Updated

BM5200 5 kV insulation resistance tester

Electrical insulation degrades over time because of the various stresses that are imposed upon it during its everyday working life. The insulation has been designed to withstand these stresses for a period of years; this is regarded as the working life of that insulation and often runs into decades.

Abnormal stresses can bring about an increase in this natural ageing process that can severely shorten the working life of the insulation. For this reason, it is good practice to perform regular testing to identify whether increased ageing is taking place and, if possible, to determine whether the effects may be reversible.

The purpose of diagnostic insulation testing is:

• To identify increased ageing
• To determine the cause of this ageing
• To pinpoint, if possible, the most appropriate actions to correct the situation

While there are cases where the drop in insulation resistance can be sudden, such as when equipment is flooded, it usually drops gradually, giving plenty of warning if tested periodically. These regular checks permit planned reconditioning before service failure, or a shock condition occurs. 

Without a regular testing program, any failures will come as a surprise, i.e., unplanned, inconvenient, and will take a great deal of time, resources, and money to fix.

As the insulation value increases, the test current decreases and becomes harder to measure with the same level of accuracy.

That depends upon the size and complexity of your plant. Even identical units can differ in the required check periods; experience is your best guide. In general, however, working apparatus – such as motors and generators – are more likely to develop insulation weaknesses than wiring, insulators, and the like. 

We recommend establishing a test schedule for working equipment, varying from every 6 to 12 months, depending on the size of the equipment and the severity of the surrounding atmospheric conditions. For wiring and the like, tests once a year are generally sufficient unless the plant conditions are unusually severe.