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In Reply to: One hears this often here, but my city inspector sees no issue with a separate audio ground for my listening room. posted by Norm on April 2, 2007 at 06:49:58:
Consider what happens if your audio system ground earth connection is some distance away from your main power entry panel ground earth connection. The earth resistance between these points will be on the order of one to ten ohms or more.A nearby lightning strike will cause thousands of amperes of current to flow through the earth. If this current flows with some lateral component that is parallel to the geometrical line connecting your two earth connection points, thousands of volts of potential difference will be developed between the two earth connection points. Your audio circuit neutral is attached to the main service earth connection point, while your audio circuit "ground" is attached to the separate earth connection point. The thousands of volts of potential difference will appear between the neutral and ground at your dedicated circuit outlets. The outlets will flash over to limit the actual voltage to about 6000 volts or so. Local surge protection devices will be useless. This is plenty of juice to destroy your audio equipment, and the destruction may start a house fire. If your fire insurance company is on the ball, they will deny your claim.
Your city inspector is incompetent. If you survive the fire, you may have grounds to sue.
Follow Ups:
I wonder about the dissipation of current through the earth, although I understand that there will be differences between dry and wet soil. I can imagine that there would be some lateral current, but also that there must be a decline by the square of distance and probably some deviation from spherical dissipation. Obviously one solution would be to have a separate 240 feed to your audio room.
What is the likelihood of a power fault? Just sitting there, a component that is made well seems like it should be safe. However, what if there was a mixup between the design departments of the marketing and OEM companies that led to a 2-watt resistor being installed where the dissipation was over 2 watts, and the circuit board was slowly being cooked? (Don't laugh, this example is based on a true story.)What is the likelihood of a guest spilling a drink into the amp during a party?
What is the likelihood of a thunder storm causing a lightning strike on or near your house? A house I lived in for 9 years was struck the year after I left: it was under a power line with a grounded top conductor, and at the bottom of a hill, so I thought it was safer than other houses around it.
My point is that you cannot dismiss this safety concern because the events that would make it an actual hazard seem to be improbable. You cannot calculate the probabilities.
I've seen "live chassis" conditions occur a few times. I had one of these so-called "surge protectors" short an MOV, and it happened to be between the hot wire and safety ground. (It also had MOVs between hot and neutral and between neutral and safety ground, which were still fine. Although in retrospect, I don't see the point in tying MOVs to safety ground if there are others tied to neutral.) The breaker tripped without the MOV blowing open (it read dead short across an ohmmeter). The chassis may have become "live" had the safety ground been physically separated from neutral at the breaker box.MOVs shorting out are not that rare. This is a common cause of ground faults.
If there God-forbid was a failure in a component, which would make the chassis "live" (the same potential as the "hot" wire, the code grounding would short the hot line to common, and trip the breaker before things become lethal. That same failure with the "isolated" ground would *not* tie it to common, but would have some "real estate" in between, providing too much resistance to trip the breaker. And the path of least resistance could end up being the unfortunate one who comes in contact with the live chassis. ("Isolated grounding" and "DIY" components can be a deadly combination.)
The resistance of the earth path between the two "ground" rods is the common factor that makes for multiple ways this practice can kill people.
Now, if you're in Mississippi and you have watery mud between your electrodes all year round, you're okay. But if you're in Nevada and you have DRY SAND between your electrodes, the guys are right - this is deadly.Although it may be permitted by some inspectors, I would not put a system with isolated electrodes into service unless I measured the GPR myself, or had a grounding specialist or commissioning company do the test. Also, if you DO live in sandy conditions, and the sand is wet from recent rains, you're going to get ultra-low readings - readings that are COMPLETELY invalid a week later during the inevitable drought!
If the code inspector allows this without a GPR test, he's opening his inspection branch up to quite a needless liability if you ask me...
Cheers,
Presto
nt
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