Measurement Categories (CATI, CATII, CATIII, CATIV)

What are Electrical Measurement Categories?

Electrical measurement categories are classifications for the measurement of electrical circuits used to help increase the safety of measurements performed. Test equipment (such as multimeters) are rated based on the maximum category that they are designed to measure.

Measurement categories divide circuits into four distinct classifications, called categories and abbreviated by the designation ‘CAT #’.

Category I (CAT I) measurements present the lowest risk, and each category above CAT I presents greater risk than the lower categories. Category IV (CAT IV) circuit measurements present the greatest risk during measurement.

A meter rated ‘CAT IV’ is designed to measure circuits of Category I all the way up through Category IV. In general, the higher the category rating, the more protection offered.

However, just because a meter is rated for a higher category than the specified circuit, it does not mean that it is safe to use for all lower categories at all voltages.

Measurement Category Specifications

CAT I

Category I are circuit measurements that present the lowest chance of a hazardous condition. This includes circuit measurements that are not connected to the power grid at all (mains electricity). Examples include electronics and circuits powered by low voltage.

CAT II

Category II measurements present a slightly higher risk than CAT I and are performed on circuits connected to low voltage supply, typically from a wall outlet in a household. This includes portable tools or household appliances.

CAT III

Category III measurements are performed on building installation, including circuit breakers, distribution boards, bus bars, wiring, switches, and junction boxes. This rating category is useful if you expect to perform household electrical work.

CAT IV

Category IV measurements are performed at the source of low voltage installation, outside cable runs and other utility level applications. While a CAT IV rating is generally not needed for household electrical work, it can be helpful to use CAT IV rated meters in order to maximize safety or simplify the meter and probe selection process.

Voltage Spikes Happen All the Time

Despite the fact that electricity supplied by the power grid (mains electricity) is rated for a certain voltage, voltage spikes happen all the time. Lightning strikes, sudden unloading, capacitors, and motors can all cause the voltage to spike momentarily.

These voltage spikes are also known as transient overvoltages. Before the effect of these voltage spikes were known, meters would frequently fail at voltages below their rated levels. Multimeters rated for 1000V would quickly fail when measuring circuits of 600V or lower. Eventually it was discovered that voltage spikes were generating voltages far in excess of 1000V and causing them to fail.

For that reason, measurement devices (like multimeters) must be rated to withstand higher voltages (for short durations) than the expected working voltage of the circuit. This is why electrical measurement categories are so important.

However, the measurement category is only one important factor; the other is the working voltage rating. For the same rated working voltage, higher category ratings mean that the meter can withstand higher transient overvoltages.

The rated working voltage and measurement category go hand in hand to determine the maximum overvoltage that the device is designed to withstand.

Determining the Maximum Overvoltage Rating

Test equipment is rated with both a measurement category AND a voltage. The working voltage rating and the measurement category rating are both needed to determine if the test equipment will provide adequate protection for a given circuit.

The following table shows the overvoltage ratings for test equipment, based on category and working voltage. To determine the maximum overvoltage rating, find the row (CAT I/II/III/IV) that corresponds to the meter’s measurement category on the leftmost column, and then go across the row to find the column corresponding to the meter’s rated working voltage:

Working Voltage	150V	300V	600V	1000V
CAT I	800V	1500V	2500V	4000V
CAT II	1500V	2500V	4000V	6000V
CAT III	2500V	4000V	6000V	8000V
CAT IV	4000V	6000V	8000V	12000V

For instance, a meter rated CATIII, 300V can withstand an overvoltage of 4000V on a Category III circuit. As the rating suggests, it is suitable for CAT III circuits up to 300V working voltage. It is also rated as suitable for CAT II circuits up to 600V. It is not, however, suitable for use on CAT II circuits of 1000V, CAT III circuits over 300V, or any CAT IV circuits at all (despite the fact that it offers a 4000V overvoltage protection, which corresponds to a CAT IV circuit of 150V rated working voltage.

Never use a meter to perform a higher category measurement than its’ rating. Even though the overvoltage rating may be high enough, it is not designed to withstand the higher current of the higher category circuit.

Check the Measurement Category Rating of Your Test Leads

Just as a chain is only as strong as its’ weakest link, a measurement is only as safe as the lowest rated component. Everything connected to the measurement circuit must be rated for the type of measurement being performed.

Test leads and probes have category and working voltage ratings just as the test equipment does. Keep this in mind if you have a collection of leads with varying ratings. Always verify the rating of your test leads to prevent accidental failure.

Recommendations for Selecting Measurement Category Rating

In general, it is always recommended to choose the highest category possible for a few reasons.

1. Adaptability– A CAT IV rated meter can be used on any category, meaning that it will function just as well for an electronics hobby as it will household electrical work. While electrical work in the house generally won’t require a CAT IV rating (only up to CAT III for most tasks), it may still be beneficial to have greater transient voltage protection while working on any circuit. CAT I meters often cannot be used for AC circuits at all. CAT III or CAT IV ratings ensure that a meter can be used for virtually any task, providing maximum adaptability for household or electronics work.
2. Simplicity– It is possible to have one meter for electronics work that is rated CAT I and another rated CAT II/III for household electrical work. But not only do you have to remember to choose the right one for the right task, you also have to keep the probes and attachments separate so you don’t confuse them either. If you accidentally use a CAT I rated probe on your circuit breaker, you are likely putting putting yourself in danger.
3. Durability– Higher rated meters and probes are often built to higher tolerances, with better quality control and independent verifications. CAT IV meters are built to be highly durable and are frequently waterproof as well.
4. Features– CAT III and CAT IV rated meters tend to have more features including True RMS, which allows for the measurement of non-sinusoidal wave patterns. In the field of electronics, True RMS can be useful for measuring square waves and diode rectifiers.

Overall, a good CAT III meter is all you’ll need for virtually every application in electronics or for household electrical work. If you prefer a more robust meter or the knowledge that you have the highest possible protection, go with a CAT IV meter. There are highly rated CAT III meters on the market at low price points (around $25 USD in 2020). Compromising safety is never worth saving a few extra dollars.

Independent Lab Verification of Measurement Categories

Manufacturers may state ratings for measurement category and working voltage and there is generally no governmental authority validating these claims. Always look for an independent testing certification such as UL or CSA. Meters feature the symbols of lab verification on the housing. Check both the front and the back of the meter for certifications.

Note: The ‘CE’ (Conformité Européenne) mark alone may not be enough. Manufacturers are allowed to self-certify that they have met the CE standards. As such, CE is beneficial but should not be regarded as a true independent lab verification.