Under what condition can resistors in Class I locations not require an enclosure?

In Class I locations, which are areas where flammable gases or vapors may be present, certain precautions must be taken to ensure safety. Resistors, which can generate heat when current flows through them, are potential sources of ignition since excessive heat can ignite flammable substances.

The correct condition that allows resistors in Class I locations to operate without an enclosure is if the maximum temperature does not exceed 80 percent of the flashpoint of the surrounding flammable material. The flashpoint is the lowest temperature at which vapors of a combustible liquid can ignite in the presence of an ignition source. By maintaining the resistor’s temperature below this threshold, the risk of ignition is significantly minimized, hence eliminating the need for additional protective enclosures.

This condition is critical in hazardous locations to ensure that equipment does not become a source of ignition. Proper temperature management is a key aspect of safety in environments where flammable gases or vapors are present. Maintaining the resistor's temperature below 80 percent of the flashpoint ensures that even under normal operational conditions, the risk of igniting dangerous vapors is low enough to warrant the use of unprotected resistors.