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