I could use some opinions regarding the use of Emergency Stop.

According to my interpretation of the NFPA 79, sec 3.52, a system of conveyors, loaders, etc, comprising an automated workcell would be defined as a single industrial machine.

Accordingly, since each "industrial machine" requires a category 0 stop circuit, would it follow that every piece of directly associated
machinery would have to be tied into a common e-stop circuit?

Or would a broader interpretation permit a long production line, for example, to be divided into zones of control, with each zone havings its own, distinct Emergency Stop circuit?

Consider the situation of a robot removing parts from a conveyor, and loading them into a machine tool, and removing finished parts from the machine tool, and loading them to the same conveyor. This fits the deninition of "a group of machines working in a coordinated manner" according the the NFPA 79. In this circumstance, an "Emergency Stop" pushbutton on the conveyor
would also have to stop the robot, and the machine tool-- correct?

Thanks very much for your input.
John Vales
Wes-Tech Automation, Inc.
[email protected]

 

John,

In my plant we run robotic assembly lines which have up to 13 robotic cells linked together. Each cell is approximately 5 foot square in area. There are also conveyors and ancillary equipment in the operation. We have a dual E-stop system where each robotic cell has a "local" E-stop and the line has a "line" E-stop. Buttons for each type of stop are available at every cell, enabling anyone working on a robot to stop only that robot or the whole line. There are additional E-stop buttons on all the ancillary equipment as well as at strategic positions on the line. We have been running this system
for quite a few years now without incident. The teach pendants for each robot also have a local E-stop button and people working on a robot tend to
poise their finger over this if they are doing something with an unpredictable outcome. This has worked OK for us and I know that it complies with UK legislation.

Regards,

Bryan Weir.

 

There are various levels of E-Stops. The most important thing to do when installing an E-Stop system is to CLEARLY identify what the E-Stop affects. Proper labeling plus an E-Stop zone map are important. The reason is that when someone hears a cry for help they may think that the E-Stop in front of them will shut down the machinery where ever the cry for help came from.
Common sense comes into play: an E-Stop for a packaging conveyor should not shut down the chemical reactor that is feeding the product to packaging: you don't want to disrupt the whole world. On the other hand - the more you can
shut down the safer you will feel plus the less chance of not pushing the needed E-Stop button: too many E-Stops can cause confusion. There are too many possibilities for NFPA or anyone else to put on paper. This is a situation where common sense, an engineering sense and your safety review
committee come into play.

- mill wide that shuts down everything.
- local area that shuts down an equipment group.
- individual equipment that shuts down one piece of equipment.

 

Good question John, and one which I don't think has a definitive answer (unless someone on the list can tell me otherwise). My opinion (not based on any legislation particularly) is as follows; Safety of personnel (and equipment) has to be the prime concern so if in doubt I think the golden rule is "stop everything". What I mean is that zoning systems can often be the most practical solution, but if it's possible to stop everything this eliminates any confusion amongst personnel as to "what stops what". I know this isn't always practical, so if you do decide on a zoning system then you should consider how people will be able to easily and quickly differentiate between zones e.g. colour coding, numbering systems, signs on the plant, training for operators etc. I think that as long as you come up with a sensible approach and document the
reasons why the system has been designed the way it has, plus you can demonstrate that there is a formal procedure of training for people expected
to operate the equipment then you shouldn't have too many problems. Good luck with it - I'd be interested to hear how you decide to do it.
Regards
David

 

If you would like to consider a reasonable approach to the situation you are describing, please check into the guidelines publishes by the the Robotics Industry Association. Also Pilz has several very good publications regarding the use of E-Stop relays.

Gene Wilkins
CALL Associates Inc.
888-323-2255

 

It is a matter for common sense.

Each application HAS to be evaluated on its own merits.

The results of the HAZOP and risk analysis will give you your answer. But remember that a project like this evolves, a common mistake is for a machine manufacture to do an assesment on the initial design, forgetting that other circumstances might impinge on the safety of the line. A machine that is installed safely in site A with one kind of interlocking and guarding may
require additional guarding and interlocking when installed in a different location and/or along with different plant.

Also, as has been stated before, you HAZOP/Risk Assesment will tell you whether or not the cat. 0 stop (which as you rightly note you must have
available) is safe to use as the main stopping method (and it can sometimes be extremely dangerous).

Getting back to your question, start by going round the plant and at each location ask yourself how is an emergency on a piece of kit likely to be
affected if the adjacent kit carries on running? what hazards arise from the integration of the unit machines?

 

I would suggest that the E-Stop stop the entire area. When in doubt, always put safety ahead of convenience.

It does get a little tricky with conveyors however, which zone does the conveyor belong to?

E-Stops should be absolute, you do not want an operator trying to decide which button to push in the time of an emergency.

Bill

 

If there an function associated between the robot and the conveyor, then you have to stop both the robot and the conveyor, which forms a group.


sudarshan.k

 

> There are various levels of E-Stops. The most important thing to do when
> installing an E-Stop system is to CLEARLY identify what the E-Stop affects.
> Proper labeling plus an E-Stop zone map are important. The reason is that
> when someone hears a cry for help they may think that the E-Stop in front of
> them will shut down the machinery where ever the cry for help came from.

I agree

> Common sense comes into play: an E-Stop for a packaging conveyor should not
> shut down the chemical reactor that is feeding the product to packaging: you
> don't want to disrupt the whole world.

If a conveyor and the reactor are in the same room, I think the Estop should also stop the reactor. What if the reactor overflows while the
operator is at the conveying station?

<snip>

Gilles

 

Not always. I've seen plastic processes where on a 100' process tower, it was not desirable to stop heating unless you were willing to get picks and shovels.

This scenario passed with a process stop mushroom type button, as opposed to a e-stop since there were no exposed moving parts. Of course, this process was not a reactor in the chemical sense, but rather a pass-through heating and curing element.

Anthony Kerstens P.Eng.

 

> We have a dual E-stop system where each robotic cell has a "local" E-stop and the line has
> a "line" E-stop. Buttons for each type of stop are available at every cell,
> enabling anyone working on a robot to stop only that robot or the whole line.
<Snippety snip snip>
>This has worked OK for us and I know that it complies with UK legislation. <

The problem here may be that European legislation say that in the case of an emergency stop there should be no ambiguity, two types of E-Stop, hmmm.

marc

 

I am not overjoyed with the idea of having two "estops" either. I suspect this could lead to confusion in case of emergency.

This may be a case where operator training better communication with operators might resolve the problem.

 

Sounds like he has a zoned E-Stop system. Each work cell has its own E-Stop and the line that they feed has and E-Stop which probably stops all of the cells.

 

I think you are fighting the nomeclature wars here. The Cell robot E-Stops, are not really Emergency Stops. They are local stop buttons, and should be marked as such. The only switches that should be marked E-Stop are the Line
shutdowns. This fixes your ambiguity problem without fighting the lawyers about verbage.

 

After reading a couple of these I agree. That suggests why those 'standards' committees can take forever.

thanks

 

This is probably true, but I think it's important to differentiate between local stop buttons (cycle stop?) and e-stops. I think this comes down to how the button functions. If it's a hard wired system that uses a safety relay (Pilz or similar) which removes power to the machine then to my mind it's undoubtedly an e-stop circuit irrespective of the terminology. Risk assessment defines the category of circuit you need to use.
If the button simply drops out a contactor or relay, or stops the machine through software then I think it's a cycle stop i.e. not a safety circuit. I think a zoned e-stop circuit is acceptable providing the system is logical
and easily identified. My preference is a global e-stop circuit but it's not always a practical solution. I guess the e-stop debate is never ending but I'm enjoying following this thread.

 

I think that the safe way is to stop everything with the e-stop. To make this system more convenient also add a cycle stop which could be used to stop by zone. This way your operators can expect all motion to stop in case of an e-stop. If the operator is not in a emergency but wants to stop a section without bothering the rest of the line he can. You might notice that operators like to stop conveyors by hitting the e-stop but if they have another way they might tend to use it.