In short circuit conditions, a circuit may run over the designated current for an extended duration before the circuit protection is activated. During this time, a wire/cable may become very hot and potentially exceed its designated thermal rating. Well designed wires/cables will not release any smoke during this fault condition.
The overload resistance test, also known as the smoke resistance test, is designed to examine the durability of the wire insulation under extended periods of internal heating caused by over-current conditions. Additionally, this test is designed to determine if smoke is generated during overload conditions. The duration and current level are wire specification dependent. The test sample is considered to have successfully completed the test if there is no persistent emission of smoke from the sample, the insulation color sustains no significant change, and there is no odor.
Visually examine the specimen and record any defects.
Estimate the necessary current to cause the specimen's conductor to rise to the target temperature. Adjust test circuit resistance to achieve desired current.
Place specimen into a draft free enclosure such that it is free-hanging and there is tension on both ends of the specimen to prevent sagging during the test.
Connect specimen to test circuit.
Attach thermocouple to test specimen.
Turn on test power and slowly raise the current until the target specimen temperature is reached. Maintain the specimen at this temperature for the specified duration.
Record any observed smoke or odor.
Remove sample and examine for any changes.
Report test current, maximum specimen temperature, and test observations.