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Re: grounding question
Original poster: "claudio masetto" <claudmas@xxxxxxxxxxxxxxx>
I'm surprised that over there you use RCD's (residual current device)
with such a low fault current.
In Australia the electrical code is such that RCD's in a domestic
situation have a tripping current of 30mA
and in medical use such as hospitals 10mA. I would think that a 5mA
fault current would cause nuisance tripping.
Claude.
----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, October 15, 2006 9:58 AM
Subject: Re: grounding question
Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
At 08:43 AM 10/14/2006, Tesla list wrote:
Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
Hi Jim,
I think I figured out the issue after replying last nite. The
issue is NOT GROUND. The issue is NEUTRAL. You can short the HOT
to earth ground and nothing happens other than some current flows
(assuming no GFI). Only if you short the HOT to NEUTRAL will the
breakers pop. If an instrument case is RF grounded and HOT becomes
shorted to case, there is no guarantee the breakers will pop
because the earth ground impedance is too high. If the case is
green wire grounded, enough current will flow to trip the breaker
hence line safety is provided. If the breaker box is ungrounded
(lets say because the ground has dried out, the green wire is still
connected to NEUTRAL at the box which is a low impedance return for
the line. Safety is provided by the green wire connection to
NEUTRAL. Any grounding to earth via a rod or a water pipe can only
serve as an RF ground and not a safety ground.
You've described the "low impedance short from line to case" safety
aspect, and as you describe, the fact that it's connected to earth
is immaterial for this situation. However, there's also the "high
impedance fault" issue.
The earth ground also helps safety from the shock hazard standpoint.
Someone with bare feet is likely at "earth ground", likewise,
someone touching two appliances at the same time. If the case is
"green wire" grounded with reasonably low impedance, then the shock
hazard is low, from a high impedance "short" (i.e. enough to make
the case "live" but not enough to trip the overcurrent protection)
from hot to case.
However it's not perfect, which is why the electrical code now
requires GFCI for receptacles where people might be expected to have
a low impedance to earth (e.g. where there's water around: kitchens,
bathrooms, outdoors). The GFCI will trip at around 5mA
Gerry R.
Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>
Anyway, if a grounding rod is good enough for the breaker
panel then why not good enough for a TC RF ground.
Lots of reasons. the breaker panel ground is designed for line
frequency grounding, primarily for human life safety....
I understand the safety reasons. The question is: how is a
breaker panel that is grounded via a rod in the ground any more
safe than a TC ground that also is a rod in the ground other than
the frequencies are different. Could it be that the heavy guage
service neutral that comes to the house is also grounded at the
distribution point and that is what really provides the line
frequency ground, and the rod in the ground at the breaker panel
is more for RF grounding???.
No, more to deal with the possibility of break in the neutral in
the service drop, and to account for voltage drop in the service
drop neutral, so the neutral (in the house) isn't something other
than "ground".
The service entrance ground also serves as a common tie point for
other grounding systems (telephone and cable TV, notably), so you
don't get voltage differences between the chassis of a piece of
equipment (connected to ground wire ground) and the shield of the antenna coax.