From the bench
CAT III vs CAT IV: pick the rating your panel demands
IEC 61010 measurement categories describe where in an electrical installation a meter may be used, not just how many volts it reads. CAT II covers plug-in loads, CAT III covers distribution inside the building, and CAT IV covers the service entrance and supply side. Choose the category for the highest point you will measure, then check the voltage, and never use a meter in a higher category than its rating.
What is a measurement category?
A measurement category tells you how much transient overvoltage a meter is designed to withstand safely at a given point in an installation. Lightning, switching and faults upstream send short surges down the wiring. Close to the service entrance there is little impedance to limit them, so more energy is available. Further downstream, the wiring and the loads absorb part of it.
That is why the category is about location. The same voltage means a different hazard at a branch socket than at the incoming supply, and the category captures that difference.
What do CAT II, CAT III and CAT IV cover?
CAT II covers measurements on circuits connected to the installation through a plug or receptacle: appliances, portable tools and similar single-phase loads.
CAT III covers the fixed installation inside a building: distribution boards, feeders, branch circuits and permanently wired equipment such as motors.
CAT IV covers the origin of the installation: the service entrance, the primary overcurrent protection, the utility meter and the supply lines coming to the building. Where exactly the boundaries fall depends on the installation, so ask your site’s electrical safety lead if a location is unclear.
Why is the voltage number not the rating?
A rating has two parts: a category and a voltage, and they belong together. SANWA’s published ratings show how easily a big voltage figure misleads. The SANWA PC7000 states CAT III 600 V, and its measuring ranges top out at 600 V. The CD5001 states CAT IV 600 V / CAT III 1000 V, so it carries 1000 V in CAT III but only 600 V in CAT IV. The YX360TRF analog multitester states CAT III 600 V and CAT II DC 1000 V / AC 750 V.
The MG102 insulation tester shows another case: CAT IV 300 V / CAT III 600 V. Its CAT IV rating is lower in voltage than the CAT III one. The SANWA DCM2000DR clamp meter states CAT IV 1000 V, and the DCL3000R flexible AC current sensor states CAT IV 600 V. A CAT IV 300 V meter and a CAT IV 1000 V meter share a category but suit different systems.
Why must a meter never be used in a higher category than its rating?
At a higher category, the surge energy that can reach the meter is greater than the meter was designed and tested for. If the internal spacing, fusing and protection are not up to it, a surge can arc inside the instrument or across the test leads and put the energy through the person holding them.
A meter rated for a higher category may be used in a lower one, but not the reverse. A CAT III 600 V meter suits a distribution panel up to 600 V and is not suitable at the service entrance. The leads, probes and accessories have to carry the same rating or higher, because the combination is only as good as its weakest part.
How do I pick the right rating for my panel?
Start from the place. Identify the highest-category point you will measure, whether it is an outlet, a distribution board or the service entrance. Select a meter whose rating for that category is at least as high as that point, with a voltage at or above the system voltage. Then check that the leads carry the same rating. If your work spans several locations, choose the rating for the most demanding one.
Frequently asked questions
Is CAT IV better than CAT III?
CAT IV is rated for a more demanding location, closer to the supply, but that does not make it the right choice by itself. You still need a voltage rating that suits the system. A CAT IV 300 V meter is not interchangeable with a CAT III 600 V meter.
Can I use a CAT III meter at the service entrance?
No. The service entrance is a CAT IV location, and a meter must not be used in a higher category than it is rated for.
What does CAT II 1000 V mean on a meter?
The 1000 V figure applies in CAT II locations, such as plug-in loads. The PC7000, for example, states CAT III 600 V, so 600 V is its ceiling in that category.
Which SANWA instruments state a CAT IV rating?
Examples include the DCM2000DR clamp meter (CAT IV 1000 V), the DCL3000R (CAT IV 600 V), the CD5001 and CD5002 multimeters (CAT IV 600 V / CAT III 1000 V) and the MG102 insulation tester (CAT IV 300 V / CAT III 600 V). Check each product page for the full rating.