Dual Line E-Stop Circuit

M

Thread Starter

Mohan Smith

I have
2 Cable winders
2 production lines

either winder can run with either line.

My options:

1. Install (2) E-Stop PB's on each machine for
both lines.

Negatives: The operator under pressure may not press the right one from the first time

2. Install one E-Stop PB on the machine, and also install a 2 pos. selector switch that will short out the opposing E-Stop switch and send a positive signal representing the current position to the PLC for use in data processing (not estop circuit)

Questions: Is it acceptable to select based on the above the line to be E-Stopped?

3. Your suggestions


Your ideas and help would be greatly appreciated.

Thank you all.

Mohan
 
G

Gerald Moore

Mohan,

In my opinion, since you are talking about an E-Stop, I would just hook all 4 together and shutdown everything. Selector switches are unreliable for E-Stops.

Or, wire each production line, so that if a production line E-Stop is hit it also shuts down the winders but not the other production line. If a winder E-stop is hit then shutdown everything.

Remember Safety First when it comes to E-Stops.

Good Luck,
Gerry
 
R
IMHO...

Install one e-stop push button on each winder with two contact blocks. One block is for use for line 1 and the other is for use on line 2. Have the PLC "know" what line each winder is associated with. Have PLC short out the non-required contact. Meaning if winder B is connected with line 1... have a contact from the PLC short out the line 2 contact on winder B so that if winder B's e-stop is pressed only line 1 is stopped.

Best Regards... Rick Kelly

Chief Technician N/C
Cheese Operations
Kraft Canada Inc.
Ingleside, Ont.

V (613) 537-8069
F (613) 537-8044
[email protected]
 
M

Michael Griffin

I don't think it would be acceptable to simply have a switch (or PLC controlled switch) to decide which e-stop circuit the the winders should be connected into. There is too much potential for things to go wrong.

You haven't mentioned a few critical things though which make a difference as to what might work.
1) If either winder can be used with either line, how is the guarding arranged to permit this? If both winders are arranged within the same guard, you really only have one zone you are trying to guard anyway, and therefore one e-stop.
2) Are you interlocking each winder with the rest of the line it is working with? If not, then I don't understand what the problem is (of course I don't know what the physical configuration is).
3) Do the winders switch operation between lines frequently, or only occasionally?

Something I have seen done to allow equipment to operate in different configurations is to have movable guards which physically isolate
zones from each other. The guards have safety switches which form part of the guard interlock circuit when they are in position. That is, a machine is allowed to operate if either guard 'A' or guard 'B' is in position, where either guard acts to isolate a hazard from an operator. If neither guard is in position, then the machine will e-stop.


**********************
Michael Griffin
London, Ont. Canada
**********************
 
Why not have one Estop stopping both lines? no confusion right?

Besides, if you see someone in danger the E-stop button nearest at hand will stop the machine.

By shorting out the "unneeded" switch, would you not introduce a possibly great danger, what if this relay or whatever you use for shorting it fails in a way shorting the e-stop always (or if the PLC-output fails driving the relay). This sounds like a really dangerous thing to consider, even if it works in theory E-stop are supposed to work in reality and always, not just in theory.

Consider: Will the E-stops be used enough often for it to really be a problem stopping both lines always?

If yes: Do something about that!

If no: Where is the problem?

Safety first when it comes to E-stop!

/Johan Bengtsson

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P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
E-mail: [email protected]
Internet: http://www.pol.se/
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J

Joe Jansen/ENGR/HQ/KEMET/US

Wouldn't that leave you vulnerable to PLC failure preventing your E-stop from functioning? ie: If the output failed closed, the E-stop button
would be disabled.

I would ask what method is currently used to select which line the winder is running on. Are they physical positioned in anyway? I would look for a safety rated selection method of some sort that would function as #2, but interlock the winders together so that if winder 1 is set to run A line, but the machine setup is for B line, then both lines are e-stopped. In other words, Make the E-Stop selector match some critical point on the machine setup. Use safety rated switches at all points.

--Joe Jansen

*****IMPORTANT*******

I am -NOT- a safety inspector, certified in any way to offer official safety advice, etc. I am actually a programmer. However, since the
original post was asking advice, I offer it as a suggestion to present to someone who -IS- licensed in these matters...

--Joe
 
L

List Manager

I would think that any scheme using PLC I/O as part of the safety circuit would not be acceptable. Mohan, instead of a selector switch use a safety switch of some kind that is actuated by a mechanical coupling between the winder and the selected line. It must be something that positively links the winder and line, and something that won't let the winder run with the
*other* line (like forgetting to turn a selector switch). Your idea to set up the winder EStops selectively is OK as long as you have a positive
mechanical or non-defeatable electromechanical means to ensure that the EStop is always correctly coupled to the line in use. You will also need to do a little FMEA on whatever scheme you come up with so that any equipment failure of the EStop system you dream up will result in the line stopping or failing to start. There is no point in doing this if you introduce an unsafe failure mode into the safety system itself.

Paul T
 
I would think that any scheme using PLC I/O as part of the safety circuit would not be acceptable. Mohan, instead of a selector switch use a safety switch of some kind that is actuated by a mechanical coupling between the winder and the selected line. It must be something that positively links the winder and line, and something that won't let the winder run with the
*other* line (like forgetting to turn a selector switch). Your idea to set up the winder EStops selectively is OK as long as you have a positive
mechanical or non-defeatable electromechanical means to ensure that the EStop is always correctly coupled to the line in use. You will also need to do a little FMEA on whatever scheme you come up with so that any equipment failure of the EStop system you dream up will result in the line stopping or failing to start. There is no point in doing this if you introduce an unsafe failure mode into the safety system itself.

Paul T
 
E
Mohan,

I would disagree with both of your methods... Too easy to get into an unsafe condition. A winder connected to the production line now becomes a "system", comprised of the winder AND production line. Pressing either E-stop on this "system" should stop the "system". This is what you want to do, right?

I assume the production line provides power to the winder (e.g. The winder cannot run independently). Swap out the power connector with a connector that provides additional contacts for your E-stop circuit (A multipole connector like Harting, or similar). Add an additional NC contact
to both E-Stop buttons, and wire these through this connector to both machines (Winder > Production Line, and Production Line > Winder). When you don't have the Winder connected, you can install a "dummy plug" in the Production Line connector that takes the place of the winder. This way you are safe. There is no way to connect the Winder without automatically tying the two E-Stop circuits together.

You will need to modify this design if the winder can be run as a "stand-alone" system. This type of interlocking is commonly used to
interface injection molding machines to handling devices (robots). Download the .PDF file entitled "EUROMAP 12" at:

http://www.euromap.org/recommendations.htm
for information on this.

Regards,

- Eric Nelson
[email protected]
Controls/Software
Packaging Associates Automation Inc. [email protected]
Rockaway, NJ, USA
 
Joe is correct in his observations. I should have placed the following in my response.

Add a "Global E-Stop" button to each winder. These buttons should be wired to both lines and have no shorting contacts from the PLC. These buttons provide a final safety and a method to get around a PLC failure that prevents a "Controlled E-Stop" button from working. The controlled e-stop will shut down the line associated with it while the global e-stop kills
both lines.

My defense for not including this in my original response is it has been nuts around here of late.

One other point... we have readers from all over the globe on this list and while a method of doing something would not be acceptable in our plants it may by reason of need be acceptable else where. We need to remember that and while we should preach superior methods... sometimes making something as safe as possible is a compromise based on available resources.

Best Regards... Rick Kelly

Chief Technician N/C
Cheese Operations
Kraft Canada Inc.
Ingleside, Ont.

V (613) 537-8069
F (613) 537-8044
[email protected]
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