Skip to content
AkerminMeasure. Monitor. Improve. Compare Products
Guide

How to Measure Resistance Safely

Resistance measurement applies the meter's own small voltage to a circuit and assumes nothing else is there. Voltage on the circuit gives false readings and can damage the meter; parallel paths give false lows; your fingers give false highs. The procedure below avoids all three.

By the Akermin Editorial Team · 3 min read

As an Amazon Associate, Akermin earns from qualifying purchases. Product links on this page are affiliate links.

Switch the circuit off and prove it dead; discharge any capacitors; disconnect at least one end of the component so no parallel path exists. Select ohms (Ω) and auto-range or a range above the expected value. Touch the leads together and note or zero the lead resistance (usually 0.1 to 0.5 Ω). Place the probes on the component without touching the metal tips, wait for the reading to settle, and read. OL means open; a few tenths of an ohm means a good connection or a short; the value in between is compared with the specification.

Why safety comes first

In ohms mode the meter is a small power source. External voltage on the circuit swamps its measurement and pushes current into its input circuitry. Most meters protect the ohms input to a few hundred volts for a moment, but repeated abuse blows fuses or damages the meter. Isolate and prove dead with a voltage test before every resistance measurement.

Procedure

  1. Isolate: switch off, unplug, or open the breaker; lock or label it.
  2. Prove dead: measure voltage across the points; expect zero. See measuring AC voltage.
  3. Discharge capacitors: motor run capacitors, power supply capacitors and camera flash circuits store charge; discharge through a resistor or the appliance’s bleed circuit per the manual.
  4. Isolate the component: disconnect one lead or lift one leg so the meter measures only it.
  5. Select ohms; auto-range or a manual range above the expected value.
  6. Zero the leads: touch the tips together; press REL if the meter has it, or subtract the reading from low-value measurements.
  7. Measure: probes on clean metal, fingers off the tips, wait for the reading to stabilize (high values and capacitance take seconds).

Reading the result

Reading Meaning
OL / infinity Open circuit: broken wire, blown fuse, open element or winding, or the range is too low
0.0 to 0.5 Ω Continuity or a short; subtract lead resistance
Expected value ±10% Component within specification
Much lower than expected Shorted turns in a winding, or a parallel path still connected
Much higher than expected Corroded connection, failing element, or finger contact
Reading that drifts Capacitance charging, moisture, temperature change, or fingers

Typical values

  • Kettle or heater element (2 to 3 kW at 230 V): 15 to 30 Ω; at 120 V, 8 to 15 Ω.
  • Oven element (2 kW at 230 V): about 26 Ω.
  • Small motor winding: a few ohms to tens of ohms; compare windings against each other.
  • Thermistor (NTC 10 kΩ): 10 kΩ at 25 °C, falling as it warms.
  • Automotive coolant temperature sensor: 2 to 3 kΩ cold, a few hundred ohms hot.
  • Spark plug lead: a few kΩ per meter.
  • Fuse: near 0 Ω good; OL blown. See how to test continuity.

Sources of error

  • Fingers on both tips: your body adds a parallel path of 100 kΩ to a few MΩ; irrelevant for low values, decisive for high ones.
  • Lead resistance: 0.2 to 0.5 Ω, significant below 10 Ω.
  • Temperature: copper windings rise about 0.4 percent per °C.
  • Semiconductors in circuit: diodes and transistors conduct one way and confuse readings; use diode test or isolate.
  • Range too low: shows OL for a good high-value component.

The four-wire method on bench meters removes lead resistance for milliohm work; a clamp meter and voltage drop test is the field equivalent for cables. See why multimeter readings jump around for unstable readings. Meters are compared in best multimeters for home and DIY.

Fluke 117 Digital Multimeter, Non-Contact AC Voltage Detection, Measures Resistance/Continuity/Frequency/Capacitance/Min Max Average, Automatic AC/DC Selection, Low Impedance Mode

Fluke 117 Digital Multimeter, Non-Contact AC Voltage Detection, Measures Resistance/Continuity/Frequency/Capacitance/Min

View on Amazon
Klein Tools MM325 Multimeter, Digital Manual-Ranging 600V AC/DC Voltage Tester, Tests Batteries, Current, Resistance, Diodes, and Continuity

Klein Tools MM325 Multimeter, Digital Manual-Ranging 600V AC/DC Voltage Tester,

View on Amazon

Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.

Frequently Asked Questions

Can I measure resistance on a live circuit?

No. The meter injects its own current and expects no external voltage. A live circuit gives meaningless readings and can blow the meter's input protection. Isolate and prove dead first.

What does OL mean on the ohms range?

Over limit: the resistance is higher than the range can display, which for an open circuit (a blown fuse, a broken wire) is the expected reading.

Why does a resistor read lower in circuit than its marked value?

Parallel paths through other components share the meter's current. Lift one leg of the component to measure it alone.

Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.