# Normally closed Emergency stop



## McClary’s Electrical

123 said:


> Emergency stops have always been N.C.
> Now they are, depending on the danger level dual channel N.C.
> The issue I see is that they are not fail safe. When an e stop fails,
> it remains closed, and you have to run to main disconnect to switch the power off.
> E stops do fail. The most common is that the contacts detach from the body,
> but the actuator part can also fail.
> To correct the fail unsafe condition, E stops have to fail to open position.
> They have to be N.O.
> To do this, the contact block will have to have 3 positions:
> 1) stable open (actuator not pushed)( detached contact block or failed actuator)
> 2) unstable closed (actuator pushed half way)(contact block installed on the actuator, but not pushed)
> 3) stable open. ( actuator pushed all the way by e stop pushed in).
> Imagine a ball on a top of a triangle. it has 2 stable positions left and right. The ball will stay
> on the top of the triangle only if it is held there.
> It is not hard to make it. I have a design.
> Some manufacturers have tried to deal with these problems.
> Their e stops are still N.C.


 
E stops should be NC so a broken wire in the circuit is immediately noticed.


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## nolabama

E stops are NC


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## MDShunk

Sounds like a solution to a problem that doesn't truly exist with enough frequency to require effort to solve.


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## RIVETER

The typical e-stop device IS normally closed, as most have said. There are other devices that have redundant connections that are supervised but I believe that they are NC, as well.


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## John Valdes

E-Stop can be NO or NC. It is up to the author of the schematic drawing to make the E-Stops the way he wants.
NO or NC contacts can be used. Many PLC programs utilize NO contact for stop and E-Stop. Contact can do most anything you want, so pick.

OP. Schematic diagrams are not written in stone and no circuit has to be wired like you said.
While an E-Stop may be required, there is nothing to stop or prevent someone from using either a NO contact or a NC contact.
Fail safes take on many shapes and forms. Mechanical or Electrical.
Fail safe is anything that can be done to prevent unwanted starting, or to prevent starting or running. How one goes about providing a fail safe is up to the author of the drawing.
If you are working without a drawing, you either need to get one or draw one yourself.
To limit a function to one type of contact is a mistake.


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## 123

MDShunk,
I have seen estops fail only few times. Overall it is not a big problem.
The guard switches with actuators also didn’t improve much.
Everybody has a cheater actuator in a pocket.
It is more about the logic. The mechanism of the e stop is not fail safe.
Some posters misunderstood the functionality.
The proposed e stop is still closed, and when pushed it opens.
but when it fails it goes to open state.
RIVETER, I have seen the one from Allen Bradley.
It has a contact to detect the actuator body. It requires monitoring circuit.
My idea is simpler.


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## McClary’s Electrical

John Valdes said:


> E-Stop can be NO or NC. It is up to the author of the schematic drawing to make the E-Stops the way he wants.
> NO or NC contacts can be used. Many PLC programs utilize NO contact for stop and E-Stop. Contact can do most anything you want, so pick.
> 
> OP. Schematic diagrams are not written in stone and no circuit has to be wired like you said.
> While an E-Stop may be required, there is nothing to stop or prevent someone from using either a NO contact or a NC contact.
> Fail safes take on many shapes and forms. Mechanical or Electrical.
> Fail safe is anything that can be done to prevent unwanted starting, or to prevent starting or running. How one goes about providing a fail safe is up to the author of the drawing.
> If you are working without a drawing, you either need to get one or draw one yourself.
> To limit a function to one type of contact is a mistake.


 
While what you are saying is true, it is not very common at all to have an e stop circuit NO. Reason being if a wire somewhere out in the field becomes broken, it will go undetected for months or years, until someone pushes an e stop and nothing happens.


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## 123

John Valdez,
Estops have to open a control circuit. 
Closing a circuit is not an option. 
I know, it can all be fixed in a PLC, but if a wire falls of, or a fuse blows,
the system failed.
Also Etops are no longer wired to PLC unless it is a safety PLC.
Most go to a safety relay.


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## Big John

The mode of failure you describe is very rare. Much more common is open circuit, so in that regard, these circuits are fail-safe. If contact welding is that much of a concern, NFPA 79 already addresses it with "positive opening contacts" that will shear open with enough force applied so that they also fail safe.

-John


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## Jabberwoky

Many of the operators in the factories I have worked in don't realise the E-stop is not this all powerful red button that stops everything all the time. I had to eventually start showing them the $3 part that holds the brakes on while they work underneath 40-50 tons of steel. There is a reason we chock wheels and brace jacks, things fail.

I think energized E-stop lines are required in this state.


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## wildleg

as long as the initiating circuit is a NC loop, I don't think it much matters IMO. It's a simple process to build a supervised loop, fire alarm manufacturers have been doing it for years at the great expense of an EOL resistor and "comparator" circuit. So if the application is critical enough, introduce a supervisory ckt and all is good. No need to redesign the wheel.


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## MDShunk

wildleg said:


> as long as the initiating circuit is a NC loop, I don't think it much matters IMO. It's a simple process to build a supervised loop, fire alarm manufacturers have been doing it for years at the great expense of an EOL resistor and "comparator" circuit. So if the application is critical enough, introduce a supervisory ckt and all is good. No need to redesign the wheel.


There's this whole issue of NFPA-79. You don't get so many choices.


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## wildleg

I'm not downloading 79 to figure this out, but why would the addition of supervisory in an estop ckt violate any of the provisions ?


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## MDShunk

wildleg said:


> I'm not downloading 79 to figure this out, but why would the addition of supervisory in an estop ckt violate any of the provisions ?


Unlike NFPA-70, NFPA-79 is not a permissive document.


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## John Valdes

Pardon my lack of knowledge regarding all the newer safety regulations.
I do understand the reasoning behind a series circuit with one or more NC contacts in it.
My only point is that you can facilitate a stop circuit with a NO just as easy as you can with a NC contact.
I have engineered many circuit that utilities both. Shut down and annunciation. (horn or light).


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## MDShunk

John Valdes said:


> Pardon my lack of knowledge regarding all the newer safety regulations.
> I do understand the reasoning behind a series circuit with one or more NC contacts in it.
> My only point is that you can facilitate a stop circuit with a NO just as easy as you can with a NC contact.
> I have engineered many circuit that utilities both. Shut down and annunciation. (horn or light).


True enough. This stuff has just been regulated to the hilt just within my career so far. When I started, safety PLC's didn't even exist. Now, if you do your safety logic with ordinary products not specifically safety rated, you're doing the wrong thing. Bunch of crap, if you ask me, but someone once told me to think of every safety code as being written in someone else's blood.


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## JRaef

The issue of the contact block assembly disconnecting from the operator and staying closed has been addressed by several mfrs, A-B being one of them. There is now a special "Self Monitoring" contact block assembly which has a special NO contact that is wired in series with the NC contact. If the contact assembly gets broken off of the operator, the NO contact will go to an Open state even though the NC contact does not go Open, thus maintaining the 'fail safe' concept (such as it is). This can be used in a hard e-stop or an e-stop circuit monitored by a Safety Relay or Safety PLC.

http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/800-pp001_-en-p.pdf

I know that Siemens has something similar too, but only for their 22mm devices, this is available for 22mm and 30mm from A-B.


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## 123

I want to repeat, that when I talk about an Estop needing to be N.O.
I don’t mean a normal N.O. contact.
This would have to be 3 position contact block. I call it N.O.,
because on a shelve, it is open. When it gets connected to the actuator,
it is closes, and when estop is pushed , it opens. It is closed only in the middle of the stroke.
I know that stuff that deals with this out there, but not as simple.
To use this in a circuit, you unscrew the old block, install new, reconnect wires, and done. No need for supervisory circuit.
We went overboard in safety in the last 15 years, with no significant overall
improvement. Safety existed before.
When we design automated machinery, the fail safe rules is carried through.
Power, compressed air, everything is considered. 
I have only seen this failure 3 times, so not a big issue. 
But the fix is easy. I have talked to a major electrical manufacturer, and they were not interested.


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## McClary’s Electrical

123 said:


> I want to repeat, that when I talk about an Estop needing to be N.O.
> I don&#146;t mean a normal N.O. contact.
> This would have to be 3 position contact block. I call it N.O.,
> because on a shelve, it is open. When it gets connected to the actuator,
> it is closes, and when estop is pushed , it opens. It is closed only in the middle of the stroke.
> I know that stuff that deals with this out there, but not as simple.
> To use this in a circuit, you unscrew the old block, install new, reconnect wires, and done. No need for supervisory circuit.
> We went overboard in safety in the last 15 years, with no significant overall
> improvement. Safety existed before.
> When we design automated machinery, the fail safe rules is carried through.
> Power, compressed air, everything is considered.
> I have only seen this failure 3 times, so not a big issue.
> But the fix is easy. I have talked to a major electrical manufacturer, and they were not interested.


Ok, we get what you're saying now. It wouldn't be so hard to understand if you'd use correct terminology. The correct term in this case is " N.O. mechanically held closed"

There's even a symbol designation for that.


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## hardworkingstiff

mcclary's electrical said:


> E stops should be NC so a broken wire in the circuit is immediately noticed.


Curious, in a kitchen, is the ansul switch considered an E-Stop for the kitchen equipment? If so, then the typical way of feeding a shunt-trip breaker with a NO contact would not meet your requirements.

I'm not arguing with you, just pointing out something that I think is ..... well ..... not quite the way I think it should be.


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## BBQ

hardworkingstiff said:


> Curious, in a kitchen, is the ansul switch considered an E-Stop for the kitchen equipment?


Not that I am aware of, NFPA 79 would not apply to that. (At least I don't think so. :jester

I would call that switch an EPO, emergency power off.


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## Jlarson

I can probably count on my hands the number of E stop failures I've had to deal with over the years. 

And most of those are due to some tard busting the whole button up with a crane hook, forklift ect.... :laughing:


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## nolabama

Jlarson said:


> I can probably count on my hands the number of E stop failures I've had to deal with over the years.
> 
> And most of those are due to some tard busting the whole button up with a crane hook, forklift ect.... :laughing:


I have had several on one circuit. Finally food the problem when we were working with divers.


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## Peewee0413

? Don't over think


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## [email protected]

Odds of E-STOP ,, Failing,, are ,, Billion to one,, 
if you do the Daily, Monthly ,, Safety ,, checks AND inspections ,,OR When operator change and shift changes inspection
AS REQUIRED ,, OSHA ,,!!!! 
all All documented ,, as required OSHA 
But if concerned add stack block ,, to your e-stop ,,
odds both failing be about 3 BILLOIN to one,,


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## DIRT27

[email protected] said:


> Odds of E-STOP ,, Failing,, are ,, Billion to one,,
> if you do the Daily, Monthly ,, Safety ,, checks AND inspections ,,OR When operator change and shift changes inspection
> AS REQUIRED ,, OSHA ,,!!!!
> all All documented ,, as required OSHA
> But if concerned add stack block ,, to your e-stop ,,
> odds both failing be about 3 BILLOIN to one,,


All systems should go to a fail safe state with lose of power or energy. That is a the purpose of using E stops that in the event of loss of power in the circuit or pushing an e-stop it will shut down the system.

Most failures you will see are going to come down to lack of proper maintenance or modifications done to the original design. I have heard of one major failure in front of an inspector. 

All systems should be properly maintained and tested.


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## Frank Mc

Have come across this type of e/stop several times ..were if the contact body becomes detached from the button it goes n/o....Personally i think its a good idea...

Frank


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