CSLB C-10 Electrical ContractorSafety and TestingEasy
An electrician is using a digital multimeter (DMM) to troubleshoot a 120V control circuit. The DMM display shows 'OL' when attempting to measure resistance across a de-energized coil. What does this reading most likely indicate?
- AThe DMM battery is low and needs to be replaced.
- BThe coil has a very low resistance, indicating a short circuit.
- CThe coil has an open circuit, meaning infinite resistance.
- DThe DMM is set to the wrong measurement function.
Show answer & explanationAnswer & explanation
Correct answer: C. The coil has an open circuit, meaning infinite resistance.
An 'OL' (Over Limit or Open Loop) reading on a DMM when measuring resistance typically indicates that the resistance is higher than the meter's maximum range, effectively meaning an open circuit or infinite resistance.
Why the other options are wrong
- A. While a low battery can cause inaccurate readings, 'OL' specifically points to an out-of-range measurement, often an open circuit.
- B. A very low resistance or short circuit would result in a reading close to 0 ohms, not 'OL'.
- D. While possible, if the DMM is set to resistance and shows 'OL', it's a specific indication of high resistance, not necessarily a function error, unless it's set to a completely unrelated function like voltage.
DMM 'OL' (Resistance)
On a digital multimeter, 'OL' (Over Limit or Open Loop) during a resistance measurement indicates that the resistance being measured is infinite or exceeds the meter's maximum range.
- Typically signifies an open circuit.
- Meter cannot complete the circuit to measure resistance.
- Common when testing broken wires or components.
Memory trick: OL means Open Line, no path for current.