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Old October 10th, 2017, 07:42 AM   #1

aand74 is offline
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Discrepancy times

In a project with a safety controller, we have several doors with safety switches and several safety light curtains.
What discrepancy times should I use for these elements (all have two channels)?
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Old October 10th, 2017, 09:25 AM   #2
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We usually use 250 ms. Quick enough and slow enough if you know what I mean.
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Old October 10th, 2017, 03:18 PM   #3
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The discrepancy time is only to deter manual bypass of the input and does not affect the reaction time when the device is activated. We always keep this value under 1 second to make it hard to bypass, but otherwise have no set standard. Usually we measure the actual discrepancy when closing the guard at a lazy pace and add 30% to that time. You don't want operators running around the machine to open and close guards and E-Stops that faulted all the time.

If the guards use magnets and they are mechanically located near the hinge of the door, the natural discrepancy time can be very large, so it is best to have the magnets on the latch/handle side of the guard.
Cycling power is a sign of weakness.
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Old October 10th, 2017, 07:37 PM   #4
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There isn't a standard. I can say that AB older GuardLogix this was hard-coded to 500mS. I typically see times of 100mS, 250mS, 300mS or 500mS.

You want it long enough that normal operation of the guard (door or light curtain in this case) causes a false indication. On the other hand you detect a failure of the system (or prevent tampering).

A lot depends on the exact nature of the design. For example two independent switches could take longer to toggle than two channels on the same switch. Location of the actuator can make a difference as well.

Winky's advice is pretty solid. It doesn't have to be exact, give yourself some leeway and round up. You don't want to get called out because someone closed a guard real slow.
An expert is a man who has made all the mistakes which can be made in a very narrow field. --Niels Bohr as quoted by Edward Teller
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Old October 11th, 2017, 05:10 AM   #5
L D[AR2,P#0.0]
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L D[AR2,P#0.0] is offline
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For Siemens safety I/O, the discrepancy behaviour is selectable so the time you have set may affect the response time.
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Old October 11th, 2017, 06:37 PM   #6
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That's interesting. I wonder what scenario would require a "hold last value" option for a dual redundant safety input.
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Old October 12th, 2017, 10:42 PM   #7
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mk42 is offline
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Originally Posted by ASF View Post
That's interesting. I wonder what scenario would require a "hold last value" option for a dual redundant safety input.
My guess is that it is something in Process safety, instead of Machine safety, but I couldn't say what.

Either that or a setting left over from a really old safety standard and only available for compatibility reasons. Siemens has NEVER done anything like that before.... (I'm still bitter about the S5 timers still being around in the 1500).
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