(EE Question) - EPO or Emergency Power Off

[trex]

Hi,

I assume I can post whatever in this SIG and I hope if anyone of you have knowledge in this question, please help me.

Now I am talking about EPO, the Emergency Power Off button on a machine. The EPO is actually a switch, when you press on it, it will "open" and when you twist and release it back, it will become "close". EPO is used to turn off the whole machine or system if any abnormality found by users. I found an interesting situation and I can't give a good reason to convince myself. Here you go:

The circuit of the EPO system in this machine shown below:
http://www.recom.org/modules/coppermine/albums/userpics/12401/Drawing1.jpg

I have 4 EPO on this machine, which is A, B, C & D. Initially when I pressed B, the machine was running fine without powering off. This situation remain same when I pressed on EPO C & D. When I pressed on A, the machine powered off.

So, from the simple circuit above, we know that all of them are connected in series, which mean anyone of them have open-circuit, the machine will turn off.

When I measure the continuity of each EPO, I found B C and D will have "open" when I press the switch and have resistance value of 5-15 Ohm when they are closed. Only A, before I pressed it, the resistance was about 10-15 Ohm and when I pressed it, the resistance value was 600-650 Ohm... The voltage value across EPO A and D is only DC 24V.

After I changed the EPO A, each of them can trigger the machine to power off. So, can anyone of you tell me what is the theory behind for this weird situation?

By the way, I am sorry of my poor English. Do inform me if you don't understand my story.

Tyrannosaurus Rex

[digimushu]

Description of problem:

I have 4 EPO on this machine, which is A, B, C & D. Initially when I pressed B, the machine was running fine without powering off. This situation remain same when I pressed on EPO C & D. When I pressed on A, the machine powered off.

Comment: Judging by what you say, all switches should be 'normally closed' unless pressed. Have you tried pressing C and D and leaving A and B to see whether it still runs? What we need to do is to narrow down to see whether the problem is mutually exclusive to one switch or a combination of switches.


Diagnosis:

When I measure the continuity of each EPO, I found B C and D will have "open" when I press the switch and have resistance value of 5-15 Ohm when they are closed. Only A, before I pressed it, the resistance was about 10-15 Ohm and when I pressed it, the resistance value was 600-650 Ohm... The voltage value across EPO A and D is only DC 24V.

Comment: Was B, C and C closed when you measure the resistance across A? if so, then you are looking at the impedance of your system as seen from point A and point B in open circuit conditions..OR, it could be switch A is faulty and short circuited(which i think it is).

Solution:

After I changed the EPO A, each of them can trigger the machine to power off. So, can anyone of you tell me what is the theory behind for this weird situation?

Comment: I think there was a manufacturing defect in A since it still has measurable impedance in open conditions. However, that does not explain why switches B, C, and D are acting weird. Mind to give us the website of the company that manufactured it so we can look at the spec sheet? Anyone else has any more input?

[trex]

Comment: Judging by what you say, all switches should be 'normally closed' unless pressed. Have you tried pressing C and D and leaving A and B to see whether it still runs? What we need to do is to narrow down to see whether the problem is mutually exclusive to one switch or a combination of switches.

Yes, all EPO are normally-closed. I have tried all possibilities. Only B, C and D didn't work before I changed A and even though A was faulty but pressing A was able to power down the system.

Comment: Was B, C and C closed when you measure the resistance across A? if so, then you are looking at the impedance of your system as seen from point A and point B in open circuit conditions..OR, it could be switch A is faulty and short circuited(which i think it is).

Actually, I won't measure the continuity with complete circuit. I unplugged the A, B, C & D circuit out from the Power Alarm Module and only measure the wire in series. So I believe no such impedance should be considered. Even if you said, A was short-circuited, it doesn't make sense that when I pressed B, C or D, it didn't trigger the "open-circuit" to the Power Alarm Module, right?

Comment: I think there was a manufacturing defect in A since it still has measurable impedance in open conditions. However, that does not explain why switches B, C, and D are acting weird. Mind to give us the website of the company that manufactured it so we can look at the spec sheet?

The EPO A is definitely faulty but it is short-circuited. So to the overall circuit, the A is something like transparent to them. It won't affect the functionality of B, C and D.

Anyway, thanks for your time. These are the information I have when I did the troubleshooting. As I have said, none of the info is strong enough to convince me but to make that system back to normal is my first priority. If you guys don't mind, I will continue post whatever I encountered here. By the way, I am not a student, so all my problems you won't find it from the books.

Thanks Digi-mushu.

[digimushu]

No prob,

Sorry i could not help much. case closed.