# Is ground the same thing as the black wire in DC?

Discussion in 'Electronic Basics' started by www.ChantCD.com, Jul 22, 2004.

1. ### www.ChantCD.comGuest

Some circuits talk about + and -, other talk about VCC and GROUND.
One even talked about "Chassis ground" as opposed to rod-in-the-earth
Ground.

If I attach only the red wire from my power supply, and attach the other end
to a metal table or something - the circuit wouldn't work would it? I would
think you'd need to have the black wire (actually called "negative") as
"ground" for that circuit.

Am I correct? I know this is a VERY basic question... So many books can
cause a bit of confusion though - the disadvantage of learning on your own.

Matthew

2. ### andyGuest

+ and - refers to the potential difference between the two wires. This
isn't absolute - as long as the supply was designed right, you could have
the -ve wire at ground+500V, and the +ve at ground+512 volts, say. But
usually the -ve wire is connected to some kind of stable ground voltage,
and then the positive wire is so many volts above or below that ground.
You can do this the other way round - have the +ve wire at ground, and the
-ve wire at ground minus x volts. or have a split rail supply which gives
ground +/- x volts.
not unless the table had a good connection to whatever ground your power
supply is using - it might be at the right voltage to start with, but
there would be no path for the current to follow to complete the circuit.
so all you would do is charge up your table to ground+12V. (i.e. the table
would be acting as a capacitor)

3. ### BanGuest

It is a basic question and should have been cast in the basic group. What
every child already knows is: For a current to flow you need a closed loop
eg. from positive to the consumer and back to the other polarity. If that
table is connected to the other polarity via the earth wire(red) a current
will flow. Many power supplies have their 0V output connected to the
earth(red or yellow/green). Any metal enclosures(computer supply) must be
connected by law to prevent built-up of dangerous voltages on the chassis,
and very often also the secondary side ground is connected there. In a house
with metal tubing the central heating or water supply is as well connected
to earth, so a gadget will sometimes work.
If the power supply is isolated (plastic casing, only two conductors to the
plug), there is no way of a loop and no current will flow.

4. ### Robert BaerGuest

The color of a wire is merely a matter of convenience and one should
never depend on a given color to have any particular meaning or use.
"Ground" is merely a reference point; one might have a current
monitoring circuit with schematic showing a "ground", but the device
might be umpteen feet above the earth, monitoring current in a 200KV
line. Where then would you say ground is??

5. ### Allan HerrimanGuest

On Sat, 24 Jul 2004 09:50:37 GMT, Robert Baer

[snip]
I'm sure the electrons don't care, but in many applications there are
*laws* regarding the colours of wire.
Still, as you say, it is prudent not to make assumptions, particularly
where safety is concerned.

Regards,
Allan.

6. ### Allan HerrimanGuest

.... particularly in -48V DC systems, where red is sometimes used for
ground and black is used for -48V. I've also seen the reverse: black
used for ground and red for -48V. Check your local regulations, then
measure it with a multimeter just to make sure.

Regards,
Allan.

7. ### Bob WilsonGuest

There are no laws when it comes to DC wiring. In North America, it is fairly
typical to use black as the colour of a negative wire and red as positive. In
Germany, it is brown for ground and black for positive. And so on.

Bottom line is that there is no absolute certainty with low voltage DC wiring.

True ground wires in an AC circuit that is UL/CDA/CE/VDE/NEMKO/SEMKO/DEMKO
(etc) approved, are either green, of more commonly green with a yellow stripe.
But because these are related to high voltage appliances, they come under the
regulations (NOT laws!) of the above mentioned regulatory agencies.

Bob.

8. ### John WoodgateGuest

I read in sci.electronics.design that Bob Wilson <[email protected]_no_spam.
In Europe, the colours of mains conductors ARE legal requirements. And
the -MKOs are now commercial test houses, not regulatory agencies.

9. ### MjolinorGuest

AFAIK there are no hard and fast laws for colours. I think maybe it depends
on which parts of the power path you are talking about. (Generator ------
lots of other bits----- TV)

10. ### John WoodgateGuest

I read in sci.electronics.design that Mjolinor <>
Premises wiring and mains flexibles.

11. ### Bob WilsonGuest

Agreed. My point was simply that low voltage wiring colours are not regulated.

Bob.

12. ### Peter O. BrackettGuest

For purposes of human "safety and harm"...

GREEN is the ONLY ground!

If y'all are lookin for RF ground... good luck!

--
Peter
Indialantic-By-the-Sea, FL

13. ### Dr Engelbert BuxbaumGuest

Bob Wilson wrote:

No: red positive, blue negative in low voltage DC circuits.
Thats safety ground.