Insulation testers are used to look for weak points in the insulation of your strings. Catch damaged wire insulation before it becomes a ground fault or, worse, a fire. Insulation testers measure very high resistance values (e.g. MOhms) which is why they are often referred to as "Meg-testers". If, for example, a PV wire was accidentally pinched between a module frame and the racking, and the insulation of the wire is partially cut through but not shorted, the resistance of the insulation at that point will be lower than elsewhere in the string. This can be very hard to detect without an insulation tester.
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