E stop switch -- best design??

leitmotif

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Nov 2004
Location
Seattle Wa. USA
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At work we have Cutler Hammer "modular panel mount" switches that can have an E stop button attached.

When the button is pushed in (in E stop position) the contacts are open. When the button is pulled out (OK to run) the contacts are closed.

To do this requires using NC contacts.

IF the contact block becomes unattached from the operate mechanism the contacts would go to NC position allowing machine to run.

Somehow this does not strike me right. The most fail safe would be to use NO contacts and have them opened when in E stop position and shut when in OK to run. THis way if contact block was detached from operator the contacts would go to NO and shut machine down.

Am I thinking right here?

Copied the NFPA 79 as posted by someone else in a separate post.

Thanks for the feedback

Dan Bentler




9.2.2* Stop Functions. The three categories of stop functions
shall be as follows:
(1) Category 0 is an uncontrolled stop by immediately removing
power to the machine actuators.
(2) Category 1 is a controlled stop with power to the machine
actuators available to achieve the stop then remove power
when the stop is achieved.
(3) Category 2 is a controlled stop with power left available to
the machine actuators.

9.2.5 Operation.
9.2.5.1 General.
9.2.5.1.1 The necessary interlocks shall be provided for safe
operation.
9.2.5.1.2 Measures shall be taken to prevent movement of the
machine in an unintended manner after any stopping of the
machine (e.g., locked-off condition, power supply fault, battery
replacement, lost signal condition with cableless control).
9.2.5.2 Start.
9.2.5.2.1 The start of an operation shall be possible only
where all of the safeguards are in place and functional except
for conditions as described in 9.2.4.
9.2.5.2.2 On those machines where safeguards cannot be applied
for certain operations, manual control of such operations
shall be by hold-to-run controls together with enabling
devices.
9.2.5.2.3 Interlocks shall be provided to ensure correct sequential
starting.
9.2.5.2.4 On machines requiring the use of more than one control
station to initiate a start, the following criteria shall be met:
(1) Each control station shall have a separate manually actuated
start control device.
(2) All required conditions for machine operation shall be met.
(3) All start control devices shall be in the released (off) position
before a start operation is permitted.
(4) All start control devices shall be actuated concurrently.
9.2.5.3 Stop.
9.2.5.3.1 Each machine shall be equipped with a Category 0
stop.
9.2.5.3.2 Category 0, Category 1, and/or Category 2 stops
shall be provided where indicated by an analysis of the risk
assessment and the functional requirements of the machine.
Category 0 and Category 1 stops shall be operational regardless
of operating modes, and Category 0 shall take priority.
Stop function shall operate by de-energizing that relevant circuit
and shall override related start functions.
9.2.5.3.3 Where required, provisions to connect protective
devices and interlocks shall be provided. Where applicable,
the stop function shall signal the logic of the control system
that such a condition exists. The reset of the stop function
shall not initiate any hazardous conditions.
9.2.5.4* Emergency Operations (Emergency Stop, Emergency
Switching Off).
9.2.5.4.1 Emergency Stop. Emergency stop functions provided
in accordance with 9.2.5.3 shall be designed to be initiated
by a single human action.
9.2.5.4.1.1 In addition to the requirements for stop, the
emergency stop shall have the following requirements:
(1) It shall override all other functions and operations in all
modes.
(2) Power to the machine actuators, which causes a hazardous
condition(s), shall be removed as quickly as possible
without creating other hazards (e.g., by the provision of
mechanical means of stopping requiring no external
power, by reverse current braking for a Category 1 stop).
Reset of an emergency stop circuit shall not initiate a restart.
9.2.5.4.1.2 Where required, provisions to connect additional
emergency stop devices shall be provided in accordance with
Section 10.7.
9.2.5.4.1.3 The emergency stop shall function as either a
Category 0 or a Category 1 stop (see 9.2.2). The choice of
the category of the emergency stop shall be determined by
the risk assessment of the machine.
9.2.5.4.1.4 Where a Category 0 stop is used for the emergency
stop function, it shall have only hardwired electromechanical
components.
Exception: An electronic logic (hardware or software) system as well
as the communication network or link that complies with both 9.4.3
and 11.3.4 and is listed for Category 0 emergency stop function shall
be permitted. The final removal of power shall be accomplished by
means of electromechanical components.
9.2.5.4.1.5 Where a Category 0 or a Category 1 stop is used
for the emergency stop function, final removal of power to the
machine actuators shall be ensured and shall be by means of
electromechanical components. Where relays are used to accomplish
a Category 0 emergency stop function, they shall be
nonretentive relays.
9.2.5.4.2 Emergency Switching Off. Where the emergency
switching off function is used, it shall be initiated by a single
human action.
__________________
 
Allen Bradley makes NC contact blocks (PN 800F-X01S)that actually is a NC and a NO contact in series. When the contact block is installed, the NO is closed. When the contact block falls off, the NO contact opens stopping the machine. The NC contact is controlled by the operator
 
If you look at the example provided in NFPA they use N.C. contacts. The idea is to have a main control power source that is removed in an emergency.
 
leitmotif said:
IF the contact block becomes unattached from the operate mechanism the contacts would go to NC position allowing machine to run.



This is why I and others have a control button, opening or closing of contacts are not enough to restart a machine, so even though your run permissive is true (e-stops), your machine will not restart until you have reinstated the control loop and hit the start button
So if the block becomes unattached you still have a stop condition
 
geniusintraining said:
This is why I and others have a control button, opening or closing of contacts are not enough to restart a machine, so even though your run permissive is true (e-stops), your machine will not restart until you have reinstated the control loop and hit the start button
So if the block becomes unattached you still have a stop condition

That is a requirement of NFPA but I think Dan was thinking about a situation where the contact block may not remain attached to the PB; therefore it may remain in a "run" condition.

I made a short reply earlier because I could not think of a way to explain "WHY" normally closed contacts SHOULD be used. Everything I know tells me why but "I do not have the words to explain it" at this time.

May take me awhile but "I WILL BE BACK"
 
leitmotif said:
to run.

Somehow this does not strike me right. The most fail safe would be to use NO contacts and have them opened when in E stop position and shut when in OK to run. THis way if contact block was detached from operator the contacts would go to NO and shut machine down.

By definition normally opened means the contacts are open in a so called "out of the box state" ie.. nothing has acted on them. When your pushbutton is seperated from the contacts, the contacts would be open. when assembled the contacts are still open until the button is pushed. A regular pushbutton only has two states, not the three required to make what you are suggesting work mechanically.

However with something like a pull rope where the operator is held in between two positions by the tension of the cable you have a higher failsafe level. If someone pulls the rope, the contacts open, if the rope breaks the contacts open.

Sure hope this makes sense, it makes sense in my head but when I read it......
 
I use AB's safety contact blocks, which were mentioned by a previous poster. If the block comes off the actuator the circuit is opened.

Edit to add: The idec button looks intersting, although I don't like 22mm buttons much. I looked over its operating mechanism - a spring opened N.O. contact locked by a cam to the the button so that it operates like a NC contact. Clever. However I wonder, if the reason the contact block comes off the actuator is because the contacts are welded and it was the need for excess acutation force that breaks the actuator popping the block off, would that spring be able to open the contacts? I suspect not. The insulation rating of the switch body is kind of low too, although I use 24V for all of my E-Stop strings so thats not really an issue for me. I gotta admit though, the design is clever. Maybe I'll try one out.
 
Last edited:
always used n/c contacts but the e-stop was connected to the control power , that way nothing could restart unless you restarted it, e-stop would effectivley shut down entire machine, this is how it should be.
 
Dan,

I think that the ideal e-stop would be one that is 2 position maintained or "Push-Pull" type. It would have a N.O. contact that is pulled closed when the button is pulled out. That way if the contact block seperates from the operator mechanism, the small spring inside the contact block would push the contacts apart. This would open the circuit just as if the button had been pushed in. I haven't seen a button that pulls the contacts closed though. The only ones I have seen just push against the contact block, or pull away from the contact block allowing the contact to close under the power of it's own spring. Never really thought about it until now. I have seen the contact blocks fall off before, not on an e-stop button. Something to think about.
BD
 
bikerdude said:
I haven't seen a button that pulls the contacts closed though. The only ones I have seen just push against the contact block...

BD, it appears from the literature that is how the Idec switch that Kevin linked to above works.
 
Dan, so the problem is with contacts falling off the back, and disabling the ability of the mushroom head operator to open the circuit.

I have had that problem with several brand on regular pushbuttons and selector switches, though don't remember E-stop ever failing due to that, but I could have easily overlooked that possibility.

At my last job, we tried to use a one piece E-Stop switch for all the single string circuits. I think it was square D.

On the regular controls, I just made sure to always install spares on hte ones that only cover half the operator, and a little blue locktite when there was extreme vibration.

90% of my problems were when only half the operator was covered, and the two screws didn't control the side-flex of the single block. I think those were CH branded PBs, and some A/B too.

You have brought up something critical and easily overlooked that I need to remember to check at my current employment where red mushroom e-stops are the most prevalent type...

Thanks, Paul.
 
Thanks guys. Seems I have my head screwed on striat aveter all,.

The last place I worked the E stops were wired such that resetting E stop (to run OK) started the equipment. THis included saws. Bugged me to no end.

So at this job I guess I am most "qualified" electrician (there is only 3 of us). Just trying to ensure I do things right and make equipment as safe as I know how.

Right now and at least for the next six months my main goal is to attach those little green wires to the metal frame of equipment or to metal parts in a plastic box. I do not understand why people do not like hooking up the grounding conductor.

ANOTHER this is a food processing facility. Water everywhere and not a GFCI in sight (or on site may be more accurate). I know what I will get told when I ask "too many nuisance trips". AND there IS hot 120 volt exposed to employee contact.

Food plant guys care to share experience?? IE lemme tap your brain??

I think I have my work cut out for me.

Dan
 
leitmotif said:
ANOTHER this is a food processing facility. Water everywhere and not a GFCI in sight (or on site may be more accurate). I know what I will get told when I ask "too many nuisance trips". AND there IS hot 120 volt exposed to employee contact.

Food plant guys care to share experience?? IE lemme tap your brain??

I think I have my work cut out for me.

Dan

I use the AB safety PB contacts as well for e-stops. I used to work in plastic injection molding, and replaced all 120VAC outlets on the machines to GFCI because of all the water around that is used to cool the molds. I didn't want to take a chance and they learned to deal with the "nuisance trips"

Better safe than sorry!

my .02
 
One major problem you will find with electrical circuits in food grade plants is poor neutrals and ground bonds due to water so as a procedure I always check the ground to neutral feedback anytime it exceeds 1 volt I have the problem addressed any higher of a value then various electronics such as scales will start to fluctuate. Also try to make usre all conduits are mounted at the bottom of the electrical boxes and try to place them above the equipment to be washed down this will help control the moisture. On critical panels we placed a heater of some form whether its an electrical heater or a high wattage lightbulb to try and keep the cabinets dry. DO NOT OVERCOMPRESS THE SEAL GASKETS. I believe the torque on electrical cabinets gaskets is around 11 PSI if you overcompress them they do not stop the water ingression as well as they should. Our plant also has a similar problem of having the equipment grounds not in place suprisingly it doesn't seem to cause nuisance tripping. I would make sure any new equipment has all devices start witha positive voltage scheme as using 0 volts to activate a device can lead to false activation in the event of a ground short after the start button. I too have been adding GFCI's on critical equipment. But its slow going.
 

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