What factor can have the greatest effect on an insulation-resistance reading?

The temperature of the device being tested significantly influences insulation resistance readings. As temperature increases, the resistance of insulating materials typically decreases, which can lead to lower insulation resistance readings. This is because higher temperatures can accelerate the thermal agitation of molecules within the insulation material, potentially making it less effective at resisting electrical current.

Conversely, at lower temperatures, the insulation may exhibit higher resistance due to reduced molecular activity, which can positively impact the readings. This relationship between temperature and insulation resistance is crucial for accurately assessing the condition and reliability of electrical devices and systems.

While factors like age, type of insulation material, and environmental humidity can also affect readings, temperature generally has a more pronounced and immediate impact on insulation resistance in practical testing scenarios.