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Ground circuits — Part 1

Monday, March 6, 2017 - 09:00
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Part 1 begins a series of online-exclusive electrical circuit articles that focuses on the ground side of the circuit.

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There is some mystery about the ground side of the circuit among many technicians today. Why is that?

The mystery is caused by thinking of positive current flow in circuits rather than negative current flow. Positive current flow states that current flows from the positive (+) terminal to the negative (-) terminal of the voltage source to explain how “current” flows through a circuit. Since the positive post of the battery is the post that produces sparks when it contacts ground, technicians assume that is where the “juice” is. They note that if you ground the negative post of the battery it doesn’t cause sparks to fly because there is no “juice” there.

It seems logical to therefore assume that when a circuit is connected to a battery, the current flows from the positive terminal, through the circuit and goes to ground. Once the current goes to ground it “disappears” and you don’t have to worry about it anymore. This idea has led a lot of technicians to ignore the ground side of the circuit as if it was not important. However, the ground side of the circuit is extremely important.

What actually happens in a vehicle circuit is electrons (negative charges) leave the negative terminal of the voltage source and travel through the circuit back to the positive terminal of the same voltage source. This is referred to as electron current and is explained in this series of articles. Electron current is measured in amperes, abbreviated as “amps.”

In my early days of training in electrical, when voltage was measured on the ground side of a circuit, I have had technicians ask the question: “How does voltage feed back into ground?” The first time I heard this question I had no answer. You can’t answer a question if you don’t understand the question. I asked other techs in the class what they thought and none had an answer but all agreed it happens. They were quick to point out they have seen their test light glow sometimes when touching the ground side of a circuit.

There is a simple answer. When the ground side of a circuit develops a bad connection due to corrosion, voltage does appear on the ground side of the circuit during circuit operation. This is called a voltage drop. Some misinterpret this phenomenon as voltage from the positive side of the voltage source “feeding back into ground.” It is clear that positive current flow doesn’t explain how circuits really work. And it certainly doesn’t answer the question of how “voltage feeds back into ground” which it doesn’t. Although it appears that it does due to misunderstanding of how a circuit works and how current flows through a circuit.

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The solution to clearing up this circuit confusion is to understand how electrons flow in a circuit and how that is different from voltage in a circuit. A schematic will help to explain the difference between voltage and electron current. Figure 1 illustrates a schematic diagram of a simple automotive or truck electrical circuit with a battery and generator as the power source.

The battery negative terminal is labeled -BATT (say “minus BATT”). The electrical charge at the battery negative terminal is described as B- (say “B minus”). The minus sign indicates a negative charge because electrons have accumulated at the negative terminal due to battery state of charge. Electrons are negative (-) electrical charges. When a large number of electrons accumulate at the negative terminal of a charged battery, it results in a negative (-) charge.

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