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Old February 9th, 2018, 01:12 PM   #1
Timeismoney08
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Help with Circuit (Control Reliable)

Okay,

So I'm designing a motor circuit that will have E-stops running to a AB safety relay.

What keeps the Contactor that the safety relay will pull in safe? How do I prevent a contactor from becoming welded?

I've went over some historic drawings of other machines I've seen and the safety relay is always force guided and control reliable, but it always seems as if the contactor were welded shut, nothing would ever know.



Thanks in advance!
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Old February 9th, 2018, 01:15 PM   #2
jstolaruk
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Check out A-B's safety contactors http://ab.rockwellautomation.com/Mot...ors/IEC/Safety
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Old February 9th, 2018, 01:16 PM   #3
g.mccormick
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Put two contactors in series.
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Old February 9th, 2018, 01:25 PM   #4
mk42
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Quote:
Originally Posted by Timeismoney08 View Post
Okay,

So I'm designing a motor circuit that will have E-stops running to a AB safety relay.

What keeps the Contactor that the safety relay will pull in safe? How do I prevent a contactor from becoming welded?

I've went over some historic drawings of other machines I've seen and the safety relay is always force guided and control reliable, but it always seems as if the contactor were welded shut, nothing would ever know.



Thanks in advance!

Generally, you use TWO contactors, so that if one welds shut, the other will still break the circuit. You should also be using contactors with an auxiliary feedback contact, to report status back to the system/safety relay.

I believe it is usually required to have a periodic test carried out (once a shift, once a day, once a week, depending), to check for hidden faults. This is often procedural, not automated.
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Old February 9th, 2018, 01:26 PM   #5
Timeismoney08
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Originally Posted by g.mccormick View Post
Put two contactors in series.

Put L1,L2,L3 through one contactor and then through another contactor in series. Then to pull in that contactor, have the output side of each chain on the safety relay pull in each contactor individually and monitor the AUX on the PLC?

Would the PLC need to be safety rated at that point to monitor for a stuck contactor? and would that make it control reliable as long as there is no one point of failure that could trick the system into always running?


Thanks!
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Old February 9th, 2018, 01:31 PM   #6
Timeismoney08
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Originally Posted by mk42 View Post
Generally, you use TWO contactors, so that if one welds shut, the other will still break the circuit. You should also be using contactors with an auxiliary feedback contact, to report status back to the system/safety relay.

I believe it is usually required to have a periodic test carried out (once a shift, once a day, once a week, depending), to check for hidden faults. This is often procedural, not automated.

How do you monitor the two contactors in series with a safety relay if they only pull in when it's in a safe condition? Would I need 1 Safety relay for the E-stops and 1 for the Motor/Contactor Circuit? Could I use Auto reset on that safety relay just for the contactors?
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Old February 9th, 2018, 01:31 PM   #7
jstolaruk
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Quote:
Originally Posted by Timeismoney08 View Post
Would the PLC need to be safety rated at that point to monitor for a stuck contactor? and would that make it control reliable as long as there is no one point of failure that could trick the system into always running?


Thanks!
No the safety relay should have those monitoring inputs.
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Old February 9th, 2018, 02:02 PM   #8
g.mccormick
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The reset for the estop relay would go through the two NC contacts on the contactors in series. So that both of the contactors have to be off before the estop can be reset.
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Old February 9th, 2018, 06:29 PM   #9
Timeismoney08
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Originally Posted by g.mccormick View Post
The reset for the estop relay would go through the two NC contacts on the contactors in series. So that both of the contactors have to be off before the estop can be reset.
Genius! Thank you everyone for the help! I was sure there was an easy solution.

What if the motor being controlled didn't cause a danger? Would I still need that high level of guarding?

Thanks!
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Old February 9th, 2018, 07:47 PM   #10
PLCnovice61
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Its unlikely that the motor parse would cause a danger, it is the machinery that the motor is driving that would be the possible danger. The best/correct method is to have done a risk assessment and then do the circuitry to the level require to meet assessed danger level.
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Old February 10th, 2018, 05:51 AM   #11
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Originally Posted by Timeismoney08 View Post
Genius! Thank you everyone for the help! I was sure there was an easy solution.

What if the motor being controlled didn't cause a danger? Would I still need that high level of guarding?

Thanks!
You are on the right track by by analyzing the system for potential safety concerns. I think about safety before I ever start, and then again, afterwards.

You can't always design to the maximum level of safety, there is always a trade-off, and as the designer of the system, it is up to you to to determine where they meet. You want to build a solid-reliable system that is easy to troubleshoot, will not hurt anyone, and is as safe as it needs to be to protect people. A wise man once told me:

1. Operator safety
2. Product quality
3. Production Efficiency
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Old February 14th, 2018, 10:17 AM   #12
mk42
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Quote:
Originally Posted by Timeismoney08 View Post
Would the PLC need to be safety rated at that point to monitor for a stuck contactor?
The feedback should go to whatever device is controlling the contactors. That COULD be a safety rated PLC, but could also be a safety relay, or something else like a safety drive.
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Old February 14th, 2018, 10:35 AM   #13
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Read ISO 13849 for how to do risk assessments and implement appropriate safety circuits to mitigate the risks identified.
A single contactor can be ok for a hazard requiring PLc mitigation.
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