Stupid question about voltage

I actually hesitate to ask this, because buried deep inside my brain is the memory of a memory of the answer, and I think I'm going to sound stupid. However, here it goes.

I have a series of cabinets providing I/O to various locations. These cabinets are spread across the platform. They are all powered with Flex I/O power supplies. My voltage, in every single cabinet, reads as such:
13 volts, positive (red) to ground.
-11 volts, negative (black) to ground.

Yes, I realize that gives me a difference in potential of 24 volts. But WHY? Why is it set up that way?

Go easy on me, it's been a bad day.

The "reference to ground " is only valid if either the - or +is grounded .

As jrwb4gbm said it means the power supply is not grounded.

In my opinion that is a good thing ( others will disagree). Ground is a very noisy environment.

Your comment about being "spread around the platform" suggests this is a oil and gas sort of application.

I'm not in that industry, but I've read that ungrounded 24V DC distribution is common when you have critical loads and don't want a ground fault from one conductor to take down the whole system.

If you have a system where neither the + nor the - side is grounded to Earth or to the chassis/frame/enclosure, then short-circuiting one cable to the enclosure won't cause current to flow through the structure to the other pole of the power supply.

That is why is was jiggling something in my head.

In the environment I grew up in (working on Radars), everything was grounded. EVERYTHING. But sometimes that connection to ground would be damaged and we would see it in symptoms like this. It's just been awhile since I've worked on an ungrounded power supply and my brain really refused to put two and two together. I just knew that the thing that was really important, from the IO modules point of view, was that 24 volts was present.

Thank you for answer a dumb question. Lol.

Ken,

You are absolutely correct, and now that everything has clicked together, it makes sense why it would be set up that way. I also just spent a few minutes reading some references I found that basically said the same thing. However, now I'm a little concerned because that is definitely not the default condition in the area I am working at. SO I'm going to have to do some checking to see what the intent was there.

Mickey said:

As jrwb4gbm said it means the power supply is not grounded.

In my opinion that is a good thing ( others will disagree). Ground is a very noisy environment.

Click to expand...

Those who disagree and have big on site experience are in minority.

The fact that you get the same voltage split in every panel suggests to me that you also have some sort of ground fault monitor on the system which has an earth connection in the middle of a high resistance potential divider. It is a long time since I worked on offshore power supplies but this used to be a common arrangement.

Those who disagree and have big on site experience are in minority.

Click to expand...

I NEVER ground my 24VDC supply from the switch mode power supply to the PLC and PLC I/O and have never had a problem in thousands of jobs since the early 1990s including in HV power stations - not a friendly place. To the contrary I have had to 'fix' others work from time to time when noise was a big problem - simple - disconnect the ground. Additionally if you ground the negative of a switch mode power supply you lose your isolation.