Lightning & Dirt
The earth itself is the return path for the current in a stroke of lightning. To protect people and equipment from lightning, we must make a connection to actual soil.
Overhead power lines are frequent targets of lightning. As a result, virtually all electric power distribution lines have one conductor connected to earth ground periodically along its length.
Before this was done, power lines effectively guided lightning inside buildings, starting fires and killing people.
The (NEC) code-required earth ground at the service entry panel serves to direct lightning to earth ground before it enters the building. For the same reason, the code requires telephone, CATV, and satellite TV cables to “arrest” lightning before it enters a building.
Because soil has resistance just like any other conductor, earth ground connections are not at zero volts with respect to each other or any other mystical or “absolute” reference point. Code allows the resistance of this earth connection to be as high as 25 Ω.
Since this is far too high to trip the circuit breaker under fault conditions, an earth ground should never be confused with a safety ground.
Figure 4: All ground rods must be bonded to the main utility power-grounding electrode.
Safety ground must be connected to neutral at the main service entry panel. If more than one ground rod is used, Code requires that all must be bonded to the main utility power-grounding electrode. (Figure 4)
Facts Of Life
Most sound (or video) systems consist of at least two devices, which operate on utility AC power. Although hum and other problems are often blamed on “improper grounding,” in most cases there is actually nothing improper at all.
Any properly installed, fully code-compliant AC power distribution system will develop small, entirely safe voltage differences between the safety grounds of all outlets.
In general, the lowest voltage differences (a few millivolts) will exist between physically close outlets on the same branch circuit and the highest (up to several volts) will exist between physically distant outlets on different branch circuits.
These normally insignificant voltages cause problems only when they occur at a vulnerable signal interface - more unfortunate than improper.
What’s all of this have to do with hum and buzz? People have a strong tendency to blame “dirty” AC power for audio-video system noises.
But in fact, AC power is a utility much like a public highway - used by huge trucks as well as sports cars.
Eliminating noise problems by “purifying” the AC power is much like re-paving the highways to fix a car’s rough ride. A much more cost-effective and practical approach is to eliminate the problem that allows the power line to enter the signal path in the first place.
This is analogous to replacing bad shock absorbers in a car to isolate it from rough roads. Finding and eliminating these coupling points will be topics of upcoming columns.
Always make electrical safety your top priority!
Additional installments of “Clear Path” by Bill Whitlock are available here.
Bill Whitlock has served as president of Jensen Transformers for 20 years and is recognized as one of the foremost technical writers in professional audio.