Allen Bradley select switch

[jraef]

Agree with the sentiment above, but if there's any risk of harm involved, selecting a PB over a switch is not even close to an acceptable risk mitigation strategy. As far as a control measure goes, that's so far down the list it's not even worth a mention. The start/stop control should only ever be for control, safety should be taken care of by a risk assessment and proper procedure - eliminate, substitute, engineer, administrative, PPE. An e/stop comes after ALL of that, and then your start/stop controls even later.

No argument really, that is sound advice. BUT, there IS a reason why extended head and mushroom head buttons exist, pre-dating all of the other more specific safety strategies, going back to the KISS* concept. In fact the nexus concept actually goes back to when, in the dawn of automation when I started out with the original "PLC" from Allen Bradley in the 1970s, we began using 2 wire control vs 3 wire control. There were arguments made as to the entire concept of a selector switch being the only local Stop method as being inherently unsafe from the standpoint of what people will look to do in an emergency situation. You can design in all the safety logic and preventative measure you need, but sometimes it comes down to basic human responses to unforeseen circumstances. That's when KISS pays off, because our "lizard brain", the amygdala, is what makes us take emergency action, and it is in essence an interrupt function, bypassing all higher logic as much as possible so as to not sacrifice speed. I can probably train a lizard to slap his tail on a button to take action to get a reward, but I can't train it to rotate a switch, let alone in the correct direction.

* For those who don't know, KISS means Keep It Simple, Stupid or Stupidly Simple, depending on where you heard it first.

 

[TConnolly]

Another option to consider if you are trying to stuff two functions into one hole is to use a single momentary button and program the PLC with a flip flop for push to start, push again to stop. Flip flop programs are a common meme here on the forum, a search will turn up lots of examples.

On the subject of safety all I'm going to say is that it is your responsibility to evaluate the suitability of any control scheme, be it selector, push/pull, or flip flop. Other than that I'm just offering alternatives.

 

[ASF]

No argument really, that is sound advice. BUT, there IS a reason why extended head and mushroom head buttons exist, pre-dating all of the other more specific safety strategies, going back to the KISS* concept. In fact the nexus concept actually goes back to when, in the dawn of automation when I started out with the original "PLC" from Allen Bradley in the 1970s, we began using 2 wire control vs 3 wire control. There were arguments made as to the entire concept of a selector switch being the only local Stop method as being inherently unsafe from the standpoint of what people will look to do in an emergency situation. You can design in all the safety logic and preventative measure you need, but sometimes it comes down to basic human responses to unforeseen circumstances. That's when KISS pays off, because our "lizard brain", the amygdala, is what makes us take emergency action, and it is in essence an interrupt function, bypassing all higher logic as much as possible so as to not sacrifice speed. I can probably train a lizard to slap his tail on a button to take action to get a reward, but I can't train it to rotate a switch, let alone in the correct direction.

* For those who don't know, KISS means Keep It Simple, Stupid or Stupidly Simple, depending on where you heard it first.

I think your trained lizard would get along well with the trained dog from the "my descriptions are in italian" thread

 

[Lancie1]

i just need to control a starter.

The widely-accepted commonly-used method is a normally-closed STOP pushbutton and a normally-open START pushbutton.

 

[angi]

thanks guys

the solution for me at this point is to change the Cam.

 

[gbradley]

You got a lot of good advice here.
And you choose to ignore it.
At least you came back and posted your plans.

 

[angi]

I did not ignore anything, changing Cam is the only thing we could do.