Which quantity can be tested with a digital multimeter while the circuit is powered?

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Measuring voltage in a live circuit is possible because you’re checking the potential difference across two points without needing to disturb the circuit’s operation. When the meter is set to voltage, its high input impedance means it draws very little current, so it doesn’t meaningfully load the circuit or alter the voltage you’re trying to measure.

Current measurement, capacitance, and resistance typically require the circuit to be de-energized or arranged differently. Measuring current usually involves inserting the meter in series, which can disturb a live circuit and poses safety risks. Capacitance readings expect the capacitor to be isolated or discharged so the meter can assess its charge behavior, not while power is applied. Resistance checks need the circuit off; applying voltage can give false readings or damage the meter.

So, the quantity you commonly test with a digital multimeter in a powered circuit is voltage.