A practice problem says a resistor is 120 Ω, but the meter reads 80 Ω across its terminals while it remains connected. Does that prove the resistor has failed? Not if there is another path between the probes. Fluke's resistance-measurement guidance says an in-circuit reading includes parallel paths, which can lower the result. For a 313A candidate studying control-system service, the first question is what circuit the meter actually sees.
What does the meter see across two parallel resistors?
In this original paper exercise, assume two ideal resistors, 120 Ω and 240 Ω, connect between the same two nodes, with no other path. The equivalent resistance is R = (120 × 240) ÷ (120 + 240) = 80 Ω. The meter is seeing the combination, not an isolated 120 Ω resistor. The result is below both branch values.
Check it another way using Ohm's law. An imagined 12 V source across the same ideal network would produce 12 ÷ 120 = 0.10 A in one branch and 12 ÷ 240 = 0.05 A in the other. Total current is 0.15 A; 12 ÷ 0.15 = 80 Ω. This is a calculation check, not an instruction to energize equipment for a resistance measurement.
How can you reject a plausible wrong answer?
Adding the values gives 360 Ω, which describes these two resistors in series. Averaging them gives 180 Ω, which does not describe their parallel equivalent. For this ideal network, a proposed result above 120 Ω should make you recheck your setup. Both branches share the same endpoints; identify those nodes before choosing a formula.
As a second exercise, put two equal 120 Ω resistors in parallel. Each carries the same current at the same voltage, so the combined current doubles and equivalent resistance halves to 60 Ω. If your first answer was 240 Ω, the arithmetic followed a series circuit instead of the drawn network.
Does a lower reading prove a component is shorted?
No. The 80 Ω result in the first exercise is completely consistent with two intact stated resistors. Fluke advises checking the schematic for parallel paths and explains that other connected components can affect the reading. You need the intended test points and the manufacturer's specified measurement conditions before comparing a meter result with a component specification.
Keep the conclusion as narrow as the evidence: “The measured network differs from the isolated component” is justified here; “replace the component” is not. This article studies simple resistors. It does not assign operating impedance, acceptable winding resistance or fault limits to an AC motor, relay or electronic control.
What should your study checklist include?
Write the two test nodes, list the paths between them, calculate the ideal equivalent, and then state what the result can establish. For real resistance testing, Fluke says to remove circuit power and discharge capacitors; CCOHS describes isolation, control of stored energy and verification within a lockout/tagout procedure. Follow the equipment maker's procedure and workplace requirements before touching or disconnecting components.
The compressor-terminal article addresses a separate winding-identification problem. For exam planning, use the Ontario 313A exam guide and current-standard explanation; Red Seal still marks exams for its newer standard as under development, checked 2026-09-26. Try the free original sample for a short concept check. The question bank remains draft pending certified-mechanic review, and this exercise is not a recalled exam item.
Sources
- Fluke: how to measure resistance with a digital multimeterIn-circuit parallel paths and de-energized measurements; checked 2026-09-26
- Fluke: what is Ohm's law?Voltage, current and resistance relationship used to verify the original calculation; checked 2026-09-26
- CCOHS: Lockout/Tag outEnergy isolation and verification; checked 2026-09-26
- Red Seal: Refrigeration and Air Conditioning MechanicCurrent versus newer exam-standard status; checked 2026-09-26
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.