Machine Safety 3

[itsmoked]

An automated saw.

Occasionally the blades must be manually hand replaced. It's being done while the machine is in standby. I suspect this isn't OSHA approved, as I understand it, "the energy source has to be removed", from the motion generating device.

At issue is the time required to power down and reboot a large machine tool including its loss of position etc., etc.

The classic method would be to put in a manual disconnect in the motor leads - probably lockable.

I'm wondering about alternatives. Is a contactor capable of providing the mechanical disconnection? If so, can something like a key switch in the control(coil) circuit of the contactor acceptable?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Many years ago, several types of key lockable devices were acceptable for lock-out safety. A number of accidents including some fatalities led to them being banned for safety lockout devices.
Better go with the classic method, lockable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[redlinej]

You can incorporate a shunt trip on the motor protection(if it have the feature) and use a key to active it .

 

[itsmoked]

redlinej: I doubt that is available.

Bill; Are you saying a contactor is okay if its coil supply is blocked and locked?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

No. Not at all. There have been fatalities in the past when a control circuit was locked out and then someone pushed the contactor in with a screw driver, or the contactor was inadvertently closed some other way. Back in the day, a common trouble shooting technique was bumping a motor by momentarily closing the contactor momentarily be pushing the mechanism with a screw driver. A blocked control circuit did not provide protection against unexpected starting.
When I was starting my training, the regulations were changing from control circuit interlocks to disconnect type lockouts due to proven issues with control circuit lockouts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[melspuds]

Bill, I remember the good old days went you work everything hot. Before OSHA came on line in about 1970.
Now all sources of energy must be off. Including power, air, gasses, hydraulics, stored energy such as springs and cumulators.
Dave

 

[jraef]

I believe you can do that with what are called a safety monitoring relay system and "safety contactors", an IEC design that has not only what are called "positively driven" contacts (meaning that all contacts are mechanically linked, not floating, so if one welds they all stay in and a safety monitoring relay would know it), but also, I believe that on a safety contactor, you cannot have access to manually drive the contacts from the outside. So what i believe you would do (but I'm no expert) is to have controlled access to the blade area with a solenoid based lock, controlled by the safety monitor relay. The relay would also be controlling the safety contactor(s) and watching the interlock devices such as e-stops, access key locks, limit switches on guards etc. so before it releases the access door solenoid to get into the blade area, it must see that all safety devices are active and functioning AND that the safety contactor has dropped out and is not welded. Only then can they open the door to change the blades. Something like that.

Here in North America we are a little behind the times when it comes to machine safety systems compared to the IEC rules and regs. We CAN use their standards, and our safety related agencies like OSHA and OHS in Canada will accept compliance to them, but they are not enforceable standards here in the same way they are elsewhere. Here, we have to provide safe machines, but how we get there is open to interpretation, which unfortunately sometimes leads to issues like what Bill mentioned.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ScottyUK]

jraef,

Those are useful components within the control system, often forming part of an ESD scheme which is 'impossible' to defeat without an undue amount of effort. Personally I wouldn't trust it with my life - I would want an air gap somewhere in the power circuit with a lockable disconnector, and the key to that locked disconnector either under my direct control or locked in a keysafe.

The old EN 954 standard was fairly straightforward to apply but it has been replaced by the far more complicated EN 13849. Good luck with the latter. ;-)

 

[itsmoked]

Cripes! Always annoying when the rules start getting absurd to prevent complicated scenarios.

This is a single machine, with a single operator, with everything enclosed behind doors that need tools to open. The "blade changer person" is not going to fondle the saw blade while someone pokes around "in back" manually engaging the "just wrong" contactor. They've been changing the blades for years without issue, they just want to be "in conformance" and not have the computer running the machine have a blade starting 'cosmic ray' hiccup. i.e. blocking the computer's contactor signal.

It really is like the OSHA medieval knight.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Old fashioned ways work because they're understandable!

 

[Compositepro]

You could put a battery back-up on the computer to avoid reboot after a short lock-out of the disconnect.

 

[flexoprinting]

Hi Keith

Could you just separate the supply to saw with a quick disconnect plug and use a plug protector such as the one's supplied with Lockout/Tag out kits? It would remove any potential energy, thus satisfying OSHA rule's..

http://www.grainger.com/product/BRA...m/rp/s/is/image/Grainger/5T831_AW01?$smthumb$

Chuck

 

[melspuds]

Keith,
The 2 steel mills that I was chief engineer for over 35 years, the computer power was always left on and with battery back ups. The power for motors, ect are locked out with the main disconnect. That shut down I/O power to the plc, cnc. Therefore the computer would remain up and no I/O. That little trick was show to me at an Allen-Brady PLC-2 training center close to 40 years ago and a lot machine tool OEM use it still today including A-B.
Hope this help,
Dave

 

[itsmoked]

Thanks for the good ideas folks, I have some plans to consider now.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Rube Goldberg found a new field of activity?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[cswilson]

The machine-tool type systems I'm familiar with virtually always have separate power control for "logic" (computing and feedback) and "motive power" (drive/inverter -- if used -- and motor). Safety arrangements require some method of withdrawing motive power, but logic typically stays alive, so recovery can be much quicker.

I'll leave it to the others to suggest the best method of removing motive power in this case.

Of course, the controller must be capable of recovering gracefully when motive power is re-applied.

Curt Wilson
Delta Tau Data Systems

 

[IRstuff]

"Always annoying when the rules start getting absurd to prevent complicated scenarios."

Don't forget that idiots are extremely ingenious.

TTFN
faq731-376

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[waross]

Hi Keith
Your best choice considering safety, OHSA, your personal liability and simplicity may be to bite the bullet and instal a disconect switch.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[3DDave]

Is ridiculing safety the best attitude to take when looking to make a system safe?

Read these

http://www.fluidpowersafety.com/fpsi_alerts.html

and see what fun there is to be made of careful and sober consideration of safety interlocks.

 

[ScottyUK]

Challenging 'safety' requirements is a good attitude. Many things are done in the name of 'safety' make the job so difficult that the job ultimately becomes less safe as a result. A couple of examples:

[ul]
[li]In the UK, scaffold has become the universal answer to access problems. It is seen as acceptable to put a scaffolding gang at risk for days erecting a platform to avoid someone working from a ladder for ten minutes. Has that improved safety overall or just exposed more people to risk?[/li]

[li]Arc flash suits. Horrible claustrophobic things with lousy visibility, poor dexterity, and which cause heat stress when worn for more than a few minutes. When doing open-door fault finding do these suits improve safety, or do they decrease it because they massively increase the probability of an incident occuring due to poor visibility, loss of dexterity, etc?[/li]
[/ul]


As for a bit of humour, if I didn't laugh I would cry.