Safety Input Opinions

We are doing a in house rebuild of some plastics grinders and we are adding a plc and removing a lot of the very ols hard wried controls because most of it can not be found anymore even on ebay.

We have 2 estops that we plan to connect dual channel to a modular safety relay from allen bradley.

We also have limit switch on all the doors going into the granulator cutter chanber( Where the rotating drum of blades are). These are hinged doors but you have to remove about 10 bolts to be able to open it and I am trying to figure out if these should be safety inputs or not?

On some of our grinders they go to the plc and shut down the granulator starter if the door is opened and on 2 grinders we have they go into a safety relay. There are all different grinders from different mfg companies.

Which way is correct? I am thinking they can be normal inouts as the bolts in the door act like a hard guard?

Set me straight please?

the best answer is to ask the questions.
Can the system be started with the cover off?
If the existing switch fails could someone get hurt?
The answer is generally one or more safety relays and
dual contactors

Well if the switch failed and the bolts were removed from the door then yes it could start and yes you could get hurt but only maintenance should be removing the bolts and opening the doors when they have a jam ,etc.

before maintenance opens the doors they should isolate all energy and do their LOTO functions before they remove the bolts to open the door.

I think the original intention of the switch was the machine would not start with the door open because of safety but primary because you would be grinding and blowing material all over the floor.

I prefer not to make these dual channel safty switch if it's not required.

Tim Ganz said:

We also have limit switch on all the doors going into the granulator cutter chanber( Where the rotating drum of blades are). These are hinged doors but you have to remove about 10 bolts to be able to open it and I am trying to figure out if these should be safety inputs or not?

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From what I've been told, if you need a tool to take off a cover or open a door then it is considered to be a permanent part of the machine and does not require a safety switch. The rationale is that if you are taking a wrench to a machine, this is not a routine operation and you should have the machine properly locked and tagged out in the first place.

Tim Ganz said:

On some of our grinders they go to the plc and shut down the granulator starter if the door is opened and on 2 grinders we have they go into a safety relay. There are all different grinders from different mfg companies.

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The first situation is in general a very improper way to do things. That said, if you need bolts to take the door off then they didn't need a sensor on it at all. But if you DO have safety switches, PLCs are not safety devices. PLCs don't have the hardware redundancy and security required for any safety category. There are safety rated PLCs that are the exception to this, though.

Ultimately, what I think should guide your decision is the question of WHO this guard will affect. Who are the ones removing the bolts? The machine operators? The production management? The Sanitation Crew? Maintenance? If it's just the latter three, and they have received LOTO training and know they need to lock out a machine, then I don't think you need safety switches at all.

There is absolutely no way to make any machine 100% safe. Not even the fanciest category 4 machine fresh from Europe is 100% safe. No safety system can account for tampering, and removing bolts from a machine is generally considered to be tampering if during a regular production run they are not touched. If these doors require a safety switch, then every bolted-down component that could possibly expose the inner workings of the machine does--potentially dozens if not hundreds of safety switches.

The only reason I can see for safety switches is if the bolts are regularly removed during the normal production run by production employees who are not LOTO trained.

Another question to ask is what are those switches protecting? People or the machine? If they're just there to protect the machine, they don't have to be safety rated.

you need a LOTO sheet on the door(s) you are to unbolt.
I would also put the safety switches in place as well to help prevent someone from bypassing controls if the LOTO was ignored.

your safety department should also do a risk assessment as well.

regards,
james

brstilson said:

From what I've been told, if you need a tool to take off a cover or open a door then it is considered to be a permanent part of the machine and does not require a safety switch. The rationale is that if you are taking a wrench to a machine, this is not a routine operation and you should have the machine properly locked and tagged out in the first place.

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This is also my opinion. Some type of sign (sticker) should also be used near the doors.

"Guards must be in place before operating machine" signs on all the panels that can be removed with tools seems pretty standard from all the OEMs I've dealt with.

two posts independant but on at the same time...
so i part posted on the other one... I need a holiday.
I agree with bolting and sinage
Be wary that sinage wears and falls off.
If there is a guard switch fitted to the bulk of the machine
then dont remove it. maybe upgrade it.

A couple of points to consider.

Necessary safety measures should be governed by a risk assessment.

The risk assessment should be carried out with a competent person who fully understands the nature of the machinery and its use.

Procedures such as LOTO are not a substitute for physical safety devices.

The "fixed guard" argument does generally hold water but because of the way that these machines need to be maintained, I would disagree in this instance.

It has been nearly 30 years since I worked maintenance in a plastics factory but from what I know of the likely outcome from starting a grinder with your hands in it, I wouldn't sleep at night if I had designed a safety system that didn't include a dual channel safety device that prevented the motor from starting.


Please... Do the risk assessment! Document your conclusions because: 1) You will be taking responsibility for the safety of the machine and anyone that goes near it. 2) If there is an accident, the authorities will want to see the risk assessment and it will harm your defence if you can't produce it.




Nick

Manglemender said:

Necessary safety measures should be governed by a risk assessment.

The risk assessment should be carried out with a competent person who fully understands the nature of the machinery and its use.

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I still have my opinion that it doesn't need a safety switch, but this right here is the answer. Get a risk assessment done. A qualified safety person will know a lot more than anybody here.

Hi

Even if you are doing this in house the person i.e this is you in this case are the owner of the safety design. Your risk assement that you must do should give you the answer you are looking for. But you can also put in more safety that want your risk assement states and no judge will look at that in a bad way but even if you leave that company and no one changes the safety circuit and then someone gets hurt remember you are still the owner of the design. It's best if you do not make the call on this on your own, talk to the operator, maintaince team etc and docement your findings.
In most machines a guard that needs a tool to open it does not need a Safety switch but if that Could be opened while a hight speed drum was spinning behind it then I would put a safety switch on it and it would probley be a lockable switch in that case

Just be carfull

Donnchadh

brstilson said:

I still have my opinion that it doesn't need a safety switch, but this right here is the answer. Get a risk assessment done. A qualified safety person will know a lot more than anybody here.

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i agree with you, but if a machine has a 'safety' device for this area. All I am saying is the manufacturer may have seen a risk.
If you remove it you take the responsibility.
that is the risk.
I too se no reason to fit one.
but would consider a formal LOTO procedure

http://www.plctalk.net/qanda/showthread.php?t=81133
just a note
this is a similar post as with many others on this site.
I think we need to encourage searching
we are covering common ground

iant said:

i agree with you, but if a machine has a 'safety' device for this area. All I am saying is the manufacturer may have seen a risk.
If you remove it you take the responsibility.
that is the risk.
I too se no reason to fit one.
but would consider a formal LOTO procedure

Click to expand...

What the MFG had in place was a standard limit switch not safey rated and on the new units it goes into the plc and on the old hard wired units it goes to the coil of a normal 120 volt ice cube relay.

We would be doing a risk assessment in house and AFAIK we have never done one here. May be a good time to start.

Is there a baoiler plate risk assessment form anywhere I could use and possibly modify with our company info ,etc?