Negative 48 Volts DC

Is anyone willing to explain Negative 48 Volts DC?
I'm trying to learn about it because we're using it for a few things at
work and I'd like to better understand what we're working with than
simply going through the motions that someone else dictates.
I've learned that -48 VDC uses what is called "Positive Ground" and that
what I'm used to is called "Negative Ground".
I'm trying to correlate and understand the typical red and black wires
with them being ground / return / common / hot / etc.
I suspect that there is more to it than simply reversing the + and -
lead from a battery or meter. But I can't wrap my head around it.
Can ~> will someone offer any comments that might help me understand better?
Reply to
Grant Taylor
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I can't offer any real words of wisdom except to say old tractors and cars used 6 volt positive ground. There's a little bit here about the disadvantages of that: I found that with a search for six volt positive ground.
Reply to
Dean Hoffman
I noticed that when doing some research about positive ground systems.
I found many such articles when researching.
I'm more after a fundamental understanding of how positive & negative ground differ, particularly from an electronics standpoint. Is there really any difference to components, like a light bulb / LED, in a positive / negative ground system? (Obviously the LED needs to be oriented correctly.) But do these otherwise electrically isolated components care if the overall system is positive or negative ground?
My extremely loose understanding is that the ground / return / common is effectively just a common reference point, which happens to be grounded. I think that components largely don't care what system they are in as long as their individual parameters are satisfied. But I'm having a difficult time finding supporting documentation.
Note: The -48 VDC that I'm working with at work are servers and telco equipment. As such, the equipment is more complicated than an older automobile's electrical system. Thus they are likely more sensitive to things.
Reply to
Grant Taylor
Try the sci.electronics.design group. I bet someone there can do you some real good. It's busy unlike so many of these usenet groups.
Reply to
Dean Hoffman
It is really just arbitrarily deciding which side of the 48v you are going to ground or use as a chassis common. From the aspect of how it works, you only need to respect the polarity of attached equipment if it cares. The best reason I have heard to do it has to do with minimizing electrolysis but simply based on the number of negative ground systems you see, it must be a minimal effect. You are right, POTS telco systems are 48v positive ground. Somebody over 100 years ago made that decision.
Reply to
gfretwell
Thank you for your input and redirect. I'll see what I can learn from the sci.electronics.design newsgroup.
Reply to
Grant Taylor
Okay. That's what I'm starting to understand. Thank you for confirming.
I don't know how minimal the effect really is.
I suspect in an optimal circuit, the effect is minimal. But I further suspect that a sub-optimal circuit the effect is considerably larger. Consider wiring with imperfect insulation buried and wet, thus forming an alternate path back to the source. If this happens on the negative side, the electrolysis effect could be considerably greater than if it happens on the positive side.
I think I read where POTS telco systems started with negative ground and then switched to positive ground to combat corrosion.
Reply to
Grant Taylor
Yes, I think that's correct. The only other reasons for earthing a CO battery terminal I can come up with were to support ringing-to-ground on party line systems and as possibly a shunt path for lightning strike. In the real old days of military field telephones, the Army would sometimes utilize a "simplex" circuit to allow a ground-return telegraph or telephone circuit to be superimposed on a full metallic telephone circuit. (Don't think that was used in commercial practice, though.) Sincerely,
Reply to
J.B. Wood
See:
formatting link
on polarity. It is to help with corrosion. The -48v migrated out of the telco world into the server world. Not sure why but I suspect as computers moved into the CO offices they wanted to use the battery supply.
Data centers are moving away from the old -48V because of the inefficiencies and cabling cost. 480 3 phase to PDUs that then provide 208 or 120 to the racks is more and more common. Better efficiency and less cost.
Reply to
Joe Chisolm
Thank you.
This is my understanding.
This is my experience too. Hence why the -48 VDC supplies and equipment that I've worked with are usually much smaller and directly associated with each other. I.e. -48 VDC supply to power a piece of equipment being tested in the lab. Usually said equipment would then be deployed to partners around the world that had a -48 VDC plant and wanted our equipment.
This is exactly what we have at my office. 13 thousand and something volts 3? to the property. 13, gets stepped down to 480 VAC just inside the building. Then 480 VAC is distributed to the PDUs on the DC floor. The PDUs step down the 480 VAC to 120 VAC. The two or three ?s of 120 VAC is then distributed to cabinets. The Cabinet Distribution Units then distribute ?-to-? of 208 VAC to equipment. Some CDUs are 1? with two hots and a neutral / ground. (I'm not sure if the 3rd wire is actually neutral or ground.) Other CDUs are 3? with three different sets of ?-to-?; Black-to-Red / Red-to-Blue / Blue-to-Black.
I'm re-using the numbers that have been used in this discussion. The quick math shows that 110 VAC ?-to-neutral results in 209 VAC ?-to-?. 120 VAC ?-to-neutral results in 228 VAC ?-to-?.
V = 120 VAC ____________________________________________________________
This brings up another long standing question of what is the typical voltage in the U.S.A. 110 /
120 / 125 / ??? But I digress.
Reply to
Grant Taylor
Nameplate voltage is 120V, sometimes I see 125V probably to say something at the "high end" is OK. My outlets are 120 or 121 volts.
Ancient stuff was rated at 110 or 115 volts, many tube type radios were rated at 117 volts. I think they increased the voltage over time in small steps. You'll still hear electricians or others refer to "220" (volts) as the voltage to high power devices on residental split phase connected to both hots rather than hot to neutral. It's really 240 volts.
3? should be 208V/120V. For three phase, phase to phase voltage is ?3 times the phase-neutral voltage.
Same for 480V. It is really 480Y/277V, but 277V single phase is rarely used except for industrial lighting.
Reply to
Michael Moroney
"Nameplate". I like that. That's very "on the tin" type description. :-)
The small desk side UPS beside me is saying 124 VAC.
Interesting. I wonder why that was done.
*nod*
Hum. Now I'm going to need to go back and re-read / re-watch some material that mentioned rectangular & polar coordinates to confirm how they were doing things. It seems like ?3 is much simpler. I wonder why trig is coming to mind. :-/
*nod*
You're writing that as ?-to-? / ?-to-neutral. Where I'm used to seeing ?-to-neutral / ?-to-?. I wonder why the difference.
Reply to
Grant Taylor
Yup. For old timers...
Probably just to get more power through a circuit by changing one big transformer upstream. Not enough of a change to need to upgrade insulators etc. Maybe.
It's trig that had to be done just once. Anyway, draw a triangle with 2 sides 120 (volts) long and the angle between them 120 degrees. Find the length of the third side.
(if you split the third side in half and draw a line from the midpoint to the vertex with the 120 degree angle, you produce two standard 30-80-90 right triangles if that's easier for you.)
It's the standard notation you'll see in specifications/instructions etc.
Reply to
Michael Moroney
In accepted nomenclature if the big number is first, it is 3 phase, if the small number is first it is single phase. 240/208 is 3p wye 240/120 is 3p delta (center tapped) 120/240 is single phase. I also assume the "?" that is showing up is just the way a news client is rendering the "root" symbol. The ratio of L/N to L/L voltages on 3p wye are a function of the square root of 3.
Reply to
gfretwell
three phase power for irrigation here in central Nebraska. The Y configuration has 277 volts to ground on all three legs. Two lines in any combination read 480. The corner ground delta has two lines reading 480 volts to ground and between each other. The third line reads 0 volts to ground. The center tap delta has one line that reads 415 to ground. The other two lines read 240 to ground. The hot lines all read 480 in any combination to each other. One of those configurations lets the utility use two transformers to supply three phase.
Reply to
Dean Hoffman
:-)
*nod*
I'm starting to re-watch some videos on 3? on MathTutorDVD.com, and I'm
there is more to this particular discrepancy. I'll respond with more later.
It sounds like you might be talking about a phasor. But I don't think I'm correctly unpacking what you're talking about.
Thank you for explaining.
Reply to
Grant Taylor
Okay. That makes quite a bit of sense.
Um, That doesn't make sense to me.
I don't know how to unpack that. (I don't know as much about ? as I do about Y.)
That makes a lot of sense to me and what I'm accustom to hearing.
Probably.
That's different than I remember. But I could be misremembering or had the wrong understanding at the time. I'll refresh myself and reply later.
Reply to
Grant Taylor
3 phase Y is 208V/120V.
I assume you mean red leg delta, where 1 transformer is residential style (ct 120-0-120) and a second 240V transformer goes from one of the 120V lines to the third red leg/orange leg/wild leg/stinger. An optional third transformer goes from the other 120V connection to the wild leg. This produces a 240V delta, (open delta if 2 transformers), 240V between any two hot legs, 120V on first transformer to neutral. But in the breaker box, don't connect lighting circuits to the wild leg phase (B phase in breaker box)! It's 208V to neutral, not 120V.
Reply to
Michael Moroney
Yes, typo. 30-60-90 right triangle. The three sides are 1/2 : ?3/2 : 1.
Reply to
Michael Moroney
You can use 2 transformers on either delta and the center tapped "delta vee" is very popular for industrial bay type operations where a significant part of the load is 120/240 single phase
Reply to
gfretwell

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