Two Hand Startup

Have any of you guys ever came across a two handed start control just to start an automatic cycle - eg, like a two handed hold-to-run device but still operates once your hands are removed?

Have this one machine coming on site that basically has two pushbuttons going back to the PLC, no safety relay etc as they dont class it as a "safety related feature" - its "just to prevent accidental start-up" apparantly.

I dont like it - i cant see the point as it cannot be classed as a Type I, II or III system and it has a light curtain protecting the hazardous zone so whats the need for it?!


Oppinions please?

Brian Hamilton said:

Have any of you guys ever came across a two handed start control just to start an automatic cycle - eg, like a two handed hold-to-run device but still operates once your hands are removed?

Have this one machine coming on site that basically has two pushbuttons going back to the PLC, no safety relay etc as they dont class it as a "safety related feature" - its "just to prevent accidental start-up" apparantly.

I dont like it - i cant see the point as it cannot be classed as a Type I, II or III system and it has a light curtain protecting the hazardous zone so whats the need for it?! Oppinions please?

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Why does it have to fit into a so called safety niche to justify this feature?? It is your machine and if you satisfy the basic safety criteria then you can do anything you please. Two hand control to start it - well I can see the justification maybe. What happens to the machine or product if it is erroneously started by someone bumping the switch etc.

However as I tell my boss it is your equipment,,,,,,,,,

Dan Bentler

If you have another primary safety control (you have a light curtain) then two hand control is not required. However its also not prohibited. Its not at all uncommon to implement as you have describe, to prevent accidental startup, for example if the operator bumps one button or drops a tool on the button, etc. Accidental starting is never a good thing. The light curtain will protect limbs, but accidentally starting the machine might destroy tooling, which is something your primary safety control is not concerned with. Since you have a primary safety control, what specifically is it that you don't like? Is it using two hands? Or is it the fact that its wired to the PLC instead?

I wouldn't worry about it, the operators will probably find use for a matchstick soon after installation.

On some critical sections of Plant we tend to display an "Are You Sure ?" style pop-up to prevent accidental start-up. Don't see what the difference is between that and two start buttons. Unless of coures you have a one-handed operator.

PeterW said:

I wouldn't worry about it, the operators will probably find use for a matchstick soon after installation.

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Omron and PF come to mind for 2 buttons setup. Very common in the Punch press industry.

On the other hand, why don't you kick in a buzzer when the button is pressed and delay the start for about 3-5 seconds??
If it is intentional, the operator has to keep the button(s) pressed for the duration in order to get the machine going.
If not they can release the button and abort the start up prior to the duration timer expiring and avoid the accidental start up.
Once the machine is running cycle, cut off the buzzer.

What is the machine classified as? If it is a mechanical power press you may have some problem withe the AHJ or Local OSHA. I have read the OSHA book for hour and hours on press controls. They no longer allow two hand anti-tie down or anti-repeat in a PLC. The local insector said no to redundant plc unless they where red. I wanted to ask him is Mitz A series qualified? I thought it may have been better not to.

At the plant I work at we have a conveyor system like this. It works well in our applications. Makes sure that the operator is paying attention.

Brian Hamilton said:

Have any of you guys ever came across a two handed start control just to start an automatic cycle - eg, like a two handed hold-to-run device but still operates once your hands are removed?

Have this one machine coming on site that basically has two pushbuttons going back to the PLC, no safety relay etc as they dont class it as a "safety related feature" - its "just to prevent accidental start-up" apparantly.

I dont like it - i cant see the point as it cannot be classed as a Type I, II or III system and it has a light curtain protecting the hazardous zone so whats the need for it?!


Oppinions please?

Click to expand...

Hi Brian, what is the actual machine?
Can you get the machine suppliers to specify why there are 2 buttons?

I used to do lots of work on paper cutting guilotines; 2 hand control was very common. We used to retrofit older machines with some units from PILZ to meet the PIAC (HSE) guidelines (http://www.farnell.com/datasheets/35622.pdf), but you had to keep your fingers on for all of the dangerous part of the cycle. The unit required both buttons pressed within 0.5 seconds, and both released before allowing another trigger.

At the company I used to work for we had dozens of ram type machinery, punch presses, stamp presses, drill presses...etc. Every one of them had two button start control. And, they had me program the PLC's so the operator's couldn't tie, duct tape or matchstick one of the buttons down.

These machines also had other safety devices over time but were changed to light curtains once they became widely available. Still the 2 button approach remained.