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.