The International Electrotechnical Commission (IEC) uses measurement category to classify live electric circuits that are employed in the measurement and testing of installations and equipment, usually in the context of a building (residential or industrial).
The categories consider the total continuous energy available at a specific circuit point as well as the occurrence of impulse voltages. Circuit breakers or fuses can limit the energy, and the impulse voltages can be limited by the nominal voltage level.
What is Measurement Category
Electrical test and measurement instruments that interact with electricity are tailored to specific purposes and circumstances. Inaccurate measurements or injuries can result from exceeding or deviating from the application parameter. As a result, there are four basic electrical tool measuring categories: CAT I, CAT II, CAT III, and CAT IV.
Measurement Category are used to assign a rating to test instruments based on their capacity to withstand a voltage spike delivered through a specified resistance.
The higher the category, the greater the possibility of a high voltage overloading a circuit and causing electrical and bodily damage.
The higher the CAT (category) rating, the safer the rating is in most cases.
Electrical measurement categories are classifications for electrical circuit measurements that are intended to help improve the safety of measurements. Test equipment (such as multimeters) is rated according to the highest category it can measure.
Circuits are divided into four unique classifications by measurement categories, which are abbreviated as ‘CAT #’.
Measurements in Category I (CAT I) pose the lowest danger, with each category above CAT I posing a higher risk than the lower categories. Circuit measurements in Category IV (CAT IV) provide the highest danger during testing.
A meter labeled ‘CAT IV’ is capable of measuring circuits ranging from Category I to Category IV. In general, the higher the category rating, the greater the level of protection provided.
Read also : op-amp ac circuit
During measurement or testing, measuring circuits are exposed to working voltages and transient pressures from the circuit to which they are coupled.
When the measuring circuit is used to measure mains, the transient stresses can be approximated based on where the measurement is made within the installation.
When the measuring circuit is used to measure any other electrical signal, the user must examine the transient stresses to ensure that they do not exceed the measuring equipment’s capability. Circuits are classified into the following measurement categories in this standard:
CAT I Measurement Category
Measurements on circuits not directly connected to the mains are classified as Category I.
Circuit measurements in Category I have the lowest risk of causing a hazardous condition. Circuit measurements that are not connected to the electrical grid at all are included in this category (mains electricity). Electronics and circuits that are powered by low voltage are examples.
Measurements on circuits not derived from the mains, as well as specially protected (internal) mains-derived circuits, are examples. Because transient stresses vary in the latter scenario, IEC 61010-1-5.4.1(g) mandates that the user be informed of the equipment’s transient withstand capabilities.
Voltages from particularly protected secondary circuits are measured in this category. Signal levels, specific equipment, limited-energy sections of equipment, circuits powered by controlled low-voltage sources, and electronics are all examples of voltage measurements.
CAT II Measurement Category
Measurements on circuits directly connected to the low-voltage installation fall into measurement Category II.
Category II measurements are done on circuits linked to a low voltage source, often from a residential wall outlet, and pose a somewhat larger danger than CAT I measurements. This covers items such as hand tools and household appliances.
Measurements on domestic appliances, portable tools, and comparable equipment are examples.
Local-level electrical distribution, such as that provided by a normal wall outlet or plug-in loads, falls into this category (for example, 115 AC voltage for U.S. or 200 AC voltage for Europe). Measurements on domestic appliances, portable tools, and similar modules are examples of Measurement Category II.
CAT III Measurement Category
Measurement category III refers to measurements made throughout the construction process.
Building components such as circuit breakers, distribution boards, busbars, wire, switches, and junction boxes are subjected to Category III testing. If you want to do electrical work around the house, this rating category will come in handy.
Measurements on hard-wired equipment in fixed installations, distribution boards, and circuit breakers fall within this category.
Other examples include fixed installation wiring, which includes cables, busbars, junction boxes, switches, socket outlets, and stationary motors with permanent connections to fixed installations.
CAT IV Measurement Category
Measurements performed at the source of the low-voltage installation fall into measurement category IV.
Electricity meters, primary overcurrent protection devices, and ripple control units are all examples.
On primary overcurrent protection devices and ripple control units, this category refers to the point of installation or utility level readings.
Measurements for Category IV are taken at the source of low voltage installations, outside cable lines, and other utility-level applications.
While a CAT IV certification isn’t required for most domestic electrical projects, it can be beneficial to utilize CAT IV rated meters to increase safety or simplify the meter and probe selection process.
Why Measurement Category is Needed
Voltage spikes occur all the time, even though the electricity provided by the power grid (mains electricity) is rated for a specific voltage. Voltage spikes can be caused by lightning strikes, sudden unloading, capacitors, and motors.
Transient overvoltages are another name for these voltage spikes. Meters would regularly fail at voltages below their authorized values before the effect of these voltage spikes was understood.
When testing circuits of 600V or below, multimeters rated for 1000V would shortly fail. Voltage spikes were eventually determined to be generating voltages much in excess of 1000V, causing them to fail.
As a result, measurement instruments (such as multimeters) must be rated to resist higher voltages (for short periods of time) than the circuit’s intended working voltage. This is why the electrical measurement category is so crucial.
However, the measurement category is simply one of several essential considerations; the operating voltage rating is another. Higher category ratings indicate that the meter can handle higher transient overvoltages for the same rated working voltage.
The maximum overvoltage that the device is designed to withstand is determined by the rated working voltage and measurement category.
Measurement Category Clearances
The importance of clearance is related to electrical insulation and the risk of an arc flash between two electrically energized objects (or between live and grounded parts). Higher voltages necessitate increased clearances. The clearances must be doubled for double insulation.
The necessary values range from 0.04 mm for CAT II, 50 V single insulation to 28 mm for CAT IV, 1000 V double insulation. The precise values are set down in international standards. During the design of the right equipment, such standards should be strictly adhered to.
Measurement Category – Overvoltage Rating
Both a measurement category and a voltage are assigned to test equipment. To evaluate if the test equipment will provide appropriate protection for a specific circuit, both the operating voltage rating and the measurement category rating are required.
The overvoltage ratings for test equipment are listed in the table below, organized by category and working voltage.
To discover the maximum overvoltage rating, look for the row (CAT I/II/III/IV) on the leftmost column that corresponds to the meter’s measurement category, and then look across the row for the column that corresponds to the meter’s rated working voltage:
The required impulse withstand voltage ranges from 500 V (CAT II, 50 V) to 12,000 V (CAT IV, 1000 V), similar to the clearances:
On a Category III circuit, for example, a CATIII, 300V meter can survive an overvoltage of 4000V. It is suited for CAT III circuits up to 300V working voltage, as the rating says. It’s also rated for CAT II circuits up to 600 volts.
It is not, however, acceptable for use on 1000V CAT II circuits, 300V CAT III circuits, or any CAT IV circuits (despite the fact that it has a 4000V overvoltage protection, which corresponds to a 150V rated operating voltage CAT IV circuit).
Never use a meter to perform a measurement in a higher category than its rating. Even if the overvoltage rating is adequate, the circuit is not designed to handle the increased current of the higher category circuit.
How to Do Measurement Category on Your Own
A measurement is only as safe as the lowest rated component, just as a chain is only as strong as its weakest link. The measurement circuit and everything linked to it must be rated for the sort of measurement being done.
Test leads and probes, like test equipment, have category and working voltage ratings. If you have a collection of leads with varied ratings, keep this in mind. To avoid an unintentional failure, always double-check the rating of your test leads.
Measurement Category Rating Selection
For a variety of reasons, it is generally advisable to select the highest category available.
Adaptability – A CAT IV rated meter can be used in any category, so it can be used for everything from an electronics hobby to residential electrical repair.
While most electrical work in the home doesn’t require a CAT IV rating (most tasks only require a CAT III), it’s still a good idea to have more transient voltage protection when working on any circuit. CAT I meters are frequently ineffective for AC circuits.
A meter with a CAT III or CAT IV grade can be used for almost any application, ensuring optimum versatility for household or electronics work.
Simplicity – One CAT I meter can be used for electronics work and another CAT II/III meter can be used for residential electrical work. Not only must you remember to select the appropriate probe for the task at hand, but you must also keep the probes and attachments separate so that they do not become confused.
You are putting yourself in danger if you utilize a CAT I rated probe on your circuit breaker by accident.
Durability – Higher-rated meters and probes are frequently designed to tighter tolerances, with superior quality control and independent verifications. CAT IV meters are built to be extremely durable and, in many cases, waterproof.
Features – CAT III and CAT IV rated meters contain more features, such as True RMS, which enables for non-sinusoidal wave patterns to be measured. True RMS can be useful for measuring square waves and diode rectifiers in electronics.
In general, a good CAT III meter is all you’ll need for practically any electronic or residential electrical application. Choose a CAT IV meter if you want a more durable meter or the assurance that you are protected to the fullest extent feasible.
CAT III meters with high ratings are available at low prices (about $25 USD in 2020). It’s never worth risking your safety to save a few dollars.