{"id":72381,"date":"2026-08-21T21:34:10","date_gmt":"2026-08-22T00:34:10","guid":{"rendered":"https:\/\/a3aengenharia.com\/?post_type=articles&p=72381"},"modified":"2026-08-21T21:34:10","modified_gmt":"2026-08-22T00:34:10","slug":"class-1-class-2-class-3-spd-tests-application-coordination","status":"publish","type":"articles","link":"https:\/\/a3aengenharia.com\/en-us\/content\/technical-articles\/class-1-class-2-class-3-spd-tests-application-coordination\/","title":{"rendered":"Class 1, Class 2 and Class 3 SPDs: differences, tests, applications and coordination"},"content":{"rendered":"\n

Class 1, Class 2 and Class 3 SPDs are not defined solely by the point in the distribution system where they are installed. The classification is linked to the tests to which the device has been subjected, the quantities that characterize those tests and the type of stress the SPD must withstand within the surge-protection architecture.<\/p>\n\n\n\n

In ABNT NBR IEC 61643-11, Class I tests are characterized by impulse current Iimp<\/code>; Class II tests by nominal discharge current In<\/code>; and Class III tests by the open-circuit voltage Uoc<\/code> of the combination-wave generator. The practical consequence is important: comparing the classes merely as \u201cstrong, medium and fine\u201d oversimplifies the problem and can lead to incorrect selection.<\/p>\n\n\n\n

Correct application depends on the origin of surges, the presence of an LPS, the share of current expected at the point, the maximum continuous operating voltage Uc<\/code>, the voltage protection level Up<\/code>, the equipment impulse withstand voltage Uw<\/code>, the effective level Up\/f<\/code>, coordination between stages and short-circuit conditions. The class is one part of the specification, not the entire specification.<\/p>\n\n\n\n

In industrial installations, corporate buildings, data centers, automation, telecommunications and other environments with sensitive electronic systems, the objective is not simply to distribute classes among panels. The objective is to create a protection chain capable of conducting the expected energy and reducing overvoltage to levels compatible with the protected equipment.<\/p>\n\n\n\n

What do Class 1, Class 2 and Class 3 mean in an SPD?<\/h2>\n\n\n\n

The classification should be read as a test classification<\/strong>. It indicates which forms of stress were used to verify SPD behavior and which quantities are used to characterize its performance.<\/p>\n\n\n\n

Class<\/td>Main quantity<\/td>Type of stress<\/td>What the class demonstrates<\/td><\/tr>
Class I<\/td>Iimp<\/code><\/td>impulse current with peak, charge and specific-energy requirements<\/td>capability to withstand stresses associated with portions of lightning current<\/td><\/tr>
Class II<\/td>In<\/code><\/td>8\/20 \u00b5s impulse current<\/td>performance under shorter-duration surges, typical of induced surges and conducted transients<\/td><\/tr>
Class III<\/td>Uoc<\/code><\/td>1.2\/50 \u00b5s and 8\/20 \u00b5s combination-wave generator<\/td>performance as a complementary stage, normally under lower-energy conditions at the protected point<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

The introduction to ABNT NBR IEC 61643-11 itself distinguishes the three groups. Class I tests seek to simulate conducted partial lightning currents. Class II and III tests use shorter-duration impulses.<\/p>\n\n\n\n

This explains why the commercial equivalence \u201cClass 1 = service entrance, Class 2 = distribution board, Class 3 = outlet\u201d is only a didactic approximation. The location may match this sequence in many installations, but actual selection depends on the source of damage, current distribution and SPM coordination.<\/p>\n\n\n\n

For an overview of the device before examining the classes in depth, see SPD: what it is, what it is used for, classes, sizing and installation<\/a>.<\/p>\n\n\n\n

Test class is not the same as protection level<\/h2>\n\n\n\n

Another common mistake is to interpret Classes I, II and III as if the class directly determined how low the voltage reaching the equipment would be. This mixes energy capability<\/strong> with voltage limitation<\/strong>.<\/p>\n\n\n\n

The parameter directly related to limitation is Up<\/code>, the voltage protection level declared by the manufacturer. Actual performance in the circuit, however, must consider Up\/f<\/code>, which also incorporates the voltage drop in conductors and components of the SPD branch.<\/p>\n\n\n\n

A Class I SPD may be capable of withstanding high energy while presenting a Up<\/code> value higher than desirable for a sensitive electronic load. A downstream Class II or III SPD may then be necessary to reduce the effective voltage level. The correct logic is to coordinate withstood energy + limited voltage + location + connection<\/strong>.<\/p>\n\n\n\n

Class 1 SPD: when impulse current Iimp is decisive<\/h2>\n\n\n\n

The Class 1 SPD, also identified in the market as T1 or Type 1, is tested for a stress represented by Iimp<\/code>. This quantity is not merely a peak value in kiloamperes. Its definition involves peak value, charge transfer Q<\/code> and specific energy W\/R<\/code> over a specified time.<\/p>\n\n\n\n

This difference is essential to avoid directly comparing Iimp<\/code> with In<\/code> or Imax<\/code> as though all were merely \u201cSPD kA\u201d. An impulse current associated with direct lightning has different energy content from an 8\/20 \u00b5s current with the same peak amplitude.<\/p>\n\n\n\n

Relationship between Class I and direct lightning<\/strong><\/p>\n\n\n\n

ABNT NBR 5419-4:2026 establishes in Annex C that when the structure requires an LPS, or when other sources of impulse current are identified in the installation, SPDs tested to Class I must be used at the corresponding points.<\/p>\n\n\n\n

The logic derives from the sources of damage:<\/p>\n\n\n\n