Dear Philip,
The UL 94 is a standard defined by the U.S. Underwriter Laboratories (UL) to evaluate and classify the burning behaviour of plastics. UL 94 actually comprises two different tests, horizontal burning (HB) and vertical burning (V).  These determine the flammability or the ability to self-extinguish the flame, respectively.
In both cases, a sample with thickness below 3 mm is placed above a 50 W, 20 mm high open flame. For HB evaluations, the material is placed horizontally and exposed to the fire for 30 seconds. The burning time is measured, and the polymer can be defined as HB if the sample burns at a maximum speed of 76 mm per minute.
V testing is conducted with the material positioned vertically and hit by a flame twice for 10 seconds. The material can be classified V0 when the flame is extinguished within 10 seconds. If it takes up to 30 seconds, it can be categorised as V1 if there is no dripping of burning drops or V2 if these are present. Alternatively, 5V tests involve a 500 W flame with a heigh of 125 mm that is placed underneath a sample five times for 5 seconds. If the material self-extinguishes the flame, does not present dripping or burn holes, 5VA standards are met. If holes are observed, 5VB classification can be awarded.
To learn more about UL tests and their methods, visit: https://www.spelsberg.co.uk/service/technical-information/technical-information/tests-and-test-methods-according-to-ul-underwriters-laboratories/
Spelsberg’s team utilises the UL 94 and many other UL standards on a regular basis. In effect, we operate an in-house test laboratory in Schalksmühle, Germany, to certify our products and ensure key capabilities, such as robustness, safety and high performance. The laboratory and devices are UL- as well as VDE-accredited. Thanks to our experience in material properties and advanced testing, we offer enclosures that provide maximum flammability rating and fire protection. In particular, we recommend using our WKE range, which is made of durable thermoset plastic. To learn more about the WKE series, click: https://www.spelsberg.co.uk/fire-protection-cable-boxes/
If you have additional questions on UL 94 tests or specific flame retardant requirements for your enclosure, please feel free to send me a private message to discuss them.