Negative 48 Volts DC

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do with convention of colors or which polarity of common you use. Connect the positive meter probe to the positive power line and you will always me asure a positive voltage no matter what.

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the common, but it will still be positive.

on". They are also grounding their common to earth ground which is likely connected to a protective earth wire.

grounded chassis. You can't get any more zero than that. Of course you ha ve to use common or ground as your reference (e.g. connect the black meter lead to common).

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there is no return path for the power circuit and anything touching neutral will be hot. The bond between protective earth and neutral must be connec ted in a way that no failure can cause the protective earth to be hot.

There is no such way. If the ground wire becomes disconnected at the panel , or the wire is cut on it's way to the grounding electrode system and you also have your disconnected neutral, the eqpt grounding system will become energized through any loads.

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norm. Telephony related gear is an exception... when powered from 48 volts , but not always even then. I have seen gear that didn't have an internal connection to common and so could be used as positive ground or negative gr ound.

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ed from the input power line, so common is whichever terminal you connect t o... common. Having a connection internally to the AC protective earth lim its the applications with no added advantage.

Of course it has an advantage, lightning protection for one. Telco offices, the DC side is connected to earth, for example. Or if an AC conductor somewhere shorts to the DC side, it's likely to trip a breaker, instead of energizing the DC side and going undetected.

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Yes, I see you said that in another post now too. And I agree, it is confusing. I would not expect red to be the negative supply side and black to be positive. That is confusing and asking for trouble.

Which end is designated the ground/return doesn't change the polarity of the terminals at the power source.

Yes, so then it's just a color choice thing. Which raises an interesting question. In the small minority of cars that decades ago used a positive ground, I wonder what color the supply wires were? Possible they were red too. I've never seen one, IDK.

Well, knowing that, just consider the red to be the supply side, black to be the return. In the case of your -48V system, the red then is the negative polarity.

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Red is positive in everything I've seen, but then all of that has been negative ground, so I share your confusion. But some of it might become obvious, even without looking at the power supply. If there are breakers or fuses for example, they would be on the supply/hot side, so if those wires were red and I knew it was a -48V system, I'd start wondering.

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Yes, at least for that particular system. The question is that what all positive ground systems do?

Yes, you've got it now.

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Yes, you have that correct too. Very normal for the return side of eqpt to be connected to earth ground, for lightning, in case an AC line side conductor somewhere gets shorted to the DC/low voltage side, etc.

A couple of years ago I helped choose colours for the input connectors of a networking product that had 12V (automotive range) and 48V versions. The 12V choice was easy. We were using Anderson Powerpole connectors, so we followed the amateur radio convention for these connectors with red for positive and black for negative in a specific configuration. For 48V, I looked at every standard I could find and found a mass of conflicting recommendations. An additional constraint was that we wanted to use the "finger safe" version of the connectors and could only use colours available from Anderson. Most customers would use positive ground, but some had negative ground systems. The final outcome was that the input was isolated with no connection between either power terminal and chassis and the positive terminal was red and the negative terminal was blue. This seemed to be in keeping with the spirit of most of the standards and less likely to cause confusion than any other combination. Pictures at the bottom of the page:

John

Alas, the innards of an automobile won't teach you anything bur red = positive. There are LOTS of color conventions, many with legal enforcement, and one must learn not to generalize.

In US house wiring, red would be AC high voltage. In a computer, red is +5V, and yellow is +12V. In European AC cables, black is neutral, but in US the white is neutral. Ground is sometimes black, sometimes green, and green/yellow stripe is almost always ground.

I say 'almost always' because I've never seen (or used) it other than on ground wires, but it's worth checking every time.

That's what I would have done too. I sure would not use red for negative and black for positive. I was thinking about saying in the other reply that if I decided to use black for the return in a positive ground system, then I sure would not use red for the supply. I would use some other color that would at least give someone looking at it reason to further look into what's going on. When one sees just red and black in low voltage DC system, one naturally assumes red is positive.

This schematic shows red assigned to the higher potential and black assigned to the lower, more negative, potential:

Red is assigned to the relatively positive power rail, at least in this schematic. What installers choose to do in the field's a whole nother story.

That schematic originates with communications vendor:

Presumably the vendor knows more than most about color code usage in the industry.

In regards to ground:

"Ground is a fantasy invented by engineers to simply their work." - Bill Whitlock

The ground fantasy as applied to this thread seems to complicate instead of simplify. It's probably best to leave it out.

Thank you,

--
Don Kuenz KB7RPU 
There was a young lady named Bright Whose speed was far faster than light; 
She set out one day In a relative way And returned on the previous night.

If you mean by high voltage 240/120 AC then yes that's true, red could be used. But Grant's example is DC and 48V. You could also have red and black in house wiring in other applications, eg HVAC,thermostats, alarm wiring. But red for positive and black for negative in DC was the issue and this is the first application where I've heard of red being used for neg, black for positive.

In a computer, red is +5V,

In every low voltage DC anything where there is just one voltage, I've never seen red and black be anything other than positive and negative. Apparently it exists, but it's not common and the convention is not limited to cars. Buy a DC, low voltage, one speed motor, I've never seen one where red and black did not mean pos and neg, have you?

I'm wondering what color was used on the negative/supply side of the wiring in early cars that used a positive ground? I took a quick google, couldn't find anything.

Earth ground, I would agree is just a further complication here. But ground in general is just a system reference point and typically the return. So if someone was working on Grant's system and hooked a meter between -48V wiring and the return wiring, it would be common to say that he's measuring between supply and ground.

whit3rd wrote in news: snipped-for-privacy@googlegroups.com:

We designed a negative HV supply. While we would typically all agree that black is ground, we all know that ground is not always negative. It is most commonly always BLACK, however (AC fault return wires do not count).

So our negative supply had a RED "HOT" wire and black was ground. Being an isolated supply it should not matter. The signal to the user is that red is HOT, not neccessarily positive.

Whoey Louie wrote in news:499044a5-a46d-45a0- snipped-for-privacy@googlegroups.com:

6 Volt automotive systems used black cables throughout. Many used terminated bare braided tinned copper cable for the grounded side.

snipped-for-privacy@decadence.org wrote in news:r0q8qe$1ivm$ snipped-for-privacy@gioia.aioe.org:

Aftermarket replacements, however, are colored red for positive side connections.

Lots of stuff about converting old Willys jeeps to negative ground.

Some folks are simply confused when things do not match their pre- conceived ideas.

g

norm. Telephony related gear is an exception... when powered from 48 volts , but not always even then. I have seen gear that didn't have an internal connection to common and so could be used as positive ground or negative gr ound.

t , w

ed from the input power line, so common is whichever terminal you connect t o... common. Having a connection internally to the AC protective earth lim its the applications with no added advantage.

It it's a power supply supplied by the same company that makes the gear it's connected to, or intended specifically for the application it's being used in, they don't give a rat's ass about how it might be used for something else. It's not a bench power supply.

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t of the power supply. They just provide the strap and you likely can use it on either output. But since the positive terminal is labeled "common", maybe not.

In AC wiring in Europe, light blue is neutral, green/yellow stripe is the protective earth (PE). It is forbidden to use the green/yellow stripe for anything else than ground/earh. If you are forced to use some other colord conductor for PE, the ends must be marked with green/yellow stripe tape.

Just to confuse things even more, the live wires in 3 phase systems in Europe (including UK) are brown, black and grey. The old UK colours were black for neutral and red, yellow and blue for the phases. There are many installations where both sets of colours are mixed! John

I think it may become crystal clear for you if you can visualize what a neg ative voltage is, and how the currents flow.

For me, the water analogy helped me visualize it. Think of a voltage sourc e (power supply output) as a water pump, which brings water uphill to a res ervoir. So a +10V supply is like pumping water up a 10 foot wall. Then wh en you connect to a load, that is like dropping the water (kinetic energy); it returns to ground and has no energy left so you continue to pump it uph ill. Currents only flow downhill.

Now for the -10V supply, dig a 10 foot hole and put the pump from the botto m of the hole to ground level. Now you connect to a load the same way as b efore, you drop the water down into the hole (kinetic energy) and then you pump it back up. Currents continue to only flow downhill.

If you have a power supply with +10V and ground outputs, you can measure th e current, it will always flow out the +10V and back on the ground terminal . If your power supply has -10V and ground output, the current will flow o ut the ground output and return on the -10V.

It sounds like your systems use black=ground and red=hot, whether red i s negative or positive. "Hot" is a good term, since current will flow thro ugh your body if you touch the red wire and you are grounded. Only differe nce for a negative voltage is the direction the current takes as it flows t hrough you.

I think it's fair to say that I was. Hence why I started this thread.

I feel like I've gained a better understanding of what's going on.

--
Grant. . . . 
unix || die

Oy vey!

And I thought old four pair phone wire color codes and new four pair phone wiring color codes were confusing / annoying, in that they take a few minutes to stop, think about, and figure out. But that is nominally low voltage and decidedly low current. Much less room for catastrophic failure than the old & new color codes for three phase.

--
Grant. . . . 
unix || die

I'd like to take a moment to thank everybody that has helped me along my journey to learn about Negative 48 Volts DC. I believe that I have learned, and unlearned, enough to have a acceptable decent understanding.

I have learned that the terms "hot" / "common" / "return" / "ground" do not correlate with the polarity without indication of "negative ground" (what I'm used to) or "positive ground".

In a negative ground system, the "hot" wire will be positive compared to "ground" / "return".

In a positive ground system, the "hot" wire will be negative compared to "ground" / "return".

--
Grant. . . . 
unix || die