Q and A: Earth testing

Electrical Tester – 06 July 2018

Author: Keith Wilson

Not long ago, the need to measure earth resistivity or the resistance of an earth electrode was, for most engineers, a rare occurrence. With the advent of small generating schemes and, in particular, solar and wind energy schemes, this situation has changed. Most of these schemes have their own earthing systems and, to ensure safe operation, these need to be checked. This has led to a large increase in the number of questions our helpline receives about earth testing; here is a selection of the most common.

Q: Many high voltage insulation test sets offer polarisation index (PI) and dielectric absorption ratio (DAR) tests as well as straightforward insulation resistance tests. Are there any benefits in using PI and DAR tests on motors?

A: In a word, yes! The results of straightforward insulation resistance tests depend to some extent on temperature, so if you want to compare measurements, you have to make sure that the motor is always at the same temperature when you make the tests. PI and DAR test results, in contrast, are largely unaffected by temperature, which means that these tests can be carried out without waiting for the motor to cool down – or heat up – to a specific temperature. Motor size also has little effect on PI and DAR test results, so measurements can easily be compared with published results to provide a rapid and dependable indication of the condition of the motor insulation.

Q: Some instruments also have a ramp test option. What does this do, and what it is it useful for?

A: As the name suggests, a ramp test applies a continuously increasing insulation test voltage, up to a preselected maximum, to the motor. This allows the response of the insulation to be assessed in detail. Small defects are readily detected and the test can be terminated at the first sign of breakdown, before serious damage occurs. Ramp tests are particularly useful for detecting defects like cracks, voids, delamination, moisture ingress and surface contamination.

Q: When I’m choosing an insulation resistance test set for use on motors, what should its maximum test voltage be?

A: That very much depends on your requirements and the types of motors you’ll be testing. For most applications, 5 kV test sets will provide good results, but users are increasingly finding that tests at higher voltages, usually up to 10 kV, are more revealing, especially when the results are trended over a period of time to aid the detection of incipient faults. Recently, 15 kV test sets have become available, and these are useful where compliance is required with certain standards, such as NETA MTS-1997 Table 10.1, applicable to the maximum voltage rating of equipment, and NETA ATS 2007 Section 7.15.1 for medium voltage motors. It’s also worth remembering that the maximum test voltage is usually related to the maximum insulation resistance the instrument can measure. For Megger insulation test sets, 5 kV models measure up to 10 TΩ, 10 kV models up to 20 TΩ and 15 kV models up to 30 TΩ. The extended measuring ranges provided by instruments with higher test voltage capabilities are particularly useful when trending results  over time, as it’s not possible to trend an  “infinity” reading!