Design Article
Getting ungrounded
Bill Schweber
10/25/2009 11:16 AM EDT
But lately, I have stopped using that term where it does not apply—and that' a lot of places. In fact, you might say that I have become "ungrounded." For battery-powered equipment, as well as automotive systems and airborne or space systems, there is a "common" but there is no ground, unless we are using the term as another word for common.
But w shouldn't. A real ground refers to a circuit point which is Earth-connected, and thus can take advantage of, or needs, the huge reservoir of electrons that the Earth offers. This ground not only acts as a signal common (not very good one, admittedly), it also provides a relief path for ESD, overvoltage, and system faults. An ungrounded appliance, for example, can be lethal if its metal enclose accidently connects to the AC mains; a grounded one will pop the local circuit breaker.
Inherently, battery-powered devices are ungrounded. Sometimes this is a virtue, as when you need to make floating measurements across individual cells in a series string of batteries, and the top cell may be at a potential of hundreds of volts above Earth ground. [An aside here: remember that "voltage" is a shorthand tem for "potential difference" and you should always keep in mind the unspoken "with respect to what" part when you say "the voltage is 'x' at this point."]
The interesting thing is that there are times when you want your system grounded and there are times when you deliberately don't. In the case of the battery-cell string, you want to make sure the instrument is ungrounded (floating) so that it can be protected against high potential in the string and also make a differential measurement across a given cell. But in a reverse example, many AC-powered medical instruments carefully isolate the patient's I/O so that any failures in the instrument itself do not ripple to the patient.
So you stand your ground, and I'll stand mine, but more importantly, don't say "ground" where what you really have is "common". Not only is ungrounded a design reality in so many cases, but using ground may lead you to unconsciously assume you actually have mother Earth at your disposal, for better and worse.♦
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Haldor
10/28/2009 11:53 AM EDT
"An ungrounded appliance, for example, can be lethal if its metal enclose accidently connects to the AC mains; a grounded one will pop the local circuit breaker."
The earth ground connection has nothing to do with electrical safety. What keeps metal enclosures safe is the third wire (green) is bonded to the return in the circuit breaker box. This way if a wiring malfunction connects the hot to the enclosure there is a return path to trip the breaker. The only reason why we connect the safety "ground" wire to earth ground is to attempt to reduce touch voltage on the outsides of metal enclosures. This often doesn't work because the local earth ground potential where you are standing may be very different from the earth ground potential at the circuit breaker box.
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Guru of Grounding
11/4/2009 6:55 PM EST
Thanks Haldor for jumping on that dangerous but all-too-common conceptual error! This "electron sump" view of dirt, especially for magically draining away system noise currents, is extremely pervasive amongst engineers who should know better. In the professional audio world, where I lecture on the subject, safety ground is often seen as a dispensable "annoyance" and many think that 3-to-2 prong adapters are made for the purpose of defeating safety ground!! Earth ground isn't even necessary for ESD protection unless the structure your body is in (and has its capacitance to) is connected to earth. Is an earth connection necessary to provide ESD protection in an airplane? Remember where the charge is and where the current will flow when a circuit is completed ... current ALWAYS flows back to the source of EMF that drove it - and that may or may not be connected to dirt. Another rampant myth is that all earth grounds are at the same potential. Not so, the dirt carries lots of currents from utility poles, buried pipes, etc. and acts as a big 3-dimensional voltage divider for these currents. Lightning protection is usually the dominant reason for earth grounding ("earthing" for Europeans).
Bill Whitlock, president & chief engineer, Jensen Transformers, Inc., Audio Engineering Society Life Fellow & IEEE Life Senior member
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