Negative 48 Volts DC

On Tue, 28 Jan 2020 22:12:40 -0700, Grant Taylor
<gtaylor@tnetconsulting.net> wrote:

>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".

If you want to measure the height of something, you need to stand
somewhere and call that "zero." There is such a thing as absolute sea
level, H=0, and electrical earth ground, V=0, but they may not always
be available.

So we clip the "negative" (better called the "reference") wire of a
voltmeter to something conductive and declare that to be "ground",
namely zero volts. Chassis, car body, metal ladder that we're standing
on, most anything. Then we measure positive or negative voltages
relative to that.

We have one Tek oscilloscope with fully isolated channels. We can clip
a scope probe ground to the hot side of the AC line and then probe
relative to that.

-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com