Underwriters Laboratories’ (UL) 94 Standard for Safety Tests for Flammability of Plastic Materials for Parts in Devices and Appliances defines acceptable flammability ratings of plastic materials for parts in devices and appliances in North America, including cord sets. The First Edition of UL 94 appeared in September of 1972. On September 16, 1988, the Department of Defense (DoD) adopted UL 94.
Two primary UL 94 burn tests are the Horizontal Burn (HB) and the Vertical Burn (V), which provide testing parameters, ratings of ignition, and burning times of plastics in electronic devices and components. An example may include the plastics in sockets where the blades of a plug are inserted, or the plastic in power strips containing several outlets. These ratings are established using tests in which small-scale samples are subjected to a Tirrill burner flame ignition source. Unlike a Bunsen burner, the Tirrill allows the adjustment of both air and gas regulation.
Tirrill BurnerConcerning flame retardancy, UL 94 V-0 is considered the “gold standard” by industry professionals for the fire safety testing of plastic components found in electronics such as housings and connectors. V-1 and V-2 are considered less effective at flame retardancy, yet those ratings may not be applicable where ignition or overheating is not a factor and thus can be safely used. Flammability ratings and other material data can be found on UL Yellow Cards via UL’s Product iQ database. An example of a V-0 is modifying a resin by adding “glass-fill” and making it flame retardant is Nylon 66.
Glow-Wire basics: A nickel-chromium wire loop is heated to specific temperatures, such as 850°C (1562°F). The glow-wire (glowing hot) is then pressed against an electrical component for 30 seconds with a force of 1 Newton (1 N) equaling 0.22 lbs. After removing the glow-wire, the component must adhere to these requirements (1) Any flame or glowing must extinguish within 30 seconds; (2) drops of flaming material must not ignite tissue paper placed underneath the test material; (3) No part of the end product or attached periphery parts may catch fire.
IEC Glow-Wire Testing is contained in a numerical series of the IEC 60695 Standard listed below. While UL 94 pertains to North America (yet its flammability testing is known worldwide as many countries and islands also use the North America’s NEMA plug pattern), Glow-Wire testing is the paramount flammability test in Europe, Australia, and Asia. Instead of UL 94’s open flame, Glow-Wire simulates overheating through faulty connections and short-circuiting.
Glow-Wire testing datasheets are compiled from three separate tests under the IEC 60695 standard:
Glow-Wire Flammability Index (GWFI) listed in IEC 60695-2-12; Glow-Wire Ignition Temperature (GWIT) in IEC 60695-2-13; and the Glow-Wire End-Product Test (GWEPT) in IEC 60695-2-11.
GWFI: The highest Glow-Wire temperature in which the material meets the criteria in three consecutive tests (no sustained flaming; no glowing greater than 30 seconds after removal from glow-wire; and no ignition of tissue paper). The test temperature set at 850°C or 960°C.
GWIT: The lowest temperature at which the material ignites and burns for 5 seconds while the glow-wire is in contact. For many appliances this temperature is 775°C.
GWEPT: A test performed on finished products. Temperature: check the IEC 60335-1 (household appliances) safety standard since appliances vary. A reliable range is 750°C or 850°C with a pass/fail result.
These tests are widely referenced in product safety standards such as IEC 60335-1 where higher requirements (e.g., GWIT 775°C and GWFI 850°C) apply for unattended devices to avoid full end-product testing.
One important note. While the Glow-Wire Test is the main test in Europe, Asia, and Australia, there is the needle-flame test specified in IEC 60695-11-5 that simulates the effects of a small flame which may occur during short-circuiting and arcing. The purpose is obvious—to see how electrical faults affect electrical equipment via a small flame. It determines if materials can resist ignition and self-extinguish to prevent flames from spreading.
UL 94 tests the reaction of plastic compositions when exposed to an open flame. UL is a very fire-preventive organization going back to its roots in the late 1800s.
The IEC Glow-Wire tests how material reacts after pressed with a glowing hot (850°C) nickel-chromium loop with a force of 1 Newton for 30 seconds to simulate overloading or short-circuiting without an open flame. However, outside of the specific Glow-Wire standard, the IEC 60695-11-5 standard describes a needle-flame test.
Also noteworthy is that while North America requires UL 94 ratings, Europe and Asia accept IEC 60695-11-10 V-0 as their standard, which is equivalent to UL 94 V-0. Also, IEC 60695 has more comprehensive testing requirements and covers a much broader scope of testing materials and criteria than UL 94.