...this light-curtain should be controlled by a separate unit, not the PLC. and the reset-switch should be in the circuitry of that light-curtain, not go to the PLC. and it should not be able to be fooled by holding it constantly pushed.
I've been watching this one with interest.
Excellent wimpiesplc, and others, for pointing out the bigger picture. Fix it! or design it better! But don't let them defeat a reset, period.
DJacobs, I can see how you are trying to use the tools available to you to overcome this issue. But really it's managements problem, not yours. I would not entertain coding this, to allow them side step the fact that operators are defeating the safety measures put in place to protect them and their co-workers.
The reset is on the back of your machine for a very good reason. If this light curtain allows full body entry/exit it is possible for an operator to pass through into the danger zone. Another operator wanting to reset and start the machine will have to go around to the reset at the back to do so. From this position they should have full view of the light curtain entry/exit points and see if there is any body in there before attempting a reset. Defeating the reset could create an auto reset which may allow an auto re-start, depending on the control design. This practice could have serious implications and should be stopped immediately. They should be trained for this.
If they are defeating a light curtain reset because it's tripping every cycle, is there something wrong with it?
It's mis-aligned?
Product is tripping on entry/exit? >>Muting sensor fault?
Machine stroke is triggering it?
Operators are triggering it?
Whatever it is, fix it. Whether it's your responsibility to do so or not, tell them, your superiors, not the operators, it has to be fixed and they need to stop them doing this immediately. This is managements problem, not yours.
If your doing this to save you fixing the problem....no comment.
Light curtain resets should be wired to the manufacturers specification. As wimpiesplc pointed out, they are nearly always controlled by a safety rated unit, or more modern LCs have built in control. Again, these resets nearly always have anti tie-down as standard.
The PLC anti tie-down methods described here are good, and you can use them to prevent this, but only if the PLC is switching the LCs proper reset, not acting as the reset.
I spent half of last Thursday with another Electrician sorting out a problem with a nuisance tripping light curtain. We sorted it eventually. It deflected us from other planned work, but it's safety, so it comes first. We don't by-pass unless there is a major fault that cannot be quickly fixed without stopping production for a long period. If we do by-pass an exemption order is signed by the shift supervisor and ourselves. Signage and barriers are put in place to prevent access and warn them of the danger. Everyone involved is informed.
Coding around this is letting management off the hook here.
We don't tolerate this behavior what so ever here.
Get your superiors to manage their operators better, train them, warn them, or fire them!
It's against the law to do what they are doing.
But not what you are doing, if the PLC is not the reset.
G.