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Measurement Category Symbols: What CAT II and CAT III Really Promise
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Measurement category markings appear on multimeters, clamps and oscilloscope inputs, and they are widely misunderstood as a quality grade. A CAT III rating states that the input can withstand the transient overvoltages expected in a fixed installation at the marked voltage; it says nothing about resolution, accuracy, or the probes supplied in the box.
Voltage and category together
The marking is always a pair: a category and a voltage. CAT III at 300 V and CAT III at 600 V are different claims with different clearance and test requirements. A marking that shows only the category is incomplete, and a marking that shows a higher voltage than the design supports is dangerous.
The probe problem
The rating of the instrument applies to the instrument as tested. A CAT III instrument fitted with a CAT II probe has the rating of the probe, because the probe is part of the circuit the user touches. Supplying unrated probes with a rated instrument is a common and serious non-conformance, and it is invisible once the probes are separated from the instrument.
Practical rule for users
Match the category and voltage of the instrument, the probes and the leads, and treat the lowest of the three as the rating of the measurement. Where a measurement requires a category above the available equipment, the correct action is to change the measurement point, not to rely on a marginal instrument.
