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Dropping In On Electrical Faults

No single electrical testing method is more important, or more effective, than voltage drop testing. This feature will focus on explaining why it’s needed, what it is, how to do it, and how to understand the resulting meter readings.
Wednesday, March 25, 2015 - 07:00
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Those who know me know I talk to myself. Actually, I just think out loud (a lot). Sometimes in frustration I might ask questions out loud, even when no one’s in the room. Fortunately, I don’t always answer my own questions. With the intention of having fun and at the same time attempting to educate, in this article I took some literary freedoms and imagined “Jaime has a conversation with his split personality.”

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Jaime, I frequently hear the term “Voltage Drop Testing.” What is it?
In its simplest terms, voltage drop testing is the measuring of the voltage differences that may be present at various points in a circuit. The truth is, every circuit has some voltage drop. Even the best made wire (conductor) doesn’t have the same number of electrons available at its end as it has at its source. The difference is considered the “drop.”

That doesn’t sound very technical. Is that all there is to it?
Is anything we write about simple? Like the commercials say, “But wait! There’s more!” Remember high school physics? (No, I don’t mean the girl we liked in that class.) We were taught, among other things, that “Energy cannot be created nor destroyed — but it can be transformed into other types of energy.” In fact, that is exactly what we’re doing when we bring a vehicle to a stop. We are converting kinetic energy (vehicular motion) into thermal energy (heat) when braking friction is used to slow us down.

More physics – “An electron at rest” (will remain at rest)…
There is one condition that must be present in order to have a voltage drop. That is, the circuit must have been intended to be in operation. In other words, the circuit was completed between the battery positive and the battery negative terminals, of course, with control circuits and a load in between. If the circuit isn’t completed, electron flow will not occur. Electron flow can be measured in Volts (V or E) and Amperes (Amps, A or I) and is affected by resistance (R). Ohm’s Law is E=I x R. Here is a simple concept to remember: If there are not enough pathways for amperage to flow, the volts will decrease in numbers (voltage drop), and usually heat is produced as well.

TMI! Too Much Information! You’re hurting my brain!
This is the same reason power companies generate a lot more electricity than their consumers use. A certain percentage of what’s produced is lost in transmission; some is lost transforming it to what we use in our homes and businesses, but typically the vast majority of energy lost is in the form of heat (just like when braking). It isn’t that the energy is gone but it has been changed into another form, leaving less energy (power) available at the end of its line.

So, what does that have to do with voltage drop testing or with auto repair?
Well, as we all know, today’s automobiles have a lot more electronic gizmos than the cars did even a few years ago. Just like the power company’s electricity, the heat generated by the loss of electrons traveling from the car battery, through all the electronic devices and back to the battery again, is why there aren’t any cars made without air conditioning anymore! Don’t believe me? Then name one car that has A/C optional. OK, just kidding.

Really! Get serious! Why should we care about a little loss of voltage?
We generally don’t have to “care” about the voltage drop that is engineered into a circuit. Think about it — if every circuit has a certain amount of electron loss then the smart engineer will build a tolerance into the circuit allowing for it. It’s only when the amount of energy lost causes something to not operate as designed that we even concern ourselves with it.

Let’s take for example, a simple circuit used to illuminate a light bulb. There should be a supply voltage, a switch, the load (bulb) and a ground. If our supply voltage is as it was intended, and is measured the same at one conductor of the bulb (assuming the other conductor is grounded), the bulb should burn brightly. But, if there isn’t the same voltage measured at the bulb as what is supplied (a drop in voltage) and all other conditions are the same, will the bulb burn as brightly? No, not as brightly as it was designed to be. If our supply voltage is correct at its source but not at the bulb, all we have to do in order to make the bulb illuminate brightly is determine where the difference lies. Doing so is performing a voltage drop test.

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