The measurement category — CAT II, CAT III or CAT IV — defines how close to the power source a meter can be safely used, and how large a transient voltage spike it can withstand without the input failing dangerously. Defined in the IEC 61010 standard, a higher CAT number means a higher-energy environment. The CAT rating is always paired with a voltage (e.g. CAT III 1000 V), and you must read the two together.
What the categories mean
| Category | Where it applies |
|---|---|
| CAT II | Single-phase receptacle-connected loads — appliances, portable tools, plug-in equipment. |
| CAT III | Fixed installation / distribution — distribution boards, busbars, motors, fixed wiring. |
| CAT IV | Origin of installation — the service entrance, the utility supply, outdoor overhead lines, the meter. |
The closer you are to the source of power, the more energy is available to drive a fault, and the larger the transient spikes — so the higher the category you need.
Why the voltage and the category must be read together
A common and dangerous misconception is that a higher voltage rating always means a safer meter. It does not. A CAT III 1000 V meter is built to withstand larger transients than a CAT IV 600 V meter, because the test impulse for CAT IV at a given voltage is higher. When comparing meters, the category is the dominant factor, then the voltage within that category. Choose the category for your location first, then the voltage.
How to choose
- Identify the highest-energy point you will ever measure with the meter.
- Pick a meter rated at that category or higher, at a voltage at least as high as the circuit.
- When in doubt, go up a category — a CAT IV meter is safe to use on CAT III and CAT II circuits.
- Check the test leads and probes too — they carry the same CAT rating and must match the meter.
It is not just the meter
The CAT rating applies to the whole measurement system — meter, leads, probes and accessories. A CAT IV meter with CAT II leads is only as safe as the weakest part. Always use properly rated leads, keep fingers behind the finger guards, and wear appropriate PPE for the energy level.
Quick reference
- Plug-in appliance testing: CAT II
- Distribution boards, fixed wiring, motors: CAT III
- Service entrance, utility metering, outdoor lines: CAT IV
Unitest Instruments carries the full range of CAT III and CAT IV rated Fluke meters and matching test leads. Tell us where you work and we'll make sure the meter and leads are correctly rated.