Does this switch exist?

[Friedandcrispy]

I need a momentary switch that will make contact when pushed but will break the contact immediately even if the button is held in, does such a switch exist?

It is for a safety system on a drilling machine and it currently has a three second timer that operates once the button is pressed but the button can be held down which cancels out the timer. What is the best way to achieve a triggering of the three second timer without being able to override?

 

[Adam W]

AFAIK no such switch exists, however there are various timer or PLC options available.

If you explain exactly what it is you're trying to achieve there is more than likely a standard solution available.

 

[Sidewinder]

Welcome to the forum.

Not sure exactly what you are looking for?

Is this for a diy project at home, or for a work job?

Normally, a start button would not be anything as clever as that without it running through a programmable device.

Can you explain what you are trying to achieve?

 

[Rob.]

I'd probably be looking at the timer rather than the switch. 

What type of timer is being used?

what do you mean when you mention safety? Be very careful in modifying a safety circuit as you will have to re work out the MTTF calculations and take responsibility for it. 

Sidewinders asked all the questions I was going to start with. 

 

[Friedandcrispy]

Thanks for the response guys, I am looking into this for a customer of mine. he has a mobile drilling rig that they want to improve the user safety on.

Basically there is a function on the rig that requires the operator to push a button which I presume loads the hydraulic system, this button starts a three second timer and allows the function to operate for that three second period before the system unloads. However, this button can be held down which basically overrides the timer and allows continuous operation which isn't desirable. I wondered if there was such a switch that would make contact when pressed but would automatically break the contact once made even if the button is held down, I'm sure there is a technical term for this.

I hope this helps explain a little.

 

[Andy™]

so youre looking at modifying a machine and the best you can come up with is you 'think' the button is for the hydraulic system. you are way out of your depth. dont forget this comes under machinery regulations. get it wrong and you will be in court

 

[Rob.]

Have you any schematics of this electrical circuit?

 

[kerching]

Have a very good look at your liability insurance if you are going to modify a safety circuit...it could get very messy

 

[Tony S]

A one shot timer is easy enough to find. Modifying a machine is a tangle of red tape.

 

[Barx]

Glue a drawing pin to front of button. They won't hold it in then......

 

[Tony S]

Then they'll just jamb a bit of wire in.

A constant bain of my life was sorting out the mess the production guys coursed. A one shot on every start P/B soon sorted that out.

 

[Sidewinder]

Friedandcrispy,

If you are going to redesign and modify the safety control system, then it must conform to EN 60204-1, and EN13849-1 & be validated against EN13849-2, unless it is a programmable system then it must be designed and validated against EN62061.

You have no options, on this as it is a requirement of law, your user has to comply with PUWER regulations, and if you are taking this on as a paying job, then you have to ensure that the work complies also.

This is fraught with liability that you may not be insured for, so please be sure that you are, and study the requirements of the standards first such that your solution complies with the legal requirements, no standards are not law, but these standards are cited by law & HSE ACoP's as their approved means of legal compliance, so, if you deviate, then you will need a raft of documentation to prove that this deviation offers the same level of safety as complying with the standards.

 

[Friedandcrispy]

Hi Guys, I should stress that this is all hypothetical at the moment and is nothing more than an enquiry so don't get too hung up on liability etc. I have no schematics, I have not seen the rig myself and have only discussed it verbally. If it is anything more than a switch then I'm out anyway as I'm a mechanical engineer not a sparky.

Any suggestions on what the most basic way to achieve this would be?

 

[Andy™]

timer would work or logo / zelio etc

 

[Rob.]

timer would work or logo / zelio etc

Depends what type of timer. 

An with it safety related, a logo or zelio wouldn't cut it dependent on the PL level required. 

 

[Andy™]

the way he describes it, it seems more of a start button than a stop / safety

 

[Lurch]

But there is already a timer on the end of this button so that would maybe hint towards what safety level would be required (although there's nothing to necessarily say that this is correct so it should only be taken as a guide).

There may well be an alteration or adjustment that can be made to the existing timer so before any solution is drawn up I would investigate what is actually there now.

 

[Sidewinder]

But there is already a timer on the end of this button so that would maybe hint towards what safety level would be required (although there's nothing to necessarily say that this is correct so it should only be taken as a guide).

There may well be an alteration or adjustment that can be made to the existing timer so before any solution is drawn up I would investigate what is actually there now.

Don't forget Lurch, many OEMs get it wrong, and I am working with a few at the moment!

If the timer is in a safety PLC, then the whole system will need re-validating against EN 62061 though.

Push buttons very, very often have a timer on them when in a controller as a debounce mechanism.

the way he describes it, it seems more of a start button than a stop / safety

Yes Andy, it does sound like a start function, but, one must ensure that it cannot under any, circumstance override a stop command.

Once you start adding timers, then this could be a possibility, which would then be a safety function failure.

 

[Sidewinder]

Depends what type of timer. 

An with it safety related, a logo or zelio wouldn't cut it dependent on the PL level required. 

Rob, with a Logo or Zelio, you would be in programale territory so SIL's and EN 62061, so I doubt they would honestly cut the mustard at all...

 

[Rob.]

Rob, with a Logo or Zelio, you would be in programale territory so SIL's and EN 62061, so I doubt they would honestly cut the mustard at all...

Fully aware of this SW, machine and process safety is a major part of my job as a system integrator.