Negative 48 Volts DC

Right, Say you have a 48V battery. You hook up the neg. terminal to ground, you've got +48V at the positive terminal. You hook up the pos. terminal to ground and it's -48V at the negative terminal.

George H.

I'm at a loss to understand what you are confused about. Do you have any specific questions???

How does any of this impact using the 48 volts to supply the equipment you list? In other words, why are you confused?

Sure. It goes back to telegraphy and telephony. The power to run the telegraphs and telephones came from batteries. One terminal of the battery went to earth ground. The other terminal went to the wires between stations. The problem was, which is better? Positive ground or negative ground.

It was decided to use positive ground. The battery positive terminal went to ground, while the negative terminal went to the stations and wires resulting in a -48VDC system. The selection of polarity was NOT arbitrary. A positive ground offers better protection against corrosion: To provide cathodic protection against corrosion of line wires, operating potential of telephone lines is typically negative with respect to ground, and the tip side is generally close to the ground potential. Thus, all power supplies for telecommunication equipment are designated to supply a negative voltage.

When the internet was being built, data communications went via telephone company services and lived in telephone company owned central offices. These offices were powered by -48VDC power. It made sense for the new internet services, switches, muxs, termination equipment, PoE, etc to be powered by the same -48VDC power.

Please note that red=positive and black=negative is an color automotive standard. Color codes are VERY different in other industries.

Also, 48VDC is considered "safe" and suitable for using "low voltage insulation". Any higher voltage requires additional protection. However, in the electric power industry less than 50V is consider "extra low voltage" while anything under 1000V is considered "low voltage". Sigh.

I had a British car once with positive ground.

I had to replace the battery and, instinctively, strapped the negative lug to the chassis. When I connected the other cable and got a big spark, I understood the situation in milliseconds. Too many milliseconds!

I blew out all the diodes in the alternator, and replaced them with a couple of big Motorola bridge rectifiers. Luckily, the radio was off at the time.

I'm working with alternators again, FADEC power supplies. You short a PM alternator to regulate the output.

I think positive ground has some sort of corrosion advantage.

Am 25.01.20 um 20:57 schrieb Jeff Liebermann:

Amen.

A few additions:

The terminology is TNV-x (TNV-1 or TNV-2 or TNV-3). Here is a nice tutorial:

The IEC/EC/UL 60950-1 is past its DoW. It has been replaced with the IEC 62368-1. Many standards still refer to the 60950-1 and it will take at least another decade until this has changed.

Michael

Michael Nagel wrote in news:r0i8pk$eq9$ snipped-for-privacy@news-1.m-online.net:

Great link... thanks.

Okay.

This seems to assume that you hook the negative terminal of your meter to ground and the positive terminal of your meter to the other battery terminal.

So with a positive ground system, the meter's negative terminal is hooked to the batteries positive terminal and the meter's positive terminal is hooked to the batteries negative terminal.

This I think that "ground" / "common" is largely irreverent in this scenario. The only way that I think it starts to make any difference is if you don't measure at the batteries terminals and instead measure off of a different circuit in the car reference to common.

Not in cars. For telephone wires, it's a different story.

It's simnple high school chemistry. Electrolysis liberates metal ions at the positive terminal and produces OH- at the negative terminal.

when copper is at the positive terminal, copper ions are released. This corrodes the copper.

When copper is at the negative terminal, there is only OH- produced. There is no corrosion.

Telephone companies can drive a large ground rod into the soil to take care of the loss of ions at the positive terminal. This protects the positive copper terminal from corrosion. Since there is no corrosion at the negative terminal, the copper wires are protected and can remain buried forever.

I have a -48 VDC power supply with an unlabeled terminal and a return terminal.

Other sets of DC equipment & associated supplies (multiple small sets of different supplies and equipment used for various things) use red and black wire.

What is return? red or black

What voltage should be read when connecting the meter's positive / red lead and negative / black lead to the red & black wires from the DC supply?

How should things be wired to a piece of equipment that has the ground symbol on one terminal and the other terminal unlabeled?

My brain is having problems aggregating all of these and making associations.

Which terminal of the DC source; unlabled and return, connects to the ground terminal on the DC load equipment?

All of the -48 VDC equipment that I've seen used have used red, black, and maybe green for a safety ground.

I think that red is the "hot" wire with -48 VDC with reference to the black "common" wire. But I'm not sure.

See my other message ~5 minutes ago for more details.

Interesting. So the ground rod is for more than just safety. It's actually used to control / prevent corrosion too.

I guess pipeline companies bury old pipe out in fields then send some current out to the buried pipe. That protects the actual pipeline they want to use.

Voltages are about differences. You have to set 'zero' somewhere... or we can't really talk about it. Ground is a common point (NPI) you can float everything above ground... but it gets dangerous to touch above ~60V (depends on source impedance) GH. Are you an EE?

Go to harbor freight (assuming you live in US) buy the cheap DMM and measure it! Otherwise you have no idea how red and black are set. GH

Here is the DC color code used by two very large US wireless carriers (i.e., cell phone providers) at their cell sites (and, signal sources like HetNet that act like "mini-cell sites"):

Coming off the batteries to the rectifier: (Input to the rectifier) (The rectifier keeps the batteries charged.) Black = Negative Red = +48 VDC

Output of the rectifier feeding all the radio & other cell site equipment: (Note: This is usually the input to a PDU, which is a fused Power Distribution Unit - a fancy name for a fused distribution block, panel-mount.) And everything at many cell sites is 48 VDC, including the overhead lights, and sometimes even the air conditioning.

Black = Negative 48 VDC There is a saying in the industry: "Black to the breaker."

Grey (or White) = Return (this is 0-VDC potential relative to Earth ground), and at the rectifier, is bonded to..

Green = Earth Ground (i.e., the main facility ground bus bar) The equipment racks are also connected to ground using Green cable.

The cables used are typically TelcoFlex-III, 600 Volt. ($$ expensive cable) I've seen anywhere from #10 AWG to #2 AWG, depending on how many cell site radios are installed.

One almost always connects a meter negative terminal to ground. That way, you can see if the voltage you're measuring is positive or ground.

Ignore ricky. He doesn't want to help, he wants to insult.

Most of what he says translates to "you are stupid."

Unless there is a connection within the power supply to earth ground or som e other reference point such as a ground for a control signal, the power su pply is just a 48 volt power supply. One terminal is positive and the othe r negative.

When you say "return" terminal, what do you see that distinguishes the two terminals? Is one labeled "return"? Is that the positive terminal? If yo u plug it in and measure the terminals to earth ground, what do you get? I f one approximately zero volts and the other positive or negative? You may need to put a 1 kohm resistor across the meter to prevent seeing stray vol tage while you do this.

In typical DC electronics, red is positive and black negative. I don't wor k with -48 volt circuits a lot, so I can't say that this applies for that.

y?

What is the existing system using? I would go with that unless you want to rewire the whole thing.

Is the equipment designed for -48 volts? If so connect accordingly. I gue ss you are asking about colors. I don't know convention other than in smal ler gear where red is positive and black is negative.

The main thing is to not think in terms of "reference" or "ground" but posi tive and negative. You won't blow up things if you keep polarity straight.

If the equipment is not presently connected (for you to measure and copy), I recommend you get a manual for the equipment and find out what polarity t hey are expecting. Getting that wrong can destroy the equipment.

Notice the troll likes to demean and doesn't actually help much himself.