Negative 48 Volts DC

And the latter is his "positive ground" system. As someone else pointed out, ground in this case just means a system reference point. As an example, cars today have a negative ground system, the battery negative is connected to the car metal chassis, which is used as part of the return path for current. Some cars years ago, instead had a positive ground system, with the battery positive connected to the chassis instead. It's like driving on the wrong side of the road, some just had to be different. :) Fortunately with grounding, I think the weirdos wised up and AFAIK no cars have been built for decades that have positive ground.

In any system of connected wires at different potentials, you can arbitrarily label one of them "ground", and then measure the potentials of the other wires relative to your ground. You will then get some that are positive, and some that are negative.

For some systems, such as a car, it can make some sense to label whatever is connected to the chassis as ground, but it is still pretty much an arbitrary choice.

And of course, with a fixed system, one can connect one point to the soil under one's feet, and call that point ground.

But it's a naming convention, and nothing more.

In the absence of clear documentation on the system you're examining, the only practical option is to take a multimeter to it, and make measurements.

Sylvia

A communications system that I worked on for the International Space Station ran on -48VDC.

I should imagine for much the same reasons used to standardise road vehicle ground reference polarity and telephone exchange equipment grounding to earth - to minimise galvanic corrosion in vital metallic structures - the inside would be a humid environment - it has to be for the sake of the astronaut's respiratory health, providing ample opportunity for the water vapour to condense out onto cold spots within the structure of the ISS.

Incidentally, regarding the question of colour coding the polarity of telecoms DC supplies, the GPO (as it was when I joined that organisation as an apprentice in late '67) which is now known as BT (British Telecom), the -52v "hot" (aka "Battery") exchange equipment bus bars were insulated with blue plastic insulation (PVC afaicr) and the grounded (actually earthed) positive bus bars were all left bare.

The fact that only the 'live' bus bars were insulated and the choice of blue rather than black, were sufficient reminder (if a reminder was ever needed) that the 'dangerous' pole of the 52v DC supply was negative with respect to anything (all the exposed metalwork including the ground return bus bars) at earth potential.

This scheme neatly eliminated any possible confusion that could arise over which was the 'live' if the convention of red for positive and black for negative had been followed (red for danger and black for the safety of a conductor at ground potential failing to apply in this case).

As to whether or not this colour coding scheme was followed outside of the UK and its dependencies (and the 48vdc power standards that came in with the later solid state comms equipment), I know not so I'll leave that as a question for old timers who've worked in telephone exchanges outside of the UK to answer. ;-)

The universe is probably absolute ground, and also defines absolute zero velocity.

In a DC system, red is positive, black is negative, by convention. Terms such as hot, return and ground are simply descriptive, and usually applied in distributed AC in North America, where black is hot.

simply rely

at

.

Yeah, if we could just find the middle of it. Well, the part we can see an yway. What if we are off in a corner of the universe expanding away from t he center at greater than the speed of light?

So if the universe started as a singularity and expanded from there, how co uld any part of it speed away and become invisible by exceeding the speed o f light?

ion ran on -48VDC.

I don't know if it is still the case, but that last (and probably only) tim e I had to install a two-way radio into a backhoe or bulldozer - that was a "positive ground" system.

I remember it because there was a circuit on the microphone hang-up clip th at would automatically un-squelch the receiver (it used ground). But in th is case, we had to isolate the chassis of the two-way radio from "ground", as to avoid a short. I forget what we did as a workaround. I think the an tenna was also one of those Motorola NMO-type mounts (also "grounded"), so we had to do something to that too.

Someone earlier mentioned that "positive ground" systems tend to slow down corrosion. I don't know if that's true with backhoes, but they do spend th eir lives outdoors in the elements? (But so do cars, mostly.)

No. I'm a systems / network administrator by trade. I'm just trying to understand (more than wrote memory) how some of the equipment I administer works.

There are other, more skilled people between me and connecting equipment.

I naively agree with you. But the negative part of -48 VDC confuses me on what the positive and negative lead is. :-/

Yes. One terminal is labeled "return" and the other terminal is unlabled.

I have not yet done this test.

Yep. I'm familiar with this.

This matches my experience.

My limited experience with -48 VDC makes me think that it does not. Hence my desire to get a better understanding of -48 VDC.

Sadly, the few pieces of -48 VDC equipment that I have are all using small AC to DC power supplies for each piece of -48 VDC equipment. Both the supplies and the equipment are unique. There's nothing really common to look at. Save for the red and black wires. But with nothing else common to match against, it's tough to tell.

Yes.

I'm asking multiple questions in the hopes of getting a general understanding. I'd like to have more than simple wrote memory.

That's where the bulk of my experience is too.

If I put on blinders and look at just the positive / negative and what each piece of equipment wants, I agree.

I'm trying to learn more about how the multiple pieces of equipment work together as a system.

Yep. I know that the equipment is -48 VDC. I know that the supply provides -48 VDC. I'm just not clear on which terminal needs to be connected to what. Yes the manuals will be involved.

Unfortunately that question is confusing to me. lol One terminal is posit ive and the other negative. Unless there is an internal connection to grou nd (like the power line safety ground - which there shouldn't be) it doesn' t make any real sense to call it a -48 volt supply.

That's the part I'm not getting. Does the power supply actually say it is a -48 volt supply or just 48 volts? Or is the -48 volts just what you see in references to the supply elsewhere?

Bottom line is there's nothing unexpected going on. If it is wired for a -

The supply could very well be intended for -48 volt use with a positive gro und and so the label "return" means positive. Or it could also be a typica l supply with a positive output and a negative return. Is there a label wi th model number, maker's name, other identification? Find that and look up a data sheet.

When you say a better "understanding" you mean of the color code convention s? Technically it's all just wires and polarity, not much to understand.

That's what I'm asking, the wiring colors. Are they consistent?

Yes, you keep saying that, but you don't really say what is not clear. May be rote memory is good enough for now and a light bulb will come on later. That has happened for me in many areas.

I don't know what you mean by "blinders". You seem to think there is some greater understanding, but in reality it's all just a matter of polarity an d the conventions are just that, conventions.

Do you have a question?

Positive to positive and negative to negative. It's literally that simple. Since the vendors of the equipment seem to be fuzzy on labeling terminals , at least on the power supply, you should measure everything.

This seems to be the question you are asking I guess.

If you are working with a -48 volt system, the ground will be positive and the power lead will be negative. If your power supply is intended for such a system the lead labeled "return" will be positive and the other output will be negative.

It would be helpful to see some photos of the terminals and the power supply label.

OK, that makes sense. Good on you for trying to figure it out. probably you can get one of the tech guys to show you the voltage on a DMM (volt meter). (and even compare to ground on the ac plug.)

George H.

Hmmm... I guess the ISS doesn't need to worry about ground leakage.

In 1971, after graduating college directly into a recession, the only job I could find was working for the local 2-way radio shop. I spend

Somewhat later, I found myself servicing and later selling marine radios. On larger metal hull vessels, both the positive and negative lines are floating. Therefore, all our early marine radios had floating grounds. Eventually, a separate 12V power system was introduced for powering radios and such, which had a negative ground. Eventually, we supplied positive ground 12V radios on special order, where all the electronics was grounded to the chassis though multiple capacitors.

Ummm... if that were Motorola, it would be called a "hangup switch". It did have anything to do with the noise squelch but did turn off the PL (Private Line) tone squelch decoder. Usually, the button on the back of the microphone was connected to ground when the PL was enabled through the "microphone hanger". However, for positive ground system, there was an isolated leaf switch inside the "hangup switch" and the button on the back of the microphone was not used.

Sounds like a later model radio that didn't have a positive ground feature or option. I've seen radios installed on wooden platforms to provide the necessary insulation. A plastic case microphone, isolated hangup switch, and capacitor coupling the antenna coax ground were also common.

I've seen what happens when someone forgets about NOT ground the antenna shield. Things mostly work ok, until someone blows the fuse in the negative power line. That makes the only path to battery negative through the coax cable ground. It usually works ok, until someone keys the transmitter. The coax shield becomes hot enough to melt the outer jacket and possibly set fire to everything nearby.

The corrosion aspect was important for telco plant, but less so for automotive. For cars, the manufacturers wanted to save money. Why waste money on copper "ground" wire when they can run the current though the body of the car. At high currents, that was also good for re-heating all the Unibody spot welds. Usually about 10% of these welds would fail, which was not enough to ruin the integrity of the chassis, but was sufficient to create enough squeaks and rattles to drive the owner nuts. With such an arrangement, it really doesn't matter whether the chassis is positive or negative polarity. Road salt is sufficient to guarantee that the body rots out at about the same time as the other major components fail. I don't know why negative ground was selected, but there are a variety of theories. Here's one that suggests it was because the original cloth insulation would leak current when wet and cause corrosion:

Incidentally, Motorola decided to run two really heavy cables directly between the trunk mounted radio and the battery under the hood. However, they got cheap and lazy with their front and dash mounted radios by supplying fairly short power cables with the radios. Installers were tempted to just ground the negative to the car chassis and were then blessed with alternator whine, generator whine, and vibrator hash on transmit.

Not wishing to repeat all the mistakes of past attempts to color code the power wiring, the solar power standard committees decided that it would be better to label these in English and use the colors in their original manner (red=danger, green=working, yellow=warning, black=all_else). So, it is now mandatory to attach stickers on everything including the DC power cables: However, the DC power labels do not mention polarity. Instead the are yellow and say "Solar Circuit". Most (not all) of the panels I've seen use red and black color coded wires. NEC 2014 wasn't bad for a first attempt at labeling, but did require a rewrite in 2017 to make the English intelligible. I expect the next generation of labels might add multiple languages, hieroglyphics, and icons.

Sorry. I'll try restating.

I naively agree with you that one terminal is positive with respect to the other terminal.

However, what this terminal is called, is confusing, particularly when dealing with negative voltage.

Aside: Yes, this equipment is powered by /negative/ 48 VDC. "Neg(ative) 48 (V(DC))" or some close variant (including the "negative polarity) and "48" volts) is quite common phrasing as an alternate power source in computer (particularly servers) and networking equipment.

Further Aside: I can't actually remember the last time I heard reference to (positive) 48 VDC in computers & networking equipment.

I believe that if I hook the positive / red lead of a voltmeter to the positive / red lead of a power supply, and the negative / black lead of the volt meter to the negative / black lead of a power supply, I expect the following voltages:

Standard bench top 12 VDC power supply: + 12 VDC Negative 48 VDC telephone power supply: - 48 VDC

So what is called the positive or red lead actually is negative with respect to what is called the negative or black lead.

I /think/ this is accurate. But I'm not sure. Nor do I actually understand /why/ this is the way that it is. This thread is my attempt to learn and understand. ?how can a positive power wire have a negative voltage on it?

See above.

I understand the confusion. Hence my trying to ask questions and learn.

Simplistically, yes, the power supply does say that it's -48 VDC.

Less simplistically, the labeling may not say it in those words. It may simply say the output is (nominally) -48 VDC. Some actually are labeled as -48 VDC supplies.

Nope. To the best of my knowledge, the "negative" is an important part of the description. I don't think it's simply how it's connected in relation to the 48 VDC.

That is my plan.

No. It's more than just color code. As you say, colors are immaterial and simply chosen based on convention.

I want to understand how the volt meter reading described above.

I want to understand how the following AC-to-DC power supplies work:

-?) AC-to-DC with a connection between neutral / ground and the - output.

If I don't know which, +? or -?, the AC-to-DC supply is and I bond the - output to ?, I'm either going to be safe (-?) or quite unhappy (+?).

The equipment all uses red and black wires. I don't know what the potential is between each of them to ? is, much less how they are labeled.

I will need to look when I get an opportunity to, probably on Tuesday.

I hope this message has cleared some of this up.

Maybe. I dislike doing such.

I'm referring to the 48 VDC supply (negative omitted on purpose) coming in on two wires to connect to a piece of equipment. I can ignore the color of the wires, use a volt meter to identify which is + and - and wire it to the equipment the way that it wants. But that ignores, or is blind to, the actual colors of the wires and how it integrates to the rest of the system.

I /think/ that it is more than just polarity. See the +? and -? examples above.

I think I've asked, or at least alluded to, many in this thread. It's difficult to word smith a question about a concept that is still unclear to me. :-( I think this email has some better ""questions in the form of "I want to understand". They aren't worded as questions per say. But I hope they convey what I want to learn.

Okay.

So what colors would you expect / assume the ground and power leads to be? Black and red, respectively? Or red and black, respectively?

Okay.

Do you think it would be acceptable for the negative power lead to be referred to as "hot"?

This does seem to answer part of what I'm wanting to know. I say "part" because I think your answer likely does provide the wrote memory type of information. And in a round about way makes it easier for me to point to the other part that I'm trying to learn.

How did you come to saying that -48 the ground will be positive and the power lead will be negative? What other information / knowledge did you rely on to make that statement? That other information / knowledge is the other part of what I'm wanting to learn and understand. The intention being for me to learn the foundational knowledge to deduce statements like you did without needing to rely on wrote memory. Does that make sense?

Agreed. I'm at home, and the equipment in question is at work. I doubt that I'll be able to get any pictures before Tuesday.

That's why I would look at the documentation for the equipment/supply. If it is a -48 volt system it certainly would be ok to call the positive termi nal "return" or "common". It would be ok to call the negative terminal "ho t".

I have seen some systems use a negative ground with a 48 volt power source.

Now you are talking color codes. I can't tell you what the color code is f or a -48 volt system.

"Called" vs. "looks" I can't say. I never talk about the "red" lead when d ealing with power circuits. I use the other terms listed above.

Now you have me confused again. Who talked about a positive wire with a ne gative voltage??? Positive/negative are electric polarities and don't chan ge when your definition of the common or ground changes.

Ok, if they say it's a -48 volt supply with a minus sign in front of the vo ltage, then it is safe to believe the hot wire will be the negative the ret urn/common wire will be the positive.

Ok, because I'm willing to bet the supply is not grounded, but it may be. If not grounded they are only calling it a -48 volt supply because that is how most people use it. There is nothing inherent that makes it minus vs. positive.

The volt meter will always tell you the voltage between the two probes assu ming the red will be positive and the black negative. If that is correct t he meter will show a positive voltage. I the red lead is negative it will show a negative voltage.

- output.

That will give you a positive voltage on the positive lead with respect to (wrt) ground.

• output.

That will give you a negative voltage on the negative lead wrt ground.

BTW, none of these supplies should have any connection to power line neutra l. That is dangerous situation and can kill. Neutral is not ground (prote ctive earth).

nd I bond the -

happy (+?).

Why would you bond any voltage line to earth? Do your equipment systems us e actual grounds anywhere? It's easy enough to measure the voltages and se e if the power is connected to ground.

labeled.

I'd say a measurement is in order... with the resistor I mentioned.

That's the part I don't get. Once you measure the voltages on the wires an d know what the terminals of your equipment need, what else is there to kno w about the "system"??? The only problem that could happen is if one of th e power leads is grounded to the chassis and a piece of equipment connects power to the chassis, but they are different connections. That's why you r ead the documentation.

That is all polarity, no? I'm not getting it.

I hope that helps.

I don't assume something I don't know about. You'll have to measure.

Hot or power. Ground/common is typically used for the "reference" of the p ower. So the other lead will be positive in this system and so, hot.

Just that the definition of -48 volt power is that the negative is the powe r or hot lead and the positive is common.

I really shouldn't use "ground" for the common lead since ground is often u sed for "protective earth" which is a different concept. They may be conne cted, but are not the same thing, just as neutral and protective earth are connected (if all is well) but not the same thing.

If I mean protective earth I will say protective earth, otherwise ground wi ll mean common.

Not really. I think you are making this a bit too complicated than it is. I'm not complaining, just stating an observation.

I expect that by the time you get to the equipment this will all be much mo re clear to you.

For digital and analog signals, the 'ground' concept relates to signal return currents, and completion of the electrical circuit. For digital outputs, the logic 'zero' refers to voltages at/near ground (negative ground for 5V or 3.3V logic), and 'one' to voltages far from ground. The

For analog outputs (such as gigabit Ethernet levels) a good solution is to have differential driven wires, with NO ground dependence, so that a voltage drop in ground wires due to power fluctuations does not interfere with the signal definition. For long wires, that is important.

Alternately, a metal chassis that has to be bonded (for safety) so as not to be electrically active and cause a shock, has to be 'ground', and will connect at various points to (for instance) the shielding on a video cable. Some signal improvement results from such safety ground being either unconnected, or simply (in only ONE PLACE) connected to the signal ground/analog ground/digital ground.

But, positive and negative power supply voltages are just... voltages that deliver power, and don't have shielding requirements, nor (in general) do they have well-defined immunity to noise and interference. That's why some logic (traditional ECL) uses negative 5.2V, the noise on a power rail would contaminate the signal if that signal were examined as a difference-from-positive-power-terminal, but not if that signal was difference-from-negative-power-terminal.

Being confused is OK, but do NOT assume 'ground' is always the negative terminal of a power supply, that is NOT OK.