In an earlier post we talked about the definition of resistance as being the opposition to the flow of electrical current measured in ohms. We can calculate the resistance of an electrical circuit using ohm’s law if we know the voltage and the current in the circuit. The amount of resistance in a circuit can vary over a very wide range, but there are two extreme limits to that resistance that require special consideration. Those two extremes are INFINITE resistance and ZERO resistance. Both are actual values of resistance which we encounter in electrical systems all the time. It is important to understand the characteristics of these extreme cases and how they affect circuit operation.

The first case, infinite resistance, is a WHOLE LOT of resistance. So much that we can’t put a number on it. However, when we use the value of infinite resistance in ohm’s law, the value of circuit current goes to zero, meaning no current will flow in a circuit which has infinite resistance. This special case is commonly referred to as “an open” circuit.

The second case, zero resistance, is completely the opposite of infinite. When we try to calculate ohm’s law with zero resistance, the current goes way up, up to infinity! In practical terms, infinite current cannot be achieved and some other circuit component limits the current. The special case of zero resistance in a circuit is known as “a short” circuit.

A point we need to remember about these two special cases is that they both represent very legitimate values of resistance which can be measured by typical multimeters, and we need to know the meaning of these two phrases, open and short circuits when we are discussing electrical systems and circuits.

Either of these two cases can be good, or they can be bad, depending on the circumstances.

We now have enough knowledge of electrical terms and circuit operation that we can start to talk about real world situations, which we will do in our next post. In the meantime let’s exercise our brains by thinking about this: which of the above two special cases would most likely result in a blown fuse in an electrical system?

We’ll talk about that in the next post. Meantime, it’s getting close to spring and warmer weather so get those RVs ready to go. Camping season is just around the corner.

## George Miller

Larry, I hope you are going to cover the proper use of a meter. I am especially interested in testing for circuit continuity. George Miller

## larrycad

George, yes, I intend to spend a lot of time discussing the use of multimeters, from selecting one for your use, and how to use it to find electrical problems. The definition of continuity is vague when it is used in electrical systems, because there is no specific value of resistance assigned to the term. In general, continuity can mean that a circuit, or a circuit component conducts electrical current, but with no specific amount of current assigned. One place where a continuity test can be useful is in checking fuses. If you have a fuse out of a circuit and check it, a good fuse will have continuity, while a blown fuse will read infinity on your multimeter. In this case, continuity of the fuse will be very close to zero ohms, or practically a short.

We will talk more about multimeters later on.

Larry