How to measure 125VDC contact inputs at Mark V DTBB terminal board

I'm sure this is a simple thing but I am having difficulty in understanding how to interpret the 125VDC voltage readings for Mark 5 switch inputs at the terminals of a DTBB board. I am troubleshooting about 5 alarms that are related to input devices landing on the CD core termination board DTBB. I am trying to isolate the problem to either Mark V hardware or real world devices. Referencing the GEH-6195 Application Manual Appendix D - Signal Flow diagrams page D-4 (Digital Inputs on DTBB Terminal Board), I am trying to determine how to measure VDC with a DMM the contact inputs in a way as to know whether they are closed or open. I would like to know what to expect to see in VDC across the POS and the RET input terminals when the contacts are closed and/or open. Also I would like to know what to expect to see referencing ground with the black lead of the DMM and the red lead at the POS and subsequently the RET terminals for both the closed and open states of a contact. Thank you for your help and your patience.

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Dear phineascage, there are others that may have more time to provide a longer response, but here are a few quick answers. Nice job explaining and asking the questions BTW, and referencing the manual.
I would like to know what to expect to see in VDC across the POS and the RET input terminals when the contacts are closed and/or open.
Measuring across the POS and RET with the contact open you should see full battery voltage ~125 vdc.
Measuring across the POS and RET with the contact closed you should see ~0 vdc.

Also I would like to know what to expect to see referencing EARTH ground with the black lead of the DMM and the red lead at the POS.
Assuming your system is free from other grounds and "Balanced" you should 1/2 of the full battery voltage or ~+62.5vdc

With the contact closed referencing earth ground with the black lead of the DMM and the red lead at the RET, 62.5
With the contact open referencing earth ground with the black lead of the DMM and the red lead at the RET, -62.5

In my opinion it is easiest when first starting to diagnose an isolated system like this to first verify the state of the system. First check overall system voltage, black leads on battery negative, red lead on battery positive, should read ~125vdc.
Using your meter with black lead on earth ground check the 125vdc positive, should read +62.5, check the same to 125vdc negative, should read -62.5vdc. If the legs are not balanced, then you have some sort of leakage to ground.

Isolated systems like this can be a little tricky to wrap your head around, in my opinion since earth ground and battery or system negative is not the same. And point of reference is very important when it comes to understanding.
Let us know what you find, or come back with more questions. Hopefully this helps, now I need to get back to my other work I should have been doing!!
 
Dear MIKEVI,

Thank you for the quick response and the kind words.

Measuring across the POS and RET with the contact open you should see full battery voltage ~125 vdc.
Measuring across the POS and RET with the contact closed you should see ~0 vdc.

Referencing EARTH ground with the black lead of the DMM and the red lead at the POS.
Assuming your system is free from other grounds and "Balanced" you should 1/2 of the full battery voltage or ~+62.5vdc

With the contact closed referencing earth ground with the black lead of the DMM and the red lead at the RET, 62.5
With the contact open referencing earth ground with the black lead of the DMM and the red lead at the RET, -62.5


I had suspected the readings as you described above but for some reason I was overthinking this or was just having difficulty comprehending what to expect to see in measurements for the isolated ground 125VDC system.

In my opinion it is easiest when first starting to diagnose an isolated system like this to first verify the state of the system. First check overall system voltage, black leads on battery negative, red lead on battery positive, should read ~125vdc.
Using your meter with black lead on earth ground check the 125vdc positive, should read +62.5, check the same to 125vdc negative, should read -62.5vdc. If the legs are not balanced, then you have some sort of leakage to ground.


Thank you for pointing this out. I failed to mention that our system is relatively well balanced. Our battery negative measures about -58VDC and the positive battery measures about +57VDC, both referencing ground. Across the two I see roughly 115VDC. Our system voltage is a little on the low side but as far as I can tell is not itself an issue currently.

Isolated systems like this can be a little tricky to wrap your head around, in my opinion since earth ground and battery or system negative is not the same.


Agreed. Very tricky. I have experience with grounded VDC and AC interfaces but have quickly learned that some of that knowledge does not translate well to the world of isolated ground VDC systems. Thank you for taking the time to help me with this. I hope I haven't inversely impacted your productivity too terribly.

Best regards,
P
 
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