to stop pull to start types and generally we use blue to identify them.

The estops are latching twist to reset type with a seperate reset button which is use to reset the estop to the operational state. The start PB then needs to be pressed to restart the process.

A master reset PB is also installed on the MCC that allow us to reset all the estops (provided they are not activated) from a central location.

 

[jshiepe]

Thanks to all those who replied. I eventually forced one vendor to change to the killing-of-output-power practice that I am accustomed to, and he shamefully admitted that he also agreed with that practice and should have caught it early on. I'm not sure if this was just to prevent an argument with a customer, but the bottom line was that I am the customer, and that was a potentially unsafe situation. I also plan on pointing it out to the other vendor who will hopefully change it without a lot of reluctance.

For me, it's just added insurance. When you get a phone call at 3AM and log on to the company network from home to troubleshoot a machine, you don't want to have to worry about hurting someone because in your state of groginess, you forgot that you were working on the one machine in the plant that doesn't have it's outputs de-energized during e-stop. Although we take all the necessary precautions like having a spotter right at the machine, why add another element of risk?

Thanks,
Jeff

 

[JesperMP]

... they want to automatically restart the process after someone breaks the light curtain or opens a safety door. My answer is always the same also NO, NO, NO.

Why not restart the proces when the light curtain tells you that it is safe to do so ?

 

[LJBMatt]

Why not restart the proces when the light curtain tells you that it is safe to do so ?

There are too many gotchas that can occur. In normal running operations that can be a very valid and safe condition for restart. But what if 2 maintenance guys get into the machine, don't lock out, and need to pull the light curtain to work on the machine. They lay the one guy working on the machine and the other guys sets the light curtains so close to each other they send a clear signal to the PLC. I've heard a few stories like this where people get hurt. In the US this can end up as a lawsuit.

Stamping presses are another good example for not restarting on a light curtain clear. Maintenance breaks the light curtain to work inside the bolster and slide. Inside that far he is clear of the light curtain. Light curtains can only detect when something or someone has entered or left, but can't distinguish between the two.

 

[JesperMP]

LJBMatt

are you saying that you know of people that use E-stop circuitry as a means to power down a machine for maintenance work !!!
This is NOT safe procedure !
E-stop or safety stop is not meant for powering down before major maintenace work. Lock Out Tag Out MUST be used.

Your 1st example of people actively fooling the safety system only shows that proper guidance, manuals and safety signs are very important. If people want to disregard the instructions, then I dont think you can be blamed (but you must have proof that you have provided adequate safety instructions). The example you provided doesnt just apply to light curtains, it applies to any safety system.

Your 2nd example of a persons entering a machine so far in that he is no longer covered by the light curtain IS certainly only applicable if there is an acknowledge button (to reset the safety stop).
But this may only be used for minor work such as removing a jammed piece or to insert a new piece, not for work that requires longer time to finish. For major work the LOTO principle applies - allways.

My company supplies equipment that require regular maintenance and where people have to enter the machinery completely. There is a special doorlock for entering the machine. There are two keys in the lock - only one can be removed at a time. When the door is locked key #1 can be removed and inserted in the associated main switch for the machine thereby releasing it to be turned ON. When the door is opened key #2 can be removed an the maintenace guy can put it into his pocket. without key #2 the door cannot be closed and key #1 cannot be removed. Everything to ensure that no one closes the door and starts the machine with someone still inside.

 

[Ken M]

I believe you should always have a definite signal to the controller that it is now OK to start/restart the process. Just assuming that because there is no observable reason to stop, therefore it must be OK to restart is not adequate. "The absence of evidence is not the same as evidence of absence." Just because you can't detect the maintenance guy's head between the jaws of the vice doesn't mean it ain't there.

Ken.

 

[JesperMP]

I must clarify that when i said it is OK to restart automatically, that it only applies when a person cannot enter the machine completely.

And I must restate that maintenance equals lock out.

 

[LJBMatt]

are you saying that you know of people that use E-stop circuitry as a means to power down a machine for maintenance work !!!

Yes,

In many factories, large and small there are many maintenance men who follow lock-out procedures before servicing any equipment. Then there are others who don't want to take the time to power down machinery, especially on a small or quick fix breakdown. It's laziness pure and simple, but it's human nature and it happens. That's just reality.

 

[JesperMP]

I have to admit that I have also just relied on the safety stop when adjusting a limit switch for example.
I guess there is a grey zone between minor adjustment and major work where people take a chance when they should not.

 

[LJBMatt]

It's a situation where the procedure will keep people safe from harm 99.9% of the time. But you and I know in reality that people can get a false sense of security and do dumb things. That's why I usually don't see machinery start up without a repressed of a pushbutton.

 

[Jeffersonian]

That's why I usually don't see machinery start up without a repressed of a pushbutton.

I agree completely. There has to be some positive break of control power to motion outputs for an E-stop to be a true E-stop. I don't want a system where any bonehead who knows how to insert a branch into a rung of logic can cost someone his hand, leg or life.

 

[CaseyK]

I have to agree with corkers, that even when using a maintained e-stop, that a pushbutton must be pressed before the machine will run.

I have seen push-pull buttons, that when the head is pulled out, the machine starts running again. I do not like this idea.

I have had some machines where if the light curtain was broke, the machine would not run until they pulled their hand (etc) out of the way. THIS IS ACCEPTABLE WHEN NO HAZARD IS POSSIBLE!

I like to put an input to the plc from the MCR or e-stop circuitry, to interupt the PLC, IN ADDITION to the MCR killing power to all the outputs.

I also like to have only one (sometimes two) reset button, typically at the operator station and/or main panel.

Someone programming a PLC online, possibly remotely, should not be able to turn on an output causing an air solenoid or ??? to operate.

SAFETY FIRST, (and Second, and Third).

Regards to all.....casey

 

[JesperMP]

I have had some machines where if the light curtain was broke, the machine would not run until they pulled their hand (etc) out of the way. THIS IS ACCEPTABLE WHEN NO HAZARD IS POSSIBLE!

Dont everyone see that a light curtain is a lot safer than an E-stop button or two-hand control or whatever (pending that a person cannot pass the light curtain completely with his body).

Unless you go to drastical measures you cannot fool the light curtain, and you cannot forget to activate it.

 

[The_Wanderer]

I would have to agree that a light curtain would be good enough to start a CYCLE once the machine was properly reset. We have done this a few times-- the machine would need a reset button to initiate itself after an e-stop or power down, but once all was okay, if the machine saw that it had the components loaded and the light curtain was clear, the assembly cycle would start. This was a small machine with no chance of anyone being completely through the light curtain and no great danger anyway.

 

[ejessen]

I wasn't going to comment on this, but on the other hand, that is how the forum keeps being interesting

First of all, E-Stop is supposed to be the last way out of trouble - and the first! Meaning that it must overrule any other control device, but it must only be operated when an emergency has occured. An ordinary stop or a quick stop button must be the normal way to operate.

E-stops must be marked so you are not in doubt, what machine will stop.

A machine's safety must be evaluated by risk assessment and categorized on bases of that same risk assessment. It is not good enough just to select the highest and then skip the risk assessment - you have to document, that you above all have made the machine as risk free as mechanical possible (within resonable limits).

The result of the risk assessment will then determine the minimum level the E-Stop circuit (and other safety circuits) must live up to. The safety circuits are then wired to the PLC to inform of their state.

For E-Stop, one level of safety limit you to just break the power to certain output modules, others require breakers in the power circuits (here one can choose to also break the power to the output modules).

Be aware that there are even requirements to how to reset and what needs to be hardwired in the reset circuit.

Conclusion: Risk assesment first. Safety circuit design second. PLC programming third.