Ground - John Dunn, Consultant, Ambertec, P.E., P.C.

There was this 12-bit analog-to-digital converter (ADC) which was set up for testing in a fixture that this fellow had assembled in a 19-inch rack that was maybe seven feet tall.

First, let's look at some numbers:

The ideal mV steps of this ADC were one part in the twelfth power of two, take away one (for zero to +5V exactly). That comes to 1/(2¹² - 1) or 1/4095. At the ADC's full scale voltage of five volts, that comes to a nominal resolution of 5/4095 volts which is very nearly 1.221 mV. The ADC tests were failing their test limits and I was asked to find the reason why which did not please the fellow who had set up that rack. I began the process anyway.

Using a handheld digital multimeter, I found offset voltages in the "ground" wiring that were as high as 50 mV, way too high to be tolerated while trying to measure values like 1.22 mV. However, when I pointed this problem out, the fellow's answer was memorably stupid right down to the present day:

"Ground is ground."

This guy's concept of ground reminded me of a probably apocryphal story I read many years ago about a television repairman who answered a call to a customer's home.

The customer was this very elegant, elderly lady who had a crystal set as her radio and the crystal set had stopped working. The repairman wasn't expecting to find something like that in this day and age, but a service call was a service call so he set about looking for the problem and sure enough, he found it.

"What did you find?" his customer asked. "I found the problem.", he replied. "Your ground is no good."

She glowered at him.

"My ground is no good? I'll have you know, young man, that my garden soil is the best in town!"

I think these two ground experts had something in common.    

 

Comments

[george storm]

Too true - see also "ground loop"
But NOT in this case (unusually) the Wikipedia article (http://www.epanorama.net/documents/groundloop/ is better but is still not formally correct in every respect).

A different issue with "ground" arises when teaching design for stability: not only is ground a (convenient or misleading?) artifact, but the only gain that has meaning on its own is... +1; any other gains can be transformed to each other by suitable choice of "ground".

[ferd]

A story I've read but been unable to verify is about a dog chained to a telephone grounding rod. The dog's owner called to complain that her telephone was noisy, and sometimes didn't ring. But she knew when it was going to ring, if her dog was outside, because he would bark.

The technician found that the line was noisy, and that indeed when he dialed that line the dog would bark. The connection to the grounding rod was corroded, and the 90V phone ringing voltage was shocking the dog. The dog would get upset enough to wet all over the area, allowing a good-enough ground for the telephone to ring.

The solution was to repair the corroded telephone grounding connection. Also, it was nice to chain the poor dog to something else. Maybe those grounding idiots should be shocked by their poor installations, to prove that ground is not “just ground”.

[Richard Neubert]

My father used to tell me of an old twin-cylinder outboard motor he and his brother used for fishing from a wooden boat. It was no puppy for starting, but one of them discovered that if he got a shock from the exposed spark plug terminal while pulling the rope, it would start right up. They employed this technique for some time, until my father had a brainstorm. He attached a wire to something (probably the boat) and let it hang in the water. To start the engine, they would hook the wire onto the spark plug terminal and pull the rope. They would then bat the wire away from the terminal (knowing them, probably barehanded) once the engine got started on one cylinder.

The problem, of course, was that the magneto was firing both plugs in series and it was difficult to get enough voltage to do this on a rope pull. I'm sure grounding the plug to any metal part of the engine (not the gas tank, please) would have worked at least as well, but the lake was "ground".

[Bill]

At an apt I rented, the neighborhood kids would take great delight in touching the water faucet on the side of the house. The side where the cable service entered. I found the cable had the earth ground attached to the short length of iron pipe holding the faucet. Then it was fitting into plastic pipe as it went under the house. Bare foot on the ground and it was quite the tickle! The cable company was not interested. The Inspectors were not interested. So I added a length of wire over the electrical panel ground.