What Is a CAT Rating on a Multimeter
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What is a CAT rating on a multimeter? It is a marking that pairs a measurement category, written as CAT I, CAT II, CAT III or CAT IV, with a voltage. The category describes where in a building’s electrical system the meter’s input is connected, and the voltage describes how much the input is rated to handle at that location. This page explains each category in general terms, why the category and the voltage must both suit the circuit, and what the marking does and does not promise.
Electrical safety
Work on mains circuits can kill. Before you measure anything, check that the meter’s CAT rating suits the circuit you are measuring, and read that rating from the listing or the manufacturer’s manual. Panel and service work belongs to a licensed electrician.
The danger is not only the voltage on the display. The GAMBICA guide describes high short-circuit currents in the circuits between a building’s supply and its first accessible isolator switch, and says they “can create an extremely dangerous, high energy arc flash through any accidental short-circuit caused whilst making measurements.” Hioki’s manual for the 3244-60 also warns that using an instrument that is not CAT-rated in CAT II to CAT IV work “could result in a severe accident.”
This page gives no steps for live mains, outlets, panels or service wiring. Its examples describe what a rating means, and the sources we opened treat the categories as definitions rather than as a procedure. A rating on a meter is not a statement that the meter is safe for household mains.
Quick answer
- The category names a location in the building’s electrical system, from outlets and portable tools (CAT II) up to the service entrance (CAT IV).
- The voltage is the working level the input is rated for in that category, and both numbers have to suit the circuit.
- A higher category means the input is designed for larger transient overvoltages.
- If neither the meter nor its manual or listing states a category, do not assume one.
Where the category numbers come from
The categories belong to the IEC 61010 series of safety standards for test and measurement equipment. The GAMBICA guide says measurement categories “are defined in IEC 61010-2-x” and summarizes them. The HBM technical note explains that IEC 61010-1 defines three overvoltage categories, CAT II, CAT III and CAT IV, on the power supply side of an instrument. It also says the lowest level is now written as “O (previously CAT I).”
The Hioki page says the categories are assigned from factors such as line-to-ground voltage, the current capacity of the circuit during a short circuit, and the transient overvoltages expected at the point of measurement.
The four categories in general terms
CAT I, now often written as O
CAT I covers measurements that are not directly connected to the mains. The HBM note gives signal levels, regulated low-voltage circuits and protected secondary circuits as examples, and it says no standard overvoltage or surge levels are defined for this group. Teledyne notes that some probes formerly rated CAT I through CAT IV are now rated CAT O, and that CAT O designates test equipment that is “not intended to be connected directly to supply mains.” A CAT I or CAT O marking therefore does not describe an outlet measurement.
CAT II
CAT II is the level of the wall outlet and the equipment plugged into it. The Hioki page places it “from the power outlet plug to the power circuit of electrical equipment via the power cord.” The HBM note lists mains sockets in household applications and portable tools as examples, and the B&K Precision note describes the level as portable equipment and appliances at the local level. A Hioki manual adds that “CAT II covers directly measuring electrical outlet receptacles.” These statements define where the category sits. They do not say that a CAT II meter is approved for all household mains work.
CAT III
CAT III sits inside the building. The Hioki page puts it “from the distribution board (main switch) through indoor wiring to the terminal behind the outlet.” The B&K Precision note calls it the level “at a fixed installation, distribution level of AC main power.” The HBM note adds circuit breakers, wiring and junction boxes as examples. The GAMBICA guide says the higher short-circuit currents in this group call for additional insulation and other provisions in fixed installations.
CAT IV
CAT IV is the point where the supply enters the building. The Hioki page defines it as “from the service line entering a building to the distribution board (main switch).” The B&K Precision note describes it as “outside of a building, main power line at service level.” The GAMBICA guide describes the circuits between the building’s supply and the first accessible isolator switch, which include the tariff meter and the service fuse, and places their short-circuit current above the other groups.
The GAMBICA figures give a rough sense of how the short-circuit current rises from one category to the next. The table below uses only the typical values the guide gives. Those values depend on the installation, and the guide gives no figure for any single building.
| Category | Examples the GAMBICA guide gives | Typical short-circuit current |
|---|---|---|
| CAT II | Household appliances, portable tools, consumer side of socket outlets | Typically below 10 kA |
| CAT III | Distribution boards, circuit breakers, cables, wiring to socket outlets | Typically up to 50 kA |
| CAT IV | Circuits from the building supply to the first accessible isolator, including the tariff meter | Typically greater than 50 kA |
Why the category and the voltage must suit the circuit
A voltage rating alone does not describe how large a sudden spike the input must survive. Each category is paired with a transient overvoltage level, and that level rises with both the category and the rated voltage. The Hioki page gives the values in the table below and attributes them to the IEC/EN 61010 series.
| Voltage at the measurement point | CAT II | CAT III | CAT IV |
|---|---|---|---|
| 300 V | 2,500 V | 4,000 V | 6,000 V |
| 600 V | 4,000 V | 6,000 V | 8,000 V |
| 1,000 V | 6,000 V | 8,000 V | 12,000 V |
The table shows that the values differ by category. At 300 V, the step from CAT II to CAT IV is 3,500 V (6,000 V minus 2,500 V). At 600 V, it is 4,000 V (8,000 V minus 4,000 V). Both differences are our own arithmetic on the table values, and both show that the same voltage number can carry a different protection level.
Teledyne states the same idea in plainer words: the closer you get to the power grid, the higher a measuring device’s CAT rating must be for it to be safe. The Hioki page gives the working rule to “use instruments rated for the measurement category (CAT II, III, or IV) or higher.”
The voltage number still matters. The B&K Precision note gives a common example of a label that reads CAT I (1000V) and CAT II (300V). One meter can therefore carry a higher voltage limit for one category than for another, and you must read both figures for the place where you plan to measure.
What the rating on a meter does promise
A CAT marking tells you that the manufacturer designed the input for a stated category and a stated voltage. The B&K Precision note says the label generally indicates how much voltage the input can withstand while meeting safety requirements. The Hioki page lists several separate ratings that can appear on an instrument, including the maximum rated voltage to earth, the rated voltage between terminals, and the anticipated transient overvoltage. The category is one part of that set, and the set is what the manufacturer states for the design.
What the rating on a meter does not promise
A CAT marking is a design limit, not a general approval. Four limits follow from the sources we opened.
- It does not cover every circuit in its category. The GAMBICA guide says the measurement terminals “must never be connected to a circuit that exceeds the rated voltage,” so the voltage figure is a ceiling, not a suggestion.
- It does not turn a meter into a general-purpose household tool. The household examples for CAT II describe where a measurement sits. They do not say the meter is suited to every task in a home.
- It does not state a current or fuse rating. The Hioki page lists voltage-related ratings, and the sources we opened do not treat the CAT marking as a current limit. Check the fuse and current figures in the manual.
- It does not describe test leads on their own. The sources we opened do not explain how a meter’s category combines with separately sold leads or probes, so the leads need their own check.
Common mistakes
- Using a CAT I product on an outlet. The Hioki safety page says a CAT I measuring product should not be used in CAT II, III or IV environments, and an outlet falls in CAT II.
- Assuming a meter is rated because it looks professional or comes from a known brand. The sources we opened do not allow a category to be inferred from appearance. The category has to be stated on the meter, in its manual, or in its listing.
- Reading only the category and ignoring the voltage. The Hioki page says the instrument’s rated line-to-ground voltage and terminal-to-terminal voltage must not be exceeded, so the category does not replace the voltage check.
Reading the rating on your meter
The B&K Precision note says that labels indicating CAT are often placed near or next to the inputs. The Hioki page says its ratings are provided under “Specifications” in the user manual. Check the label first, then the manual, and then the listing. If none of these states a category, the listing does not state a CAT rating, and the meter should not be assumed to carry one. Keep that meter away from mains measurement until the rating is confirmed.
When you compare models, read each model’s category before its features. The site’s multimeter roundup covers the models in general, and each model’s listing or manual remains the only source for its own category.
FAQ
What does the number after CAT mean?
It is the voltage the manufacturer states for the input in that category. The Hioki page lists it as one of several separate figures, next to the maximum rated voltage to earth.
Does a CAT IV rating make a meter safe on any circuit?
No. A higher category means the input is designed for the energy found in the location it names. It does not remove the voltage ceiling, and the sources we opened describe that ceiling as one that must not be exceeded.
Is a CAT II meter safe for household mains?
The sources do not describe CAT II as a general safety approval for household mains. They describe CAT II as the level of outlets and portable tools. Distribution boards and the building supply fall in CAT III and CAT IV, according to the Hioki definitions.
What does CAT O mean on a newer probe?
Teledyne says some probes once rated CAT I through CAT IV now carry CAT O, and that a supplier may print CAT O alone or alongside another category. The label describes equipment that is not intended to be connected directly to supply mains.
Can a meter with a lower category be used on a higher-category circuit?
No. The Hioki safety page says a CAT I measuring product should not be used in CAT II, III or IV environments. A lower category does not cover a higher one.
Does a CAT rating protect me from a faulty circuit?
The sources describe the rating as a design limit for the input. They do not describe a meter as a fix for a faulty circuit, and a meter with a CAT rating is no substitute for the checks a licensed electrician makes on panels and service equipment.
Bottom line
Read the category and the voltage together, choose a category that matches or exceeds the highest circuit you might meet, and do not treat a CAT marking as approval for household mains. Voltage testers are a separate tool, and the site’s guide to how voltage testers work covers them. Once the category is confirmed, the auto-ranging digital multimeter roundup covers the features to compare.
How we chose
The criteria were narrow. Each source had to be an IEC-based explanation, a manufacturer page or manual, or a trade-association guide that summarizes IEC 61010. Each had to open with curl on 2026-10-22 and return readable text. Retailer pages were not used for definitions, and Wikipedia was not used for any safety statement. Every quote was searched for in the saved text of its page, and any quote that did not match was paraphrased or dropped.
Fifteen URLs were tried. Three returned errors, two with HTTP 403 and one with HTTP 429, and were excluded. Twelve returned readable text, and seven of those are cited in the sources list. The cited sources come from manufacturers and one trade association, not from retailer blogs.
What we could not verify
The pages we opened leave the points below open, and this page does not fill them with assumptions.
- The IEC 61010 text itself was not opened. The pages describe the standard secondhand.
- No rating is given for any named meter, because this page names no model.
- Automotive electronics and low-voltage DC systems are outside the pages we opened.
- The sources do not say how a meter’s category combines with separately sold test leads or probes on a given model.
- The short-circuit current figures are typical values that depend on the installation. No source gives a figure for a specific building or panel.
- No source gives a current or fuse rating that applies to a meter discussed here.
- Local electrical codes and workplace rules are outside the sources we opened.
- The sources do not all use the same labels. Some still use CAT I for what is now written as O, and we quote each label as its source gives it.
Sources
- B&K Precision, “What do the input CAT ratings mean?”: https://www.bkprecision.com/knowledge/technical-notes/5/what-do-the-input-cat-ratings-mean
- GAMBICA, “Guide to Overvoltage Categories & Measurement Categories”: https://www.gambica.org.uk/static/d96bb734-348d-478e-b0cba3b50fc35940/Guide-to-Overvoltage-Categories-and-Measurement-Categories.pdf
- Hioki, “Ensuring Safe Operation”: https://www.hioki.com/br-pt/support/warranty/safe-operation
- Hioki, 3244-60 instruction manual: https://www.hioki.com/download/30665
- Hioki, safety usage pages: https://www.hioki.com/download/28195
- HBM, “GN610, GN611 Isolation and Type Testing”: https://www.Hbm.com/fileadmin/mediapool/files/GN610_GN611_Isolation_and_type_testing.pdf
- Teledyne LeCroy, “Probe Safety Demystified: CAT Ratings”: https://blog.teledynelecroy.com/2020/06/probe-safety-demystified-cat-ratings.html
