Generating Insights: UL Certification

Published On: September 22, 2021

Underwriters Laboratories (UL) is an independent organisation that certifies, tests, inspects and audits for product compliance. UL is chiefly concerned with the safety aspects of products.

Manufacturers submit products to UL for testing and safety certification on a voluntary basis. There are no laws specifying that a UL mark must be used. However, in the United States there are many municipalities that have laws, codes or regulations which require a product to be tested by a nationally recognised testing laboratory before it can be sold in their area.

Basic UL standards:

■ UL 2200 - UL standard for generator sets
■ UL 1004 -1 & 4- applies to electrical rotating machines/alternators
■ UL 1446 - evaluation of electrical insulation systems

UL 1446 covers all electrically insulating materials in the wound components of the electrical rotating machine. This includes main stator/rotor, exciter stator/rotor and PMG stator. These materials are divided into two categories:

Major Insulation includes materials that are relied upon to prevent a risk of electric shock or fire. These materials are used between the magnet wire and ground or between different phases of the windings.

Materials under this category are evaluated through a long term thermal aging motorette test. Test specimens are subjected to a series of environmental conditions, temperature, cold shock, vibration, humidity and electrical test to simulate an accelerated life cycle and the tests are repeated till the specimens fail. Though the duration of this test is typically one year but the total project time to complete the test is about 18 months.

Minor Insulation includes materials that are typically used for mechanical properties and are not relied upon for insulation.

A sealed tube test is conducted after completion of a long term motorette to add minor insulating material to determine if the gaseous degradation by-products of these materials are detrimental to the performance of major components. Materials are sealed in glass tubes and aged for two weeks. Following the test, dielectric strength testing is conducted on the magnet wire to determine if the materials have caused any additional degradation.

Motorette-Test Motorette test


Sealed-Tube-Test Sealed tube test


The total time to complete a sealed tube test takes several months due to time taken for collecting samples from all plants.

For alternators to be UL 1004 recognized the insulation system is required to be UL 1446 compliant. A product investigation by UL typically involves testing of product samples.

Through the UL witness test data programme, performance tests are conducted at Cummins Generator Technologies test facilities under the supervision of UL personnel. A product technical file is established to include all the product construction features in accordance with UL 1004 requirements.

Components such as AVRs, space heaters, RTD/ thermistors, CT/isolation transformers and plastic components are also evaluated separately.

An initial UL production inspection is conducted at the manufacturing location following the successful completion of a product investigation. UL then provides an authorisation to apply the UL mark on products.

UL carries out follow up audits at each manufacturing plant, typically quarterly, over life time of UL certification of products. If the auditor records any non-compliance during this audit, UL will issue a variation notice which must be addressed.

UL 1446: A plant specific label is installed on the machine to indicate compliance with UL insulation system

UL 1004: A label common to all plants is installed to indicate that all relevant components in the machine comply with UL 1004 requirement.

Note: It is required that these labels are installed on products at the plants before shipment.

For more information please contact the Applications team