Moisture entering faults in the insulation of subscriber lines provides so‐called “leakage” paths which reduce the insulation resistance. Testing the insulation resistance of all lines under the environmental conditions which tend to produce these leakages is a maintenance technique, relatively new, for detecting the insulation defects. The faults can then be corrected before they become serious enough to affect the customers' service. Subscriber reports are thereby reduced and the correction of the faults on a preventive maintenance basis lends toward a more uniform work load for the repair personnel. Rapid testing of the lines is necessary, otherwise the environmental conditions may change and the leakages will disappear without detection. Rapid line insulation testing is practiced quite generally in all the switching systems throughout the Bell System, but the testing arrangements used are wholly or partially manually controlled in the testing and recording operations. While the benefits derived from rapid line insulation testing apply to all systems alike, this article is confined to a discussion of the entirely automatic testing and recording arrangements which are now being introduced in the No. 1 and No. 5 crossbar systems.

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