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Old May 4th, 2021, 03:15 AM   #1
Kataeb
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Safety doors with safety relay

Kindly, how can we keep the safety ensured on safety doors at high speeds, without stopping the machine at low speeds (if a door is opened).
Our customer needs to run at low speed (with doors opened and alarm horn on), so he can do some operation settings. Of course, to enable the run at high speeds, the doors should be closed.
We are using a safety switch installed on each door, and the switch contacts are wired to a safety relay (double circuit).
So, it is not clear how to enable or disable the safety doors, depending on machine speed, without endangering the safety.
Thank you
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Old May 4th, 2021, 03:40 AM   #2
alan_505
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You will need a safety device to monitor speed, if the speed is at an acceptable value disable the safety door, I think you will need some form of programmable safety controller.


Alan
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Old May 4th, 2021, 03:53 AM   #3
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If you are changing speeds by using a VFD, then the VFD must support the safety function SLS (safe low speed).
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Last edited by JesperMP; May 4th, 2021 at 03:57 AM. Reason: "low" not "slow"
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Old May 4th, 2021, 08:24 AM   #4
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Several manufacturers make safety interlock switches with an internal solenoid used to keep a door/gate locked. Check with AB or Telemecanique.

I am curious how it is deemed safe to enter the workspace at low speed. Are there any pinch points or rotating things then....?
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Old May 4th, 2021, 08:52 AM   #5
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Quote:
Originally Posted by dogleg43 View Post
I am curious how it is deemed safe to enter the workspace at low speed. Are there any pinch points or rotating things then....?
This is a relatively recent method that has become popular with VFD control, especially in robot cells. There is usually also a mobile hand operated release button that has to be pressed when the drive is to operate at the slow speed with the door open.
The button is made so that it has to be pressed in a half-depressed position. If you release the button or press the button fully, the drive must stop.
With that and the slow speed, if you feel that you are about to be pinched you let go of the button and the drive will stop before harm is done.
edit: And the speed must be so low that you cannot be harmed by impact alone.

All of this shall be carefully considered in the risk assessment.
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Old May 4th, 2021, 11:38 AM   #6
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Safe Slow Speed most easily done in a drive but can also be done with a speed monitoring safety relay such as Pilz Safe speed monitor PNOZ s30.

Some advice: Ask for a copy of the signed Risk Assessment which indicates that it is safe to open the guards below speed X.

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Old May 4th, 2021, 12:58 PM   #7
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Quote:
Originally Posted by JesperMP View Post
This is a relatively recent method that has become popular with VFD control, especially in robot cells. There is usually also a mobile hand operated release button that has to be pressed when the drive is to operate at the slow speed with the door open.
The button is made so that it has to be pressed in a half-depressed position. If you release the button or press the button fully, the drive must stop.
With that and the slow speed, if you feel that you are about to be pinched you let go of the button and the drive will stop before harm is done.
edit: And the speed must be so low that you cannot be harmed by impact alone.

All of this shall be carefully considered in the risk assessment.
Ok, right, kind of like going into a robot cell holding a dead man’s switch. Thanks.
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Old May 5th, 2021, 09:00 AM   #8
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Quote:
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The button is made so that it has to be pressed in a half-depressed position. If you release the button or press the button fully, the drive must stop.
Yes, this device is usually called "enabling switch" or, more expressively, "dead man's switch". To comply with the safety standards, it should be dual-channel with force-guided contacts.
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Old May 5th, 2021, 09:14 AM   #9
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Originally Posted by LadderLogic View Post
Yes, this device is usually called "enabling switch" or, more expressively, "dead man's switch". To comply with the safety standards, it should be dual-channel with force-guided contacts.

I've heard "live man switch" and "3 position enable switch" as well.
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Old May 5th, 2021, 11:12 AM   #10
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Awww, evidently “deadman’s switch” isn’t appropriate any longer. From a 2011 article.
https://machinerysafety101.com/2011/...-term-deadman/

“Do you use the phrase ‘deadman’ or ‘deadman switch’ when talking about safety-related controls on your machinery? I often run into this when I’m working with clients who use the terms to refer to ‘enabling devices’ – you know, those two or three-position switches that are found on robot teaching pendants and in other applications to give the operator a way to stop machinery, even if they have already been injured or killed by the equipment. Calling these devices a ‘Deadman Switch’ or even a ‘Live-Man Switch’ as the three-position devices are sometimes called, sends entirely the wrong message to the user as far as I’m concerned. The objective of our work as machinery safety engineers is to prevent injuries from happening in the first place......”
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Old May 5th, 2021, 11:48 AM   #11
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Quote:
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Awww, evidently “deadman’s switch” isn’t appropriate any longer. From a 2011 article.
More years ago than I'm prepared to admit, I served my apprenticeship on the railways. Early electric trains had a "Dead Man's Handle" to control the speed of the train: Holding the handle at the correct height you could rotate it to alter the power (voltage) applied to the motor via a device called a tap changer. It was designed such that if the driver had a heart attack and died, even if he slumped forward onto the handle, the train would still stop.

I guess they are all consigned to museums now along with the travelling post office carriages that I used to maintain.

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