Controlling environmental conditions for testing relays or, for that matter, making any insulation resistance measurement is critical for accuracy and consistency. Subtle changes in humidity, dust, or temperature can cause major changes in IR measurements. Typically, customers will require a leakage current at a specified voltage in addition to an insulation resistance requirement. The external paths on sealed vacuum or gas filled relays are the areas on the surface of the relay exposed to the atmosphere, between high voltage terminals or from the high voltage terminals to the base of the relay.
Leakage current is specified at a higher voltage, the operating or test voltage of the relay.
Together these specifications provide a clear picture of how well a relay will provide isolation. However, it is important to provide a controlled environment when making measurements to assure consistency.