Negative 48 Volts DC

[whit3rd]

On Sunday, January 26, 2020 at 10:23:27 AM UTC-8, Grant Taylor wrote:

On 1/25/20 2:34 PM, George Herold wrote:
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?

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.

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
5V logic and 3.3V logic can share a ground connection, and interconnect,
thus that negative ground is the common connection (so 'common' often means 'ground').

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.

 

[Rick C]

On Sunday, January 26, 2020 at 7:43:42 PM UTC-5, whit3rd wrote:

On Sunday, January 26, 2020 at 10:23:27 AM UTC-8, Grant Taylor wrote:
On 1/25/20 2:34 PM, George Herold wrote:
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?

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.

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
5V logic and 3.3V logic can share a ground connection, and interconnect,
thus that negative ground is the common connection (so 'common' often means 'ground').

You seem to be saying there is a difference in the concept of ground for analog and digital circuits. Digital circuits are still analog electrically. They just have a much higher noise tolerance. But noise is still a significant issue and should not be ignored.

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.

Exactly the same as digital. Differential pairs are commonly used to convey signals between boards or boxes.

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.

Don't confuse the concepts of protective earth and signal ground. They are not always connected directly. When passing signals between boxes, even in the same rack, the "commonality" or protective ground is at question. Directly connecting grounds can cause large currents to flow in the ground conductor creating voltages that interfere with the signal.

Most of this has little or nothing to do with getting the polarity right on negative voltage power supplies. No need to overload the guy.

--

Rick C.

-+- Get 1,000 miles of free Supercharging
-+- Tesla referral code - https://ts.la/richard11209

 

[gray_wolf]

On 25/01/2020 8:31 pm, Sylvia Else wrote:

On 26/01/2020 6:21 am, Grant Taylor wrote:
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
(servers and switch equipment) and I'd like to better understand what we're
working with than simply going through the motions that someone else
dictates.  Read:  I want to understand and not simply rely on wrote memory.

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?




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

Years ago the Seeburg jukebox used a ferrite core memory to keep track of the
selections of records to be played. It replaced a lot of electro-mechanical
crap. It was all tube circuitry and the sense lead was was connected to a pulse
pulse amplifier in it's own little plugin metal box with a 12AX7, IIRC, and a
small ferrite looking pulse transformer contained therein. The module plugged
into the main control chassis. It's the only +V grounded circuit I've ever seen
except for some PNP germanium audio amplifiers and PNP computer stuff. I never
found the reason for this. I suspect it was done to increase the noise immunity.
I will say that the -V wire had quite a layer of dust attached to it. The
later NPN solid state versions used a -V ground with the sense lead went to the
base of a SCS, silicon controlled switch. Once it fired it stayed on until the
current flow was interrupted. Other than convention, I never cared which end of
the power supply was connected to ground. I never touch bare terminals to check
their potential to ground. I don't care what color they are.

 

[gray_wolf]

On 26/01/2020 3:42 pm, Grant Taylor wrote:

On 1/26/20 1:37 PM, Rick C wrote:
Unfortunately that question is confusing to me.  lol

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?

One terminal is positive and the other negative.  Unless there is an internal
connection to ground (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.

See above.

That's the part I'm not getting.

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

Does the power supply actually say it is a -48 volt supply

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.

or just 48 volts?  Or is the -48 volts just what you see in references to the
supply elsewhere?

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.

Bottom line is there's nothing unexpected going on.  If it is wired for a -48
volt supply it will have the positive terminal wired to ground and the other
terminal will be -48 volts.  I can't tell you anything about how the terminals
are labeled and I would not assume anything unless the labels on the supply
are a bit more clear.

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

That is my plan.

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

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.
+⏚)  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 (+⏚).

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

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.

Yes, you keep saying that, but you don't really say what is not clear.

I hope this message has cleared some of this up.

Maybe rote memory is good enough for now and a light bulb will come on later.
That has happened for me in many areas.

Maybe.  I dislike doing such.

I don't know what you mean by "blinders".

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.

You seem to think there is some greater understanding, but in reality it's all
just a matter of polarity and the conventions are just that, conventions.

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

Do you have a question?

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.

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.

I think you have a poor understanding of basic current flow theory and likely
very little practical experience. This may be a good place to start. So your job
is connecting equipment to other equipment and you have no idea how to do this?

 

[Whoey Louie]

On Sunday, January 26, 2020 at 3:37:40 PM UTC-5, Rick C wrote:

On Sunday, January 26, 2020 at 1:36:16 PM UTC-5, Grant Taylor wrote:
On 1/25/20 3:54 PM, Rick C wrote:
Unless there is a connection within the power supply to earth ground
or some other reference point such as a ground for a control signal,
the power supply is just a 48 volt power supply. One terminal is
positive and the other negative.

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

Unfortunately that question is confusing to me. lol One terminal is positive and the other negative. Unless there is an internal connection to ground (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.

Sure it does. It's intended for use in a SYSTEM that uses -48V as
the power source.

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?

He clearly stated that one terminal is marked -48V, the other return.

 

[Whoey Louie]

On Sunday, January 26, 2020 at 4:42:38 PM UTC-5, Grant Taylor wrote:

On 1/26/20 1:37 PM, Rick C wrote:
Unfortunately that question is confusing to me. lol

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.

Why is it confusing? One terminal is the negative polarity, the other
the positive. On your supply, one terminal is marked -48V, the other
return. I would also make the -48V black, the return one red. but
even with just -48V on one and return on the other, it's clear which
is negative, which is positive. If you had a 9V battery and one terminal
was marked -9V, or 'neg", would you be confused about the polarity?

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

Woah there Pilgrim! Are you saying on that -48V supply has a
RED terminal or wire for the -48V and BLACK for return? Now that I would
agree would be confusing. I would expect black to be on the -48V,
red on the return.

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

It's either one polarity or the other, positive is never called negative.
What is called the "hot", the "source" or the "return" could be either
depending on the system polarity.

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?

With one voltage, two wires like you have, it can't.

One terminal is positive and the other negative. Unless there is an
internal connection to ground (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.

See above.

That's the part I'm not getting.

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

But it does make sense. That power supply is designed to be used in a
positive ground system. So the supply terminal is labeled -48V.

Does the power supply actually say it is a -48 volt supply

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.

or just 48 volts? Or is the -48 volts just what you see in references
to the supply elsewhere?

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.

Bottom line is there's nothing unexpected going on. If it is wired
for a -48 volt supply it will have the positive terminal wired to
ground and the other terminal will be -48 volts. I can't tell you
anything about how the terminals are labeled and I would not assume
anything unless the labels on the supply are a bit more clear.

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

That is my plan.

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

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

So what color is the -48V terminal? Is there a color? I would expect it
to be either black or no color identification and the return to be either
red or no color.

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

Why are you now adding neutral to this? Neutral is on the AC side.

 

[Rick C]

On Monday, January 27, 2020 at 11:31:43 AM UTC-5, Whoey Louie wrote:

On Sunday, January 26, 2020 at 3:37:40 PM UTC-5, Rick C wrote:
On Sunday, January 26, 2020 at 1:36:16 PM UTC-5, Grant Taylor wrote:
On 1/25/20 3:54 PM, Rick C wrote:
Unless there is a connection within the power supply to earth ground
or some other reference point such as a ground for a control signal,
the power supply is just a 48 volt power supply. One terminal is
positive and the other negative.

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

Unfortunately that question is confusing to me. lol One terminal is positive and the other negative. Unless there is an internal connection to ground (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.


Sure it does. It's intended for use in a SYSTEM that uses -48V as
the power source.



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?

He clearly stated that one terminal is marked -48V, the other return.

As usual, you are wrong again, always wrong.

--

Rick C.

-++ Get 1,000 miles of free Supercharging
-++ Tesla referral code - https://ts.la/richard11209

 

[Rick C]

On Monday, January 27, 2020 at 11:59:28 AM UTC-5, Whoey Louie wrote:

On Sunday, January 26, 2020 at 4:50:41 PM UTC-5, Grant Taylor wrote:

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?

Did he say that? I hope not.


With your -48V supply, the -48V terminal would be called the "hot"
the "supply", but not the positive.

Please don't confuse the guy by not reading what he writes.

--

Rick C.

+-- Get 1,000 miles of free Supercharging
+-- Tesla referral code - https://ts.la/richard11209

 

[Grant Taylor]

On 1/26/20 3:36 PM, Rick C wrote:
> I don't assume something I don't know about. You'll have to measure.

I'll try to make a point to do so tomorrow.

> Hot or power.

Okay.

Unless anyone objects, and provides an explanation of why they object,
I'm going to try to make a point to use the term "hot".

> Ground/common is typically used for the "reference" of the power.

I'm learning that.

Previously, I had assumed that ground / common also made reference to
the negative side. I'm having to unlearn that assumption as I go
through my Negative 48 Volt DC journey.

> So the other lead will be positive in this system and so, hot.

I want to agree, but I'm reticent because of the word "positive" in there.

I agree that hot will be positive in a negative ground system.

I'm learning that hot will be negative in a positive ground system.

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

Thank you.

I really shouldn't use "ground" for the common lead since ground
is often used for "protective earth" which is a different concept.

Agreed.

I think chassis is another similar thing that can be used as a
reference, which may also be distinct from ground (protective earth).

They may be connected, but are not the same thing, just as neutral and
protective earth are connected (if all is well) but not the same thing.

Agreed.

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

Okay.

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

Fair.

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

It's starting to.



--
Grant. . . .
unix || die

 

[Grant Taylor]

On 1/27/20 9:46 AM, Whoey Louie wrote:

Why is it confusing? One terminal is the negative polarity, the other
the positive. On your supply, one terminal is marked -48V, the other
return. I would also make the -48V black, the return one red.

Having looked at the equipment earlier today, you'd be wrong.

Negative had the red wires.
Positive / ground had the black wires.

The confusion is understanding why things are wired the way that they are.

but even with just -48V on one and return on the other, it's clear
which is negative, which is positive. If you had a 9V battery and
one terminal was marked -9V, or 'neg", would you be confused about
the polarity?

In a negative ground system, no, I would not be confused.

In a positive ground system, I apparently am.

Woah there Pilgrim! Are you saying on that -48V supply has a
RED terminal or wire for the -48V and BLACK for return?

Yes!!!

That is EXACTLY what I'm saying. (See above.)

> Now that I would agree would be confusing.

Hence this thread.

> I would expect black to be on the -48V, red on the return.

Nope.

I don't know what I would have expected / guessed this time last week.
I'm loath to guess, lest I confuse myself more this week.

It's either one polarity or the other, positive is never called negative.
What is called the "hot", the "source" or the "return" could be either
depending on the system polarity.

I'm slowly coming to the conclusion that I have naively assumed that red
≈ positive. Which is obviously wrong for positive ground systems.

I think I need to retrain myself that red = hot and that what is hot is
dependent on the type of system it is; negative ground or positive ground.

> With one voltage, two wires like you have, it can't.

I think that I need to reword my statement to be "a hot wire has a
negative voltage … in a positive ground system".

But it does make sense. That power supply is designed to be used in a
positive ground system. So the supply terminal is labeled -48V.

I checked, and one terminal is labeled "negative" and the other terminal
is labeled "positive".

So what color is the -48V terminal? Is there a color? I would expect it
to be either black or no color identification and the return to be either
red or no color.

There is no color (save for polished / chrome steal).

The red wire was connected to negative. The black wire was connected to
positive.

I've not had the opportunity to measure the voltage yet. I'm planing on
doing that tomorrow.

> Why are you now adding neutral to this? Neutral is on the AC side.

I'm fairly certain that neutral and ground are bonded together somewhere
upstream. Thus I typed "neutral / ground" as "neutral or ground".
Because, from an electrical conductivity standpoint, they are at the
same potential and in some ways the same.



--
Grant. . . .
unix || die

 

[Grant Taylor]

On 1/26/20 8:22 PM, gray_wolf wrote:

I think you have a poor understanding of basic current flow theory and
likely very little practical experience.

I /thought/ I had an acceptable understanding, particularly for a server
/ network administrator.

This may be a good place to start.

So your job is connecting equipment to other equipment and you have
no idea how to do this?

That is a bit ambiguous.

My job is working with server and network equipment, including racking /
stacking / cabling (data and power). AC power is quite simple and to be
honest, hard to mess up. DC power is much more rare and requires being
more careful. Hence this thread.

Copper data connections can be tricky, particularly if equipment is
powered from different legs / phases and something about the power is
not correct. That's a great way to burn out ports. Sneak current's are
a bitch. Ground loops can be a problem if you're not careful. Optical
cables are safer to work with.

So, "connecting" is a bit ambiguous and can involve a number of
different things.



--
Grant. . . .
unix || die

 

[Grant Taylor]

On 1/26/20 4:02 PM, mpm wrote:
> Grant, let me just offer this as consolation:

…

> I guess I'm saying, don't feel bad about being confused.

~chuckle~

Thank you. I needed that. It's been a rough day.

> Even the big guys fuck it up.

Yep.



--
Grant. . . .
unix || die

 

[Grant Taylor]

On 1/26/20 3:28 PM, Rick C wrote:
> That's why I would look at the documentation for the equipment/supply.

That is the plan. I've not had the opportunity to do so yet.

If it is a -48 volt system it certainly would be ok to call the
positive terminal "return" or "common". It would be ok to call the
negative terminal "hot".

Okay.

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

Fair enough. I've not had the exposure to (positive) 48 volt power.

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

I wasn't meaning to focus on the color code. Allow me to modify my
statement slightly by removing the colors from it.

I believe that if I hook the positive lead of a voltmeter to the
positive lead of a power supply, and the negative lead of the volt meter
to the negative 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

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

Okay.

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

I believe it was a permutation of "hot" -> "red" -> "positive" in a
system that has negative voltage.

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

Okay.

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.

I had a few minutes to look at the -48 VDC power supply today.

¡ The negative terminal was connected to the red wires.
¡ The positive terminal was connected to the black wires.
¡ The positive terminal was connected to the chassis.
¡ The chassis will make electrical contact with a rack that is
connected to earth ground.

The volt meter will always tell you the voltage between the two
probes assuming the red will be positive and the black negative.

The implication of polarity with volt meter leads doesn't help this
thread. But I generally agree with your statement.

If that is correct the meter will show a positive voltage. I the
red lead is negative it will show a negative voltage.

I believe the red lead does show a negative voltage.

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

I still need to measure, but I believe the positive lead has 0 volts to
ground.

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

Agreed.

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

I agree that ground and neutral (in typical 120/240 VAC 1ɸ wiring) are
separate conductors and should not be used interchangeably. However,
ground and neutral are bonded together in the main electrical panel.
(At least every main panel that I've ever looked in. Sub-panels are
decidedly different with ground and neutral isolated from each other.)
So, there is a conductive path between ground and neutral.

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

Ground is quite prevalent in networking equipment and servers. Some
vendors refuse to honor warranty if the equipment is not properly
grounded. Static is a (big) concern. So ground is used to dissipate
static before it builds up and causes problems.

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

After looking at the -48 VDC power supply earlier today, I am quite
convinced that the positive terminal has electrical conductivity with
the building's earth ground. (Positive terminal strapped to chassis
which is bare metal connecting to the rack it mounts in (also bare
metal) which is earth grounded.)

That's the part I don't get. Once you measure the voltages on the
wires and know what the terminals of your equipment need, what else
is there to know about the "system"??? The only problem that could
happen is if one of the 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 read the documentation.

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

I hope that helps.

--
Grant. . . .
unix || die

 

[Rick C]

On Monday, January 27, 2020 at 9:37:34 PM UTC-5, Grant Taylor wrote:

On 1/26/20 3:36 PM, Rick C wrote:
I don't assume something I don't know about. You'll have to measure.

I'll try to make a point to do so tomorrow.

Hot or power.

Okay.

Unless anyone objects, and provides an explanation of why they object,
I'm going to try to make a point to use the term "hot".

Ground/common is typically used for the "reference" of the power.

I'm learning that.

Previously, I had assumed that ground / common also made reference to
the negative side. I'm having to unlearn that assumption as I go
through my Negative 48 Volt DC journey.

Yes, that is the only real difference.

So the other lead will be positive in this system and so, hot.

I want to agree, but I'm reticent because of the word "positive" in there.

I'm talking about negative ground systems which I consider "typical".


> I agree that hot will be positive in a negative ground system.

Yes.


> I'm learning that hot will be negative in a positive ground system.

Yes.

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

Thank you.

I really shouldn't use "ground" for the common lead since ground
is often used for "protective earth" which is a different concept.

Agreed.

I think chassis is another similar thing that can be used as a
reference, which may also be distinct from ground (protective earth).

No, any exposed metal is always connected to protective earth. That way no matter what happens with the power circuit (AC or DC) the exposed metal parts won't become "hot". Either a fuse will blow or the power source will blow up. lol

That may also be connected to the common power rail, but the point is it MUST be protective earth.

They may be connected, but are not the same thing, just as neutral and
protective earth are connected (if all is well) but not the same thing.

Agreed.

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

Okay.

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

Fair.

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

It's starting to.

Yeah, it's not hard, but takes a bit of work to get the mental connections correct.

--

Rick C.

++- Get 1,000 miles of free Supercharging
++- Tesla referral code - https://ts.la/richard11209

 

[Rick C]

On Monday, January 27, 2020 at 9:09:27 PM UTC-5, Grant Taylor wrote:

On 1/26/20 3:28 PM, Rick C wrote:
That's why I would look at the documentation for the equipment/supply.

That is the plan. I've not had the opportunity to do so yet.

If it is a -48 volt system it certainly would be ok to call the
positive terminal "return" or "common". It would be ok to call the
negative terminal "hot".

Okay.

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

Fair enough. I've not had the exposure to (positive) 48 volt power.

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

I wasn't meaning to focus on the color code. Allow me to modify my
statement slightly by removing the colors from it.

I believe that if I hook the positive lead of a voltmeter to the
positive lead of a power supply, and the negative lead of the volt meter
to the negative 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

Positive and negative are electrical terms of physics. It has nothing to 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 measure a positive voltage no matter what.

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

Okay.

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

I believe it was a permutation of "hot" -> "red" -> "positive" in a
system that has negative voltage.

Let's forget color. If it is a -48 volt system the positive wire will be the common, but it will still be positive.

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

Okay.

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.

I had a few minutes to look at the -48 VDC power supply today.

¡ The negative terminal was connected to the red wires.
¡ The positive terminal was connected to the black wires.
¡ The positive terminal was connected to the chassis.
¡ The chassis will make electrical contact with a rack that is
connected to earth ground.

Ok, so they are using the convention that red is "hot" and black is "common". They are also grounding their common to earth ground which is likely connected to a protective earth wire.

The volt meter will always tell you the voltage between the two
probes assuming the red will be positive and the black negative.

The implication of polarity with volt meter leads doesn't help this
thread. But I generally agree with your statement.

If that is correct the meter will show a positive voltage. I the
red lead is negative it will show a negative voltage.

I believe the red lead does show a negative voltage.

I was talking about the voltmeter leads. But whatever.

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

I still need to measure, but I believe the positive lead has 0 volts to
ground.

You just said above the positive power lead is common and connected to a grounded chassis. You can't get any more zero than that. Of course you have to use common or ground as your reference (e.g. connect the black meter lead to common).

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

Agreed.

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

I agree that ground and neutral (in typical 120/240 VAC 1ɸ wiring) are
separate conductors and should not be used interchangeably. However,
ground and neutral are bonded together in the main electrical panel.
(At least every main panel that I've ever looked in. Sub-panels are
decidedly different with ground and neutral isolated from each other.)
So, there is a conductive path between ground and neutral.

And that is the hazard. If the neutral connection to the panel is lost, 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 connected in a way that no failure can cause the protective earth to be hot.

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

Ground is quite prevalent in networking equipment and servers. Some
vendors refuse to honor warranty if the equipment is not properly
grounded. Static is a (big) concern. So ground is used to dissipate
static before it builds up and causes problems.

That would imply the equipment cabinet be grounded and the equipment connected to that. The power source can be floating and the equipment should not care. Do the equipment vendors define "properly grounded", that is, any specific instructions?

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

After looking at the -48 VDC power supply earlier today, I am quite
convinced that the positive terminal has electrical conductivity with
the building's earth ground. (Positive terminal strapped to chassis
which is bare metal connecting to the rack it mounts in (also bare
metal) which is earth grounded.)

Is this strap at the power supply or the equipment being powered? Braided copper strap or just a regular wire? Just curious. As I've said, I've not worked with this sort of equipment.

--

Rick C.

+-+ Get 1,000 miles of free Supercharging
+-+ Tesla referral code - https://ts.la/richard11209

 

[Grant Taylor]

On 1/27/20 8:17 PM, Rick C wrote:

Positive and negative are electrical terms of physics. It has nothing
to 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 measure a positive voltage no matter what.

Presuming the negative meter probe is connected to negative power line,
and I agree.

The thing that I've learned is that there is no correlation between
"hot" / "common" / "return" / "ground" with positive or negative like I
have naively thought for the last 30 years.

That would imply the equipment cabinet be grounded and the equipment
connected to that. The power source can be floating and the equipment
should not care. Do the equipment vendors define "properly grounded",
that is, any specific instructions?

Yes. The better documentation calls out very specific directions on how
to bond ground to the equipment. They are quite specific about earth /
safety ground.

> Is this strap at the power supply or the equipment being powered?

It is part of part of the power supply.

Braided copper strap or just a regular wire? Just curious. As I've
said, I've not worked with this sort of equipment.

It is a piece of solid (what looks to be) steal that fits over the bolts
of the positive terminal where wires get bolted on top, thus sandwiching
the steal between the terminals and the restraining bolts. The other
end is bolted to the metal chassis.



--
Grant. . . .
unix || die

 

[Grant Taylor]

On 1/27/20 8:39 PM, Rick C wrote:
> Yes, that is the only real difference.

*nod*

> I'm talking about negative ground systems which I consider "typical".

Fair enough.

I agree, negative ground has been about 99% of what I've seen and worked
around.

No, any exposed metal is always connected to protective earth.
That way no matter what happens with the power circuit (AC or DC)
the exposed metal parts won't become "hot". Either a fuse will blow
or the power source will blow up. lol

Sorry I mistyped, I was thinking where common and hot were floating
inside of and insulated from the chassis and the chassis itself is
grounded to earth. Thus chassis and and common can be at two different
potentials.

That may also be connected to the common power rail, but the point
is it MUST be protective earth.

*nod*

Yeah, it's not hard, but takes a bit of work to get the mental
connections correct.

Yep. Now that I've bifurcated ground / common from the negative
polarity, along with some better understanding of terms, I think I'm
starting to understand.



--
Grant. . . .
unix || die

 

[Rick C]

On Monday, January 27, 2020 at 11:20:11 PM UTC-5, Grant Taylor wrote:

On 1/27/20 8:17 PM, Rick C wrote:
Positive and negative are electrical terms of physics. It has nothing
to 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 measure a positive voltage no matter what.

Presuming the negative meter probe is connected to negative power line,
and I agree.

The thing that I've learned is that there is no correlation between
"hot" / "common" / "return" / "ground" with positive or negative like I
have naively thought for the last 30 years.

That's because positive ground systems are the exception rather than the 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 ground.

That would imply the equipment cabinet be grounded and the equipment
connected to that. The power source can be floating and the equipment
should not care. Do the equipment vendors define "properly grounded",
that is, any specific instructions?

Yes. The better documentation calls out very specific directions on how
to bond ground to the equipment. They are quite specific about earth /
safety ground.

Is this strap at the power supply or the equipment being powered?

It is part of part of the power supply.

That surprises me a bit. A AC/DC power supply output is typically isolated from the input power line, so common is whichever terminal you connect to.... common. Having a connection internally to the AC protective earth limits the applications with no added advantage.

Braided copper strap or just a regular wire? Just curious. As I've
said, I've not worked with this sort of equipment.

It is a piece of solid (what looks to be) steal that fits over the bolts
of the positive terminal where wires get bolted on top, thus sandwiching
the steal between the terminals and the restraining bolts. The other
end is bolted to the metal chassis.

That sounds very external then. That makes sense. That's not really part 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.

--

Rick C.

+++ Get 1,000 miles of free Supercharging
+++ Tesla referral code - https://ts.la/richard11209

 

[Michael Terrell]

On Sunday, January 26, 2020 at 4:30:12 PM UTC-5, Jeff Liebermann wrote:

On Sun, 26 Jan 2020 10:16:41 -0800 (PST), mpm <mpmillard@aol.com
wrote:

On Sunday, January 26, 2020 at 2:10:41 AM UTC-5, Michael Terrell wrote:
A communications system that I worked on for the International Space Station ran on -48VDC.

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

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

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
2 months installing Motorola Motrac radios in concrete trucks. I
think they were either Mack or Peterbilt but I'm not sure. Most had
positive ground system. There were also some 24 volt systems (2
batteries in series) but we were able to tap off one battery.

There were plastic cases with Mobile CB radios mounded inside for Truckers, back in the '70s. They had a 6A or 25A full wave bridge on the DC input, and a mag mount antenna with a thicker than normal insulating pad on the mag mount, or a 1:! RF transformer to isolate the output for Mirror mount antennas. They were popular with Fleet truckers who never knew if the truck would have positive or negative ground. They were a pain in the ass on either grounding system, but truckers bought them.

 

[Michael Terrell]

On Sunday, January 26, 2020 at 4:30:12 PM UTC-5, Jeff Liebermann wrote:

On Sunday, January 26, 2020 at 2:10:41 AM UTC-5, Michael Terrell wrote:

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

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

They used some COTS -48VDC items, so the entire station was designed to operate off the batteries and solar cells at that voltage.

The equipment that we supplied used a pair of Vicor 120/240 input supplies. One +5VDC and one +12/-12V dual output supply. It was a simple modification to replace them with a single, triple output,-48VDC input power supply. The biggest problem was that we only had one power supply at the factory that could power it, so it had to be moved from station to station through final test with the electronics.