Choosing a Bench Multimeter by Safety Rating, Not by Digits

Multimeters are bought on digits, accuracy and features, and the safety rating is treated as a formality. For anyone measuring inside a live installation, the rating is the most important specification on the datasheet, because it describes the transient the instrument will survive without flashing over.

The rating pair

Read the category and the voltage together: CAT III 600 V is a different instrument from CAT II 600 V even if both carry the same accuracy claim. The category describes where the instrument may be connected, so the correct choice follows from the measurement point rather than from the instrument budget.

The accessories

Probes, leads and clips carry their own ratings, and the measurement is only as safe as the weakest item in the chain. A rated instrument with unrated leads has the rating of the leads. Check that the probe has finger guards, that the lead insulation is intact, and that the current jacks are separate from the voltage jacks, because a measurement made with the lead in the wrong jack is the classic cause of an arc flash.

Inspection and retirement

Check the instrument periodically for cracks in the case, damage at the jacks, and a functioning fuse in the current range. An instrument that has been dropped or that has been used in a category above its rating should be withdrawn from service rather than re-tested, because the internal damage that matters is not visible from outside.

Related technical notes

• What IEC 61010-1 Covers and What It Leaves to the Part 2 Standards
• Pollution Degree 1, 2 and 3: The Number Behind Every Clearance