Maintenance Stop Alternatives 2

[Matt A]

So I have a question about a problem I am trying to solve. Currently, an eight contact push button is being used to stop machines in case of a fire or other catastrophe, however there are not enough contacts for all the machines that need to be turned off in such a situation. I need to know if there are methods that I can research that would be able to allow for more machines to be connected to a maintenance stop without the use of relays. All the devices that will be turned off by this push button involve three wire start stop circuits.

Thank you,

Matthew A.

 

[itsmoked]

Do tell why you can not use relays??

You're having a problem because you're not using the solution everyone else does.

I'd use as many relays as needed and run the power for them thru the E-Stop. E-stop pushed power gone, no-way for the relays to remain ON. Any wires open again it stops, fail-safe.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Matt A]

The person I am working for doesn't want to use relays simply because they need to be powered. He wants me to look into alternative solutions like stack-able contacts or shunt trip breakers. I'm having a hard time researching and coming up with anything relevant though. Thank you for the help so far.

 

[3DDave]

If there's a disaster then why not go farther upstream and cut the connection there? Power has to come into the building somehow and there will be an interrupter there. If it's manual, replace with a remotely operated one.

 

[Matt A]

Good idea, but my employer only wants certain shops shut off by this maintenance stop each one installed at a shop.

 

[jraef]

You are, in my opinion, skating on thin ice with this. Is this a machine safety issue, or a maintenance issue? If it is for machine safety, what you are doing is likely to get someone injured or worse. If it is strictly for maintenance purposes, and a full Lock-Out/Tag-Out power lockout routine will follow the big push button, then OK, but it doesn't sound like that.

As to your multi-contact buttons, what you are experiencing is the limitations of the physics involved in making push buttons work. Each set of contacts has an amount of force needed to not only separate them, but to push them together when you want them closed. So to make that happen consistently, contacts are "floating" on springs and the amount of movement of the actuator is absorbed by those springs. But with each contact set, there is a 'slop" in the movement and once you get too many stacked on top of each other, the cumulative slop results in the ones at the end not having enough movement to operate reliably.

That's why people use relays... If this is not a machine safety issue, then you can use what are called "mechanically latched" relays, they maintain their state regardless of whether power is available or not by virtue of having two coils, one that you energize to latch, another one that unlatches. Of course if there is a fire and power is lost, you can't operate the relays but then again, the machines would be powered down as well.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[itsmoked]

I feel for you Matt. Your boss is nutz.

We're talking about a couple of watts here. Turn the overhead lights off 30 seconds earlier each day to completely erase the power consumption of the relays. Oh wait, you're probably working in complete darkness.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[thebard3]

jraef is correct. NFPA79 requires a circuit which must be reset after returning to normal. If this could be considered a machine safety, and someone got hurt later, then OSHA would be ecstatic about doling out fines and prison sentences, which, I'm sure in hindsight would make the installation of a relay seem cheap by contrast.

BTW, as far as I know, there is usually no (mechanical) limit as to how many contacts you may add to most industrial pushbuttons (Allen-Bradley Bulletin 800 for example). I'm sure there is a practical limit, though.

Brad Waybright

It's all okay as long as it's okay.

 

[Matt A]

Thanks everyone for the advice. I am still waiting on a response, but I messaged my employer about this to further see why he doesn't want relays and maybe ask about mechanically latched relays as you recommended. There are, I'm sure, actual emergency stops in place. I think the purpose of these are maintenance stops in case of fires or other "catastrophes". Not sure why he doesn't consider those to be emergencies though...

 

[thebard3]

Another thing;
If this is to be considered a machine safety, it should NOT be used as a normal stop. If it is a normal stop circuit, then it doesn't have to meet the requirement of NFPA79. It should not be considered as both.


Brad Waybright

It's all okay as long as it's okay.

 

[controlsdude]

I think the OP is confusing maintenance mode with just the regular start/stop.

I have done maintenance mode conditions on machines which is just running motors individually for maint purposes.

There is a button either puts the machine to Automatic or Maint Mode.

When you toggle between conditions you stop the machine.
Push start again.
Now your in maint mode.
Then you usually have run/jog buttons for each motor or cycle part of machine.

Most of these maint mode functions are done thru a HMI due to amount of buttons that you need and programming that needs to be done on machines or conveyors.

Of course Estop and other safety devices still work and are not bypassed due to maint mode.