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A part from the fact that good insulation resistance is an essential condition for maintaining service, the regular recording of insulation resistance values is undoubtedly the best method of detecting deterioration and of indicating when remedial action is desirable, or perhaps essential, in order to prevent complete failure.

Insulation resistance (IR) should accordingly be measured and recorded at regular intervals, the recording being preferably on a separate log-sheet for each important machine or circuit, so that each fresh reading can be compared with previous values and any downward trend immediately observed. It cannot be too strongly emphasised that, subject of course to reasonable minimum values being maintained, trends are more important than actual values; a single value is of comparatively little significance. The intelligent interpretation of results is therefore of great importance and a general appreciation of the factors involved is essential.

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