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Timeismoney08 February 9th, 2018 01:12 PM

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!

jstolaruk February 9th, 2018 01:15 PM

Check out A-B's safety contactors http://ab.rockwellautomation.com/Mot...ors/IEC/Safety

g.mccormick February 9th, 2018 01:16 PM

Put two contactors in series.

mk42 February 9th, 2018 01:25 PM

Quote:

Originally Posted by Timeismoney08 (Post 768164)
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.

Timeismoney08 February 9th, 2018 01:26 PM

Quote:

Originally Posted by g.mccormick (Post 768167)
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!

Timeismoney08 February 9th, 2018 01:31 PM

Quote:

Originally Posted by mk42 (Post 768170)
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?

jstolaruk February 9th, 2018 01:31 PM

Quote:

Originally Posted by Timeismoney08 (Post 768171)
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.

g.mccormick February 9th, 2018 02:02 PM

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.

Timeismoney08 February 9th, 2018 06:29 PM

Quote:

Originally Posted by g.mccormick (Post 768180)
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!

PLCnovice61 February 9th, 2018 07:47 PM

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.

sparkie February 10th, 2018 05:51 AM

Quote:

Originally Posted by Timeismoney08 (Post 768222)
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

mk42 February 14th, 2018 10:17 AM

Quote:

Originally Posted by Timeismoney08 (Post 768171)
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.

AustralIan February 14th, 2018 10:35 AM

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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