Re: Ping: DC

On Wed, 18 Mar 2009 20:19:03 -0400, "Phil S." <psymonds@xxxxxxxxxxx>
wrote:


"Dave Curtis" <dbaudiotech@xxxxxxxxxxxxxx> wrote in message
news:t8t2s4dat5lqrvmc6evr0c9r8o2k1kmtkq@xxxxxxxxxx
On Tue, 17 Mar 2009 21:54:24 -0400, "Phil S." <psymonds@xxxxxxxxxxx>
wrote:

Dave: this is for a licensed master 8^/.

I'm reading on another forum...a guy thinks it's OK to connect the mains
ground to the circuit ground...that is the green wire from the wall supply
and all the circuit grounds on the same bolt.

I'm thinking NEC doesn't permit this. True or false? If true, can you
give
a reasonably simple explanation of why or what the potential problem is?

Hmmm. I'd have to look it up, but I believe the safety ground bonding
point is a sacred place in a piece of equipment that no other piddly
little conductor should share. It's quite possible that I won't find
it in the NEC, but someone well-versed in UL standards would be able
to say for sure. Shoot Bruce an email: http://www.missionamps.com/, he
also hangs out on Music Electronics Forum.

Damn near every factory-wired amp with a 3-wire supply I've ever been
in (probably all of them) has the safety ground point separate from
the signal grounds.

Tangent: that ground wire from the cord should be the longest of the
three. That way, if the cord gets pulled out it'll be the last to be
ripped loose, so that if the hot wire contacts the still-grounded
chassis, the OCD (over-current device; CB or fuse) will trip.

I'm thinking, in the event of a failure the chassis + the circuit are all
at
wall voltage potential. This couldn't be a good thing. Might as well
lick
your fingers and stick 'em right in the wall socket. It'd be quicker and
more sure. Maybe I'm overthinking this?

This could happen even if the safety ground is on a separate bolt. If
there's a failure and the chassis becomes hot, the OCD should trip,
provided the circuit upstream is wired properly.

The book is in the van. I'll see if I can find the article and get
back to you.

Thanks, Dave. I think the person who intends to do this will probably do it
anyway. I was challenged when I said combining the signal and chassis
ground point was "foolish and dangerous." Some people can't be helped, as
you know.


It's not in the NEC, although it does refer to UL listing
requirements:

ANSI/UL 813-1996 Commercial Audio Equipment
ANSI/UL 1419-1997 Professional Video and Audio Equipment
ANSI/UL 1492-1996 Audio-Video Products and Accessories
ANSI/UL 6500-1996 Audio/Video and Musical Instrument Apparatus for
Household, Commercial, and Similar Use

The NEC deals mainly with the wiring to and from the equipment, not
the equipment itself. That's UL's jurisdiction.

Although:

"Article 250.126 Identification of Wiring Device Terminals. The
terminal for the connection of the equipment grounding conductor shall
be identified by one of the following:

(1) A green, not readily removable terminal screw with a hexagonal
head.

(2) green, hexagonal, not readily removable terminal nut

(3) A green pressure wire connector"


I don't know about "foolish and dangerous"; they are at the same
potential, after all. I keep it separated.

-DC
.

Relevant Pages

  • Re: Ping: DC
    ... and all the circuit grounds on the same bolt. ... that ground wire from the cord should be the longest of the ... chassis, the OCD ... wall voltage potential. ...
    (alt.guitar.amps)
  • Re: Ping: DC
    ... and all the circuit grounds on the same bolt. ... I'd have to look it up, but I believe the safety ground bonding ... that ground wire from the cord should be the longest of the ... wall voltage potential. ...
    (alt.guitar.amps)
  • Re: Ping: DC
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