Chip C wrote:
On Aug 18, 10:12 am, wrote:
On Tue, 18 Aug 2009 07:07:06 -0700 (PDT), Chip C
wrote:
On Aug 18, 8:31 am, stan wrote:
On Aug 18, 9:43 pm, (Doug Miller) wrote:
In article , Ashton Crusher wrote:
On Sat, 15 Aug 2009 08:33:57 GMT, (Doug Miller)
wrote:
Orange = isolated ground (used for sensitive electronic equipment, usually
medical or laboratory instruments, or high-end computer gear)
How is isolated ground different then "regular" ground?
http://tinyurl.com/n937l4
Thanks to all posters. Learned something. And made one think. Thank-
you.
Sounds like 'Isolated' Ground might also be called an 'Individual'
Ground!
In other words the grounding wire from this type of outlet (orange!)
is run individually to the grounding point; not using the ground used
for a 'run' or group of 'regular' outlets.
Have only used an orange outlet once, connected to the output of a UPS
located in our basement and wired up stairs to the room with the main
PC etc. AFIK took the ground back to the output of the UPS.
But as one poster pointed out if we have other computer type gear (say
printers or scanners etc. plugged into regular outlets) their grounds
may inadvertently be connected via the various cables connecting them
to the main computer with its isolated ground. Thus possibly defeating
the purpose of the individual ground?
The http://tinyurl.com/n937l4 was a useful explanatory.
That's a good article about the hoo-hah surrounding isolated ground.
I'm convinced that architects put them in out of rote habit. I've
personally never seen a computer with documentation that requires
them.
I believe that audio and signal processing equipment might benefit
from them, since in some places (at some time) I hear was ok to use
the metallic conduit as the circuit's ground, and not pull a bare or
green wire. The conduit picks up a lot of noise, so it causes problems
for equipment that uses the ground as a signal reference level. The
orange outlet at least guarantees you a real copper ground all the way
back to something earthed. As others have said, today's codes,
especially for hospitals, now guarantee this for all outlets.
I am also told that the orange outlets can indicate a circuit fed by a
(nearby) isolation transformer, on the output side of which the
neutral is re-referenced (ie, tied) to ground. This guarantees that at
the outlet, ground and neutral are close together in voltage.
Otherwise, in large commercial buildings, as you get further from the
point of grounding you can find that ground and neutral not only have
noise relative to each other but are far apart in DC levels.
As others have said, the read outlets are on circuits that can be fed
by the generators if the utility power cuts out. (And yes, the
breakers for these circuits are in bright read service panels.) But be
warned, a lot of places do generator and cutover tests at some regular
schedule, usually at some early morning hour, which cause outages of a
second or so. Much hospital equipment (fridges, lighting, elevators,
plus patient support equipment that has internal batteries) ride
through these fine, but computers don't. And of course, when a utility
outage happens, it takes time for the generators to ramp up (the
nominal standard is generally 15 seconds, in the real world can be
longer). Bottom line: don't plug a computer in to a hospital red
outlet without a UPS.
Chip C
Isolation transformer is there for safety.
Yes, in cases where its output is not referenced to earth. The two-
prong shaver outlets that used to be in bathroom light fixtures were
like this. Also I hear that in the UK (and other 240V places??) they
use 120V isolated power on outdoor construction sites.
The UK transformer has a centertap that is earthed. The hot wires are
60V from earth potential.
If the one of the transformer output legs is tied to earth, then I
don't see the safety benefit.
It keeps the hot wires from being at 2000V with respect to earth. Or
with 120V primary and secondary one secondary wire could be at 240V with
respect to the earth. When servicing electronic equipment an isolation
transformer with a completely floating secondary may be used. There can
be significant hazards working on equipment with the DC power system
tied to the neutral.
Almost all systems are earthed.
--
bud--