Infinite resistance means a complete block to the flow of electric current. In practical terms, it describes a break or gap in a circuit where no electrical path exists, so zero current flows regardless of the voltage applied. You’ll encounter this concept when learning basic electronics, troubleshooting circuits with a multimeter, or trying to understand why a component has failed.
How Ohm’s Law Explains It
The relationship between voltage, current, and resistance is described by a simple formula: voltage equals current multiplied by resistance (V = I × R). If you rearrange that to solve for current, you get current equals voltage divided by resistance (I = V/R). As resistance climbs higher and higher toward infinity, the current shrinks closer and closer to zero. At truly infinite resistance, no current flows at all, no matter how much voltage you apply.
This is the opposite of zero resistance, where even the tiniest voltage would theoretically push an infinite amount of current through a circuit. In real-world terms, a superconductor approaches zero resistance, while an open circuit (a wire that’s been cut, for example) approaches infinite resistance.
Open Circuits and Broken Paths
The most common real-world example of infinite resistance is an open circuit. When a wire breaks, a switch flips off, or a fuse blows, the physical path that electrons travel through is interrupted. With no continuous conductor connecting one side of the circuit to the other, resistance becomes effectively infinite and current stops.
A closed switch provides a low-resistance path for current to flow. An open switch does the opposite: it creates a gap with no continuity, blocking current entirely. This is the basic principle behind every light switch in your house. Flipping it off introduces infinite resistance into the circuit, cutting power to the light.
Why No Real Material Has Truly Infinite Resistance
In theory, a perfect insulator would have infinite resistance because every electron stays tightly bound to its atom, leaving none free to carry current. No such perfect insulator exists. Even the best insulating materials, like specialized glass and certain plastics, have measurable (though extremely high) resistance. Exceptional insulators can reach resistivities as high as 1016 ohm-meters, while materials above 108 ohm-meters are generally considered good insulators. These numbers are enormous, but they aren’t infinite.
Air is a useful example. Under normal conditions, air acts as an insulator with very high resistance. But push the voltage high enough, around 3 kilovolts per millimeter, and air molecules become ionized and start conducting electricity. This is called dielectric breakdown, and it’s what happens during a lightning strike. The air that was effectively blocking current suddenly becomes a conductor. So even “infinite” resistance has limits when enough energy is involved.
What It Looks Like on a Multimeter
When you measure resistance with a digital multimeter, the display shows “OL” (overload) to indicate infinite or unmeasurably high resistance. On older analog meters, the needle stays pinned at the far left of the scale, which represents the infinity end. Before you even connect the test leads to anything, a multimeter in resistance mode displays OL because the open air gap between the unconnected leads has effectively infinite resistance.
This reading is useful for diagnosing problems. If you’re testing a component that should allow current to flow and the meter reads OL, something is broken. If you’re testing something that should block current, like insulation around a wire, OL is exactly what you want to see.
Using Infinite Resistance to Find Faults
One of the most practical applications of understanding infinite resistance is troubleshooting blown fuses. To test a fuse, you remove it from the circuit, set your multimeter to resistance or continuity mode, and touch the probes to the fuse’s metal contacts. A working fuse shows very low resistance (near zero) and may trigger a continuity beep. A blown fuse reads infinite resistance, meaning the thin metal strip inside has melted and broken the circuit path.
The same logic applies to broken wires, failed heating elements, and corroded connections. Any time a conductor loses its continuous path, resistance jumps toward infinity. In composite materials used in engineering, for instance, breaking enough conductive fibers inside the material progressively increases resistance. If enough fibers snap, the material fractures completely and resistance goes infinite, which engineers can use as a damage indicator.
Infinite Resistance vs. Very High Resistance
There’s a practical distinction worth understanding. True infinite resistance means absolutely zero current flow, and it only exists in idealized physics problems or fully open circuits. Very high resistance, on the other hand, means current flow is so tiny it’s negligible for most purposes, but it isn’t actually zero. A rubber glove protecting an electrician doesn’t have infinite resistance, but its resistance is high enough that the tiny current passing through it poses no danger.
For everyday troubleshooting and circuit design, the distinction rarely matters. If your multimeter reads OL, the component is either open or its resistance exceeds what the meter can measure. Either way, current isn’t getting through, and that’s the information you need to act on.