The Measurement Category (CAT) rating: Why it matters


Working with electricity every day is a hazardous existence. Safety equipment, such as clothing, gloves, boots, etc., is critical in helping protect electrical workers from potential accidents. A less obvious safety precaution is the design of any test equipment being used to maintain the electrical system.

The International Electrotechnical Commission (IEC) 61010 standard outlines the safety requirements for electrical equipment for measurement, control, and laboratory use. It includes requirements for testing and measuring circuits and for instruments.


A key aspect of the standard is maintaining protection against electric shock in normal and single fault conditions (short circuits, open circuits, ground faults, overloads, etc.). Where the instrument is used plays a vital role in determining the level of protection required. This paper examines the Measurement Category (CAT) rating and its crucial role in instrument design and user selection.


Overvoltage transients on power lines have the potential to harm connected equipment, resulting in failures that could pose safety risks to users. While large transients, such as those caused by lightning strikes, are rare, moderate ones (1.5-2.0 times the nominal voltage, and sometimes up to 4.0 times) occur regularly because of load switching on the distribution system. They can injure and kill when they occur if the operator is not using a properly rated instrument.

Two key words in electrical safety are ‘creepage’ and ‘clearance.’ Creepage is the shortest distance between two points at different electrical potential along the surface of an insulating material. Clearance is the shortest distance in the air between two conducting parts. Conductive parts at significantly different voltages must be a safe distance from one another, or an arcing failure may occur. Arcing failures cause circuit damage, overheating, and even fire. Most importantly, they cause an electrical shock hazard for the user, especially with handheld instruments.

What constitutes a safe distance is dependent on where in the supply system the instrument is being used.   

Measurement Category (CAT) Rating

Measurement Category is a way of classifying test and measurement equipment terminals according to the type of power circuit to which they will be connected. Measurement categories consider transient overvoltage peaks, short circuit current levels, the location in the building installation where the test or measurement is to be made and some forms of energy limitation or transient protection that may be included in the building installation.

The CAT rating defines the level of transient (spike or surge) that the instrument has been designed to withstand. Transients vary in size and duration depending on the source. A transient may be several kV in amplitude, but its duration is notably short, a typical interval being 50 µsec (microseconds). Its main danger is that when it rides on top of the sinusoidal voltage it can initiate an arc, which will continue until the end of the cycle.

Measurement categories are defined in IEC 61010-2.

Measurement Category II: Applicable to test and measuring circuits connected directly to utilisation points (socket outlets and similar points but excluding installed lighting) of the low voltage power installation.

Measurement Category III: Applicable to test and measuring circuits connected between Measurement Category II and Measurement Category IV of the building’s low voltage power installation.

Measurement Category IV: Applicable to test and measuring circuits connected between the source of the building’s low voltage supply and the first accessible isolator switch able to disconnect all line and neutral connections. The high potential short circuit currents, typically greater than 50 kA, existing in these circuits can create a dangerous, high energy arc flash through an accidental short circuit caused while making measurements.

Each category also has a voltage rating to show the maximum safe phase-to-earth system voltage, normally 50 V, 100 V, 150 V, 300 V, 600 V or 1000 V. A CAT rating without an associated voltage is useless.

The following graphic shows what CAT rating is required based on the area of work:

In a CAT IV environment, the available short circuit current can exceed 1000 amps. Inside an instrument that happens to be testing the circuit, this can generate hundreds of kilowatts of heat in a small space for a few milliseconds. The rapid expansion of air can cause the instrument to disintegrate or explode, leading to fire, burns, and dangerous flying debris.

An instrument with a CAT IV rating requires greater creepage and clearance distances than one with a CAT III rating. Instruments that are not connected to live circuits must meet a category rating relative to the input voltage that drives the instrument. The standard is CAT II 300 V. In addition, they must meet the creepage and clearance distances for the test voltages for secondary circuits (output voltage) derived from the mains circuit.

Using the CAT rating

The CAT rating is given in two parameters: one indicating system level, and the other specifying rated operating voltage. A designation of ‘CAT IV 600 V’ means that the unit is safe to operate in any electrical environment up to and including CAT IV, on cable or apparatus rated up to 600 volts. Understanding the test environment and the highest voltage rating of the apparatus to be tested dictate the required CAT rating.

Beware of products that specify the CAT rating while failing to list the voltage level. Not giving a voltage level means incomplete information and the absence can be costly in terms of safe operation. Also think twice about any instrument where the data sheet simply states “CAT rated”. This term means nothing in the real world of electrical testing. Reputable instrument suppliers will clearly define a category rating and a voltage rating.


Understanding the CAT rating for a piece of test equipment is a critical part of ensuring your safety. Where you are working impacts the required CAT rating on any test instrument you use. The higher the CAT rating, the greater the creepage and clearance distances within the instrument, which can impact instrument design.

Here are key considerations to keep in mind when evaluating test instruments:

  • Base the required CAT rating off the worst-case testing environment and apparatus.
  • Even instruments that test ‘dead’ apparatus should have a CAT rating (output voltage derived from the input voltage).
  • A CAT rating should have two parts, a measurement level and voltage level. Beware a CAT rating without an associated voltage level.
  • Beware the general term “CAT rated” as the only CAT rating reference.

An instrument with the proper, or higher CAT, rating for the test environment will be the safest instrument to use.