Tug the chords?■

Yup, pull the cords on each switch unit on both sides of the belt once a month from a to Z while the belt is running empty preferably.

no, using the PLC to test them does not work or count as the coated aircraft cable must be

tensioned a certain amount to pull the switch handle with very lttle effort half way between two switches and the normally closed contact MUST open ■

I don't think the plant could afford pulling all cords on a running conveyor of any reasonable length.

During a standard maintenance shift, the plant can pull a large number that show status change at the PLC and if with dual contacts, at the MCC. No need to run the conveyor.

Once a month or once every 3-6 months. A number of the micro switches are of questionable reliabillity. Know the MTBF of your supplier. ■

I think what you need is an integrated system which combined with those functions, like realtime alarm, realtime monitoring 24/7, Detect the splices and broken cords and etc. ■

I am afraid the answer depends on the quality of the pull cords. There is a real variety of levels of dependability. But chosing one that has an approval sticker from an outside agency assures some one checked its quality. For example, CE Mark requires a pull cord be cycled 10,000 times without any missed trips, damage, or signs of wear. I find the most common damage to these type of switches is caused by water or something entering through conduit entries. If a good seal is present, every 3-6 months as a previous blog suggests, when the conveyor is not running, I think a plant manager would be very upset if these tests were done stopping the conveyor each time. ■

Copper wire was allegedly invented by 2 Dutchmen fighting over a penny.

Why are you trying to reinvent the wire? ■

Originally Posted by sganesh

Dear experts,

Pull-cord switches are very important life & equipment saving devices.

But are you confident that the switches installed in your conveyors are always reliable?

How do you ensure the reliability of these switches?

Request your advices please.

Thanks & Regards,

If any equipment is flagged as PSM (Process Safety Management) Critical or CSE (Critical Safety Equipment), then your safety assurance procedures will require them to be tested and the results recorded at a pre-determined interval. But it is not just the switch that needs to be tested, so does the cord itself. Take a bursting or rupture disc. These need to be replaced every 5 years or so. Now there would be many systems that have these devices that are older than 5 years but have never had them replaced.

But even here there are issues. For example, on a pressurised system you will have safety relief devices. These have to be tested and the results recorded at pre-determined intervals. Now, these generally have to be removed and the tests carried out on a test rig. The problem is in reinstalling the device, you can introduce factors that actually render the device worse than when it was removed for testing! Again, this is why you will have two such relief devices in a system so that if one fails, there is at least another one in the circuit. Yet something more. How many who have specified safety relief devices have actually specified the flow through the device when activated?

On things like pull cord switches, generally you will have the cord anchored at one end and the other will be connected to the switch. If the system demands it, you have a switch at either end of the cord. This way you hope that at least one will work. And that is all you can do. You can have two or more checks, and all can fail. On a pull cord system, you can have the switch fail in either position so even a "fail safe" setup might not be effective. It could be that it always indicates fail so plant operators bridge it out! This is exactly what happened when one of two identical oxygen analysers always indicated high oxygen from new. The operators tried everything they could, but could not get one to work correctly. In the end, they bridged it out and ran the plant for over 15 years with just one operational! It has only now been rectified.

I went to a seminar on Safety Assurance where one of the slides had something like this.

"Engineers fail to grasp the ingenuity of fools when they design fool-proof devices and systems" ■

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Originally Posted by waiwhite

If any equipment is flagged as PSM (Process Safety Management) Critical or CSE (Critical Safety Equipment), then your safety assurance procedures will require them to be tested and the results recorded at a pre-determined interval. But it is not just the switch that needs to be tested, so does the cord itself. Take a bursting or rupture disc. These need to be replaced every 5 years or so. Now there would be many systems that have these devices that are older than 5 years but have never had them replaced.

But even here there are issues. For example, on a pressurised system you will have safety relief devices. These have to be tested and the results recorded at pre-determined intervals. Now, these generally have to be removed and the tests carried out on a test rig. The problem is in reinstalling the device, you can introduce factors that actually render the device worse than when it was removed for testing! Again, this is why you will have two such relief devices in a system so that if one fails, there is at least another one in the circuit. Yet something more. How many who have specified safety relief devices have actually specified the flow through the device when activated?

On things like pull cord switches, generally you will have the cord anchored at one end and the other will be connected to the switch. If the system demands it, you have a switch at either end of the cord. This way you hope that at least one will work. And that is all you can do. You can have two or more checks, and all can fail. On a pull cord system, you can have the switch fail in either position so even a "fail safe" setup might not be effective. It could be that it always indicates fail so plant operators bridge it out! This is exactly what happened when one of two identical oxygen analysers always indicated high oxygen from new. The operators tried everything they could, but could not get one to work correctly. In the end, they bridged it out and ran the plant for over 15 years with just one operational! It has only now been rectified.

I went to a seminar on Safety Assurance where one of the slides had something like this.

"Engineers fail to grasp the ingenuity of fools when they design fool-proof devices and systems"

what is the egus os loso sis slks

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Originally Posted by waiwhite

"Engineers fail to grasp the ingenuity of fools when they design fool-proof devices and systems"

Isn't that the truth - just when you think you have seen all the fools they build a bigger fool. ■

Originally Posted by guddu

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what is the egus os loso sis slks

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Dear sir

Please see below clarification

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Originally Posted by sganesh

Dear all,

But for very minor jobs ( say replacing a side trough roller), taking LOTO , many times look not practical.

Regards,

Can appreciate your position somewhat, though how long will an investigation into a fatality due to a failure in isolation take?

In saying that, unfortunately, it wasn't that long ago this was standard practice in AU also (and probably still is, albeit illegal, in some facilities).

Regards,

Lyle ■

Originally Posted by sganesh

Dear all,

Thanks a lot for your contributions.

Pull cord switches have complicated mechanisms inside.

In stead of pull cord switches, please imagine proximity switches ( the switches used in speed sensors ) are installed and connected in series. There would be pendulams like plates hanging in front of them.

All these plates are connected by pull cord. During any emergency, the cord will be pulled by the operator. So the pendulam will move away from the proximity switch. The conveyor will be stopped by the corresponding proximity switch.

The advantages will be :

1. There are no moving parts inside the switch. So no wear & tear. Malfunctioning will be eliminated.

2. The proxy switches do not require any sealing arrangements. Maintenance Free.

3. The defective switch would not allow the conveyor to start. ( Please correct me if I am wrong. )

Have you ever seen or used such arrangements ? Requesting your earliest reply.

Regards,

Dear Sir,

Greetings

We have developed the switch and request you for a appointement for a demo.

Rgds

Sunil cornelio

09167905766

AG SYTEM CONTROLS

Mumbai India ■