What this site does
It compares what manufacturers publish about instruments: categories, voltages, ranges, fuse types, ingress ratings. It reports those as manufacturer statements and does not assess conformity to any of them.
What it does not do
Tell you how to work on an installation. Isolation, verification, testing procedure and the competence required for each are governed by the manufacturer instructions for your instrument and by the rules that apply where you live. Where a job belongs to a qualified person, it belongs to a qualified person.
The three things about instruments worth knowing anyway
One: the category and the voltage are one specification. CAT III 600 V and CAT II 1000 V are different instruments, and the larger-looking number is not the safer tool. Across ten electrical instruments we compare, two listings state the complete form. Read the manual.
Two: prove the tester. On a known live source, before and after. A voltage tester that has failed looks exactly like a dead circuit — that is the entire hazard of the instrument, and proving is the only defense.
Three: a contactless pen never confirms absence of voltage. It says “there is probably voltage here”. Insulation thickness, screening and how you hold it all change what it senses. Silence from a pen is the absence of evidence, not evidence of absence.
The mistake that damages meters
A lead left in the current socket while measuring voltage puts a near short circuit across the supply. The fuse is what interrupts it, which is why the fuse specification is a safety matter and why some instruments now light the correct sockets for the selected function.
Leads back in the voltage sockets, every time.
And the sentence that governs every page on this site
Nothing here replaces the manufacturer’s own instructions or the judgment of someone qualified to do the work.
What gets asked here
What is a measurement category?
A manufacturer statement of the transient over-voltage an instrument is built to survive at a given point in an installation. CAT II covers appliances and socket outlets, CAT III fixed installation and distribution, CAT IV the supply side. It is a safety rating, not a quality grade.
Is a higher category always better?
No. It describes where an instrument may be used. A socket tester rated CAT II 300 V is correctly rated for a socket outlet.
What does proving mean?
Confirming your tester works on a known live source immediately before and immediately after the measurement that matters. A tester that failed between the two checks is the one case a single test cannot catch.
Last reviewed 31 August 2026
