Electric-motor manufacturers seeking Underwriters Laboratories’ (UL) “explosionproof” certification face a Dec. 11, 2010 deadline that could change the way they design and build their products. On that date, UL will require that motors fully comply with the UL 674 standard before being labeled explosionproof.
Explosionproof does not mean the motor cannot explode or will survive a catastrophic external explosion. It means the motor will prevent an explosion or spark from within the motor from propagating into the surrounding atmosphere, which may contain flammable gases and vapors.
The UL standard breaks explosionproof motors (EPMs) into classes, divisions, and groups defined by the environments and locations in which they can be used. Class denotes the general environment, with Class I indicating the presence of flammable gases and vapors.
Division refers to the EPM’s location. Division 1 locations are those where, according to the National Electrical Code, “ignitable concentrations of flammable gases or vapors can exist under normal operating conditions; or in which [such concentrations] may exist frequently because of repair or maintenance operations or because of leakage; or in which breakdown or faulty operation of equipment or processes might release ignitable concentrations and might also cause simultaneous failure of electric equipment.”
The specific gases, dusts, or fibers in the environment are listed in the Group classification. One of the most challenging environments is one with Group D substances: Glacial acetic acid, acetone, ammonium hydroxide 20%, ASTM Reference Fuel “C,” diethyl ether, ethyl acetate, ethylene dichloride, furfural, n-Hexane, methyl ethyl ketone, methanol, 2-nitropropane, and toluene.
EPM designers must look for leak paths where vapors could enter or leave the motor and initiate an explosion. The Achilles’ heel of EPMs is usually the point where electrical and signal wires enter the motor housing. Differential pressure between the interior of the motor and the outside could cause gases to leak into or out of any voids, gaps, or spaces.
Designers typically use a low viscosity polymer that flows into voids and creates a solid contact with wire surfaces and entry-point edges to create a gas and moisturetight barrier. These polymers are most often thermoset urethanes, epoxies, or silicones. Many thermosets resist standard chemicals, but Group D substances aggressively degrade even chemical-resistant polymers.
EPM manufacturers generally use materials that pass UL’s testing on a stand-alone basis because it speeds the UL-approval process for the overall EPM assembly. UL waives additional testing if all components withstand the chemical environments of the Class and Group for which the manufacturer is seeking approval.
Most EPM-ready adhesives and potting polymers meet UL 1203, but no adhesive polymers are approved to UL 1203, Class I, Division 1, Group D without restrictions. According to the spec, the cured adhesive must retain 85% or more of its original compression strength when exposed to Group-D gases with less than 1% shrinkage or swell.
A typical two-part, room-temperature-cure epoxy will crack, split, degrade into powder, swell, or gain weight when exposed to Group D vapors, failing the UL-674 requirements for sealing compounds. Even materials with no visible changes during exposure may weaken internally and fail the compression test.
Polymer designers looked at resin and curative chemistry, cured cross-link density, accelerators, and fillers to develop an epoxy that would pass the UL-674 requirements. Henkel Corp., Rocky Hill, Conn., recently had its E‑40EXP potting material approved to UL 1204 as meeting compression strength requirements after exposure to the Group D solvents and vapors.