Industrial Control circuit problem

Thread Starter

strantor

Joined Oct 3, 2010
5,160
I've created a skills assessment test for the purpose of hiring maintenance technicians. The question below is one of the questions on my test. So far out of 10 candidates nobody has gotten it right. So now I'm doubting myself; maybe I'm not clear or maybe I'm just plain wrong. I don't want to divulge the answer just yet (I will later) because I don't want to set up any preconceptions that will alter anyone's thought processes as they try to work it out.

If you have nothing better to do, please answer the question and give a brief explanation of why you chose it.



image001.png
 

MaxHeadRoom

Joined Jul 18, 2013
19,186
I much prefer JIC/NEMA symbol DWG's as opposed to IEC

For a N.A. questionnaire I would think that JIC versions would be better for a test.
I know there are imported machines with associated diagrams in IEC format/symbols.
Also, the JIC conforms to the PLC screen style presentation, even European PLC vendors display this way, i.e logic progresses from left to right, rather than top down.
Max,.
https://www.reddit.com/r/PLC/comments/75izvn
 

Thread Starter

strantor

Joined Oct 3, 2010
5,160
The Whole thing is confusing to start with, no wonder no one is getting it right..
Ok now we're getting somewhere. Thanks for the feedback. What specifically is confusing about it? It's important for me to get this right because it's worth a significant portion of the grade.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,160
It certainly is for me - I don't understand the symbols. What is K00?
Understanding of the symbols is part of what the question is meant to test. I can see where, if you didn't know what you're looking at, it would be impossible to solve. While I am trying to suss out symbol knowledge and troubleshooting process logic at the same time, maybe the safety relay K00 symbol is too specific to be a fair assumption of prior knowledge.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,160
I much prefer JIC/NEMA symbol DWG's as opposed to IEC

For a N.A. questionnaire I would think that JIC versions would be better for a test.
I know there are imported machines with associated diagrams in IEC format/symbols.
Also, the JIC conforms to the PLC screen style presentation, even European PLC vendors display this way, i.e logic progresses from left to right, rather than top down.
Max,.
https://www.reddit.com/r/PLC/comments/75izvn
I totally agree. JIC is a WAY better system. But 80-90% of our machinery is German and the ability to read IEC schematics is a must.

Well maybe not a "must." If a guy had "the knack" and was fluent in JIC, he could with a few days training, be more than capable of navigating our machinery and its schematics. But if I have a pool of candidates who already know what they're looking at, I don't have much incentive to look outside of it.
 

MaxHeadRoom

Joined Jul 18, 2013
19,186
Understanding of the symbols is part of what the question is meant to test. I can see where, if you didn't know what you're looking at, it would be impossible to solve. While I am trying to suss out symbol knowledge and troubleshooting process logic at the same time, maybe the safety relay K00 symbol is too specific to be a fair assumption of prior knowledge.
K00 shows that it is a Safety Relay, which often come with unique attributes and are selected at the time of order when selecting such as Safety Relay's.
On a test, the characteristics would not normally be available.
Max.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,160
K00 shows that it is a Safety Relay, which often come with unique attributes and are selected at the time of order when selecting such as Safety Relay's.
On a test, the characteristics would not normally be available.
Max.
True but in my experience the basic operation is always the same. There's a N.O. circuit to set it and a N.C. circuit which, if opened, resets it. These circuits can be single (as drawn) or double/redundant circuits.

Have you ever seen a safety relay circuit which deviates from that?


EDIT:
I'm not trying to argue in favor of the validity of question I posted. I'm inclined to flush it if it's revealed to me that it's simply a bad question and its fault lies in my own preconceptions based on my own limited experience.
 
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MaxHeadRoom

Joined Jul 18, 2013
19,186
Have you ever seen a safety relay circuit which deviates from that?
There are different version, some with timers includes etc, designed to accommodate different shut down requirements, along the lines of the three basic modes of E-Stop as outlined in NFPA79 etc.
As well as to implement the old standard E-Stop shut down.
I assume that there are compatible modes use by the IEU etc.
If not already included in the current NFPA79, I imagine that Safety Relays will be mandatory soon in NA.
Max..
,
 

noweare

Joined Jun 30, 2017
107
I am comfortable with ladder logic and plc's and I was lost on the diagram.

If you are going to give a test, give it in a more general purpose way so people can understand the problem.
 

Jony130

Joined Feb 17, 2009
5,022
Well, if the voltage is present at X82/11 terminal, and on S33 when S03 is pressed, then there must be an open circuit outside the schematic (page 64).
 

MaxHeadRoom

Joined Jul 18, 2013
19,186
I am comfortable with ladder logic and plc's and I was lost on the diagram.

If you are going to give a test, give it in a more general purpose way so people can understand the problem.
As to my take, the diagram presents a 'black box', not a simple E-stop relay as implied in the schematic.
Without reference to the device in my link, there is no way of knowing the internal connection I/O to the K00 without the info sheet.
Max.. ,
 
While I'll admit I have no clue about the difference in symbols and all the variables with e stop relays I do know given the conditions listed and tracing the paths would lead me to believe the issue is in another circuit not shown. I spent some time in a factory and looked through whatever technical stuff I could find laying around just to stay awake at times. If that is the correct answer possibly people are just making it harder than it needs to be.

I decided to add that since you do not specify whether or not there is voltage after the NC e stop it may cause some confusion, but also probably give the answer away. Maybe have people describe how they would troubleshoot it also?
 
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MisterBill2

Joined Jan 23, 2018
4,054
I agree with those stating that the drawing is confusing. not only does it use symbols that I have not seen in 45 years of designing controls circuits, it is also incomplete, with wires leaving that page and not one single note as to what they are connected to. I would refuse to accept a system documented so poorly. And those funny things that I assume intend to be pictures of a connector have inadequate descriptors. And it looks like terminal numbers are given, but no wire numbers, so how is a tech to know what circuit a wire is part of? And indeed, that "black box" of a safety relay MUST have the internal contacts shown to be of any value. There is a big reason for ladder logic drawings and it is because they need to be useful to service equipment. Drawings that do not convey enough information to allow the diagnosis of functional problems are not even worth the paper that they are printed on. I know that this is a one-sided criticism, but it is also correct. Such poor documentation would greatly extend the time that a production machine was out of service, and cost the company far more than having proper drawings.
 

be80be

Joined Jul 5, 2008
1,944
Maybe you should just say the answer so we can check your work LOL just kidding but there is know way to
see what's going on there.

But on a side note if you want a good maintenance technician watch them take the test see who really trying to find the fault
hire that person on the spot.
There the only one that going to try to get the job done and the only one that will learn what's needed for there job in a short time.

stop.png
That's more like what i look at thanks Max
 
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Thread Starter

strantor

Joined Oct 3, 2010
5,160
I agree with those stating that the drawing is confusing. not only does it use symbols that I have not seen in 45 years of designing controls circuits, it is also incomplete, with wires leaving that page and not one single note as to what they are connected to. I would refuse to accept a system documented so poorly. And those funny things that I assume intend to be pictures of a connector have inadequate descriptors. And it looks like terminal numbers are given, but no wire numbers, so how is a tech to know what circuit a wire is part of? And indeed, that "black box" of a safety relay MUST have the internal contacts shown to be of any value. There is a big reason for ladder logic drawings and it is because they need to be useful to service equipment. Drawings that do not convey enough information to allow the diagnosis of functional problems are not even worth the paper that they are printed on. I know that this is a one-sided criticism, but it is also correct. Such poor documentation would greatly extend the time that a production machine was out of service, and cost the company far more than having proper drawings.
I agree with your criticism, which is easy, because it isn't a criticism of me or my work. This is an actual schematic (that I didn't draw) from an actual machine (that I didn't build), and the scenario is plagiarized from a real-world experience that happened just over a week ago. Over the phone I had a mechanic take the measurements which became the bullet points beneath the diagram, and from those measurements I was able to determine that the problematic component was not drawn here. I had him check the e-stop switch contacts on page 64 (not shown) and he found there was a bad e-stop contact elsewhere on the machine.

While this drawing format IS confusing, it IS compliant with the IEC drawing standard and it IS complete, albeit not all on one page as you and I would like to see it. IEC drawing format is quite common across the world except here in N.A. and I need people who know what they're looking at. Or at least that can figure it out.
 
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