Testing Reliability of Pull-cord Switches

Conveyors are usually in dirty dusty conditions so a build up on the sensors makes the stop inoperative.

Can you guarantee that all plates will clear the light path on a single pull?

I'm sorry but a trip back to the drawing board is required

dear,

I think, the Electro magnetic pulse type proximity switches may help out the situation more better. Definitely, IR basewd will create the operational problems due to environment.

I think he means Hall effect sensors not optical or IR

Nice idea but VERY impractical.

you mention "Wear and tear".. but as the "Pull cord" is for emergency how often would it be used? Unless.... you have operators riding the conveyor and they lean over pull the cord to stop the conveyor to get off???

I left school and starting in the local steel works as a trainee electrical engineer, where raw material was moved by conveyor, the safety pull cords were maintained on a regular basis, and on opening the individual units for maintenance, inside was as new, because it was never used, and I have NEVER known these units to malfunction.

Furthermore, what is complicated about these units? Two springs, a lever, a small weighted rod, couple of split pins & washers and moving electrical contacts... simplicity itself!!

I've seen these units get damaged from material falling off the conveyor, but your suggested equipment can also be damaged in that way too! So unless your operators use the pull switch frequently (retrain the operators) and you're had units fail, your argument does not hold water.

Would your alternative suggestion be maintenance free?? NO it would not. It would STILL require a function check and cleaning. Your safety regime should dictate that you routinely check all safety equipment and its function/operation during regular maintenance

Finally... ALL "conventional safety pull switches" as fitted to conveyors, if faulty or broken, or the pull cord is broken/cut will stop the conveyor or prevent the conveyor from starting. If the ones you have at the moment don't... you got a BIG safety problem

One last thing.. the safety pull switches on conveyors have been in use for decades, if it isn't broke.. don't try and fix it. Focus your energies and talent into something else

Thank you all.

Yes. The sensors I have mentioned is of Hall Effect Sensors.

The moving parts need a very little amount of lubrication once in a while ( by sprays). The covers need to be opened and fixed back. Also we should ensure that sealants are fixed properly to avoid moisture going inside. RTV sealants are required before monsoon.

Last year when an operator pulled the pull cord switch, the conveyor was not stopped, though from outside, it was found operated. But inside the pull cord switch, the electrical micro switch was found not operated.

The supplier was called and shown the switch. He said that the design had been changed long back. It was not communicated.

The pull cord switches are designed for certain number of operations. But who would count the operations? For a plant which has more than 1000 pull cord switches, how to ensure the reliability of PMI on the switches done by technician ?

Ensuring the pull cord rope condition can be checked by the operators. But reliability of each and every switch is a very uphill task. We cannot physically check each switch , though it is online or offline. What I am trying to seek is that if the pull cord switch is faulty, the conveyor should not be started.

Thanks & Regards,

PS : I had a similar demand for safety sirens, which need to blow the sound, before starting the conveyor. Last year this requirement was fulfilled by one of my engineers.

Now we are sure that if siren is faulty, the conveyor will not be started.

There are many more requirements set out by ISO 13850 that your system would need to comply with. For example, once an emergency stop switch is activated, it needs to latch in the off position until someone physically inspects the area and manually resets it. Your system would automatically reset itself as soon as the pendulum moved back when the rope is released or cut. You would also have many more nuisance trips due to vibrations moving these pendulums.

Relying on operators to periodically ensure that the ropes are still attached also does not comply with safety standards. The system needs to ensure that a rope was not removed for any reason and not replaced. 1000 switches means 1000 ropes. Your operators will not be able to see them all at all times. Also, a loose rope or one just about to fail may break when needed and not trip your switch.

You also claim that mechanical switches cannot be checked for operation without actually checking it. This is true even for solid state switches. And solid state switches most often fail in the shorted condition. This means a failed switch will allow the machine to run anyway. The only way to test any switch is to test it.

you are assuming that they conform to or even know about ISO....

It must be pointed out that the "rope" is not really wire rope but high strength wire braided cable with either 2 or 4 conductors so that the "rope" carries the electrical connection between each trip switch, so if the rope is cut or removed then the conveyor will not start, an added safety feature in addition to the switch tripping if the rope breaks away from its attachment to the trip switch.

All safety trip switches for conveyors are designed to "open contacts" if they fail for whatever reason.

In response to Masu.... experiments with reed switches proved to be unreliable due to vibration and the fact that as you state, " if the magnet moves far enough away the reed switch will open". Not a fail safe idea.

As far as the environment goes filled with metallic dust and the like, you have never worked in a Steel works.... ALL conveyors MUST have trip switches, as there will be operators working around them wearing the required PPE as per their risk assessment!!

I would ask why someone is trying to re-design a faultless system, that has proven itself for decades, preventing damage to equipment and more importantly saving lives.... and why are we daft enough to be drawn into it?

• "if the magnet moves far enough away the reed switch will open". Not a fail safe idea."

You can get normally open as well as normally closed and three terminal change over reed switches so you can set it up so that it's a failsafe with either open or closed contacts and with or without magnetic fields present.

You could, therefore, make it failsafe by selecting the appropriate configuration of contact type and magnet mechanism.

The vibration is a good point though and I have no idea how reed switches take to vibration, although I would have thought that it would take a fair old acceleration to move those contacts without a magnetic field. However, you can get some very large short duration forces with vibrations so without doing some further research and or testing I couldn't say one way or the other with any certainty.

Ummm - maybe include in that why would one deviate from a simple festoon of switches, in series with the contactor coils - which is inherently 'fail safe' - compared to a system that also requires a dedicated power supply and inherent 2nd tier reliability issues?

I'm sorry but you're trying to re-invent the wheel.

Pre-start warning and E-Stop pull cords are established and reliable technology. What is the problem with using what's available?

This is sort of along the lines of a proximity switch but rather than using induction or IR beam type technology how about going for a magnet and reed switch.

Reed switches are pretty reliable and while fragile can be encapsulated as they are with security systems, to make the virtually unbreakable. Provided there is a small magnet in proximity to the reed switch the contacts will be closed (or open if you so decide) and the machinery will operate will, if you set it up so the pull cord moves the magnet far enough away from the reed switch the contacts will open and stop the process.

There could be a problem in an environment where there is airborne ferrous metal dust particles as they would be attracted to the magnets an could ultimately clog them up with metal dust that could jam the mechanism. However, I if the environment were contaminated that badly with airborne ferrous metallic dust then I doubt it would be fit for human habitation and thus obviate the need for a manually operated emergency pull switch.

Dear all,

Thanking you for your replies. I am a mechanical engineer. Please bear with me if my question is absurd to you.

I would come up with examples in future when I face problem with pull cord switches if I face in future.

Requesting others to continue contributing this thread.

Regards,

Sqanesh, no such thing as a stupid question, its only stupid not to ask!

And the division by Zero is infinity !

On behalf of the people in the field. Don't feel so bad. Everyone get's a new asshole ripped once in a while and today, you drew the short straw.

I have worked as both an electrician and engineer in automation systems.

Every point I was ready to make after reading your blog was rbought up by someone.

Once think I realized and you have to as well is the with engineering it is not difficult to complicate something but to simplimize a system. Note that with pull chord EMG stops, they are hard wired into the system and not through a processor or PLC. If my arm gets caught. I don't want my safety system to have a chance to freeze up.

Like mentioned, those pullchords are built though for a reason and look great once opened. Their are monthly PMs performed on the systems (slack, damamge to the cable, ,proper operation,etc).

Good try thou,

Solar powered flashlight anyone?

Dear all,

Many conveyors are remotely located from the control room and substation where its feeders are installed. They are all remotely operated.

Please assume 650 mm width conveyor. Replacing its carrying roller hardly takes few seconds. Or you may consider removal of an unwanted foreign material from the carrying side. Going as per LOTO may not be always practical.

So we need to depend sometimes on the reliability of pull cord switch and sirens for a very minor jobs. We do follow LOTO, where it is very unsafe and potential hazard to life.

May I compare (Pull cord switch + siren ) to my car pedal brake and LOTO to the (hand brake and wedges for all the four wheels, stopping the engine & taking out the key)? Both are necessary and important for the safety of the car, passengers and the pedestrians.

I just wanted to know is there anyway to have 100% reliability of pull cord switch by avoiding moving parts & their maintenance.

Thanks & regards to all,

Whats a LOTO?

LOTO.. Lock Out - Tag Out, used widely all over the world as a method of isolation, where a worker or a group of workers can lock off a item of machinery with their INDIVIDUAL padlocks, that MIGHT move or start working as they are working on it, the TAG is the label associated with the padlock identifying the person who has completed the LOCAL isolation and this TAG in many cases, combines a PTW (Permit to Work) or CoI (Certificate of Isolation) number with responsible persons' (RP) name etc.

If when the time has come to sign off the PTW, ALL LOTO padlocks should be returned to the responsible person, thus telling the RP that the work area is clear, so he/she can close the PTW & CoI and the machine can be started.

However if one padlock is missing then the PTW and or CoI will not be signed off, because it will be be an indication of someone still working.

If it's not practical to lock out, then you need to reconfigure the hardware until it is

Safety is not an option

What does Lock Out/Tag Out have to do with safety pull chords on a conveyor?

Safety pull chord is not a disconnecting means or an ON/OFF "button".

it is common to use a safety device as a shortcut instead of locking out, not legal, but common...

generally safety devices operate on control circuits

lock out should be placed on disconnects that interrupt the main power source to the drive motor [feel free to correct me, if you know of exceptions]

Lock outs may also be used for pneumatic or hydraulic power sources in some cases