ET online
November 2007
How much is enough?

How much is enough?

14 November 2007

Jeff Jowett - Senior application engineer

There are so many ways of testing insulation that it’s often hard to know which tests to choose in order to get dependable results. While there’s no universal answer, work done by leading electric  locomotive manufacturer, GM Electro Motive Division (GM EMD), gives some useful insights.

There are so many ways of testing insulation that it’s often hard to know which tests to choose in order to get dependable results. While there’s no universal answer, work done by leading electric locomotive manufacturer, GM Electro Motive Division (GM EMD), gives some useful insights. A failed locomotive always means inconvenience and expense, so locomotives returned to GM EMD for maintenance are routinely subjected to insulation testing. In the past, the tests involved either using a 1 kV insulation tester, or highpotting, which essentially means applying a high voltage – in this case 1,080 V AC – across the insulation and waiting to see what happens. 

These tests cause a lot of problems, but there were still some costly and annoying failures when locomotives were returned to service. Let’s see why.
High-potting is a crude test, but it’s popular because it apparently gives unambiguous results, usually in the form of a failure light and audible alert. In fact, a high-pot tester makes its pass/fail decision on the basis of a trip setting for breakdown, leakage current, or both. Highpotting undoubtedly gives an indication of the insulation condition at the time of testing, but crucially it doesn’t give any information about future performance.

If, for example, the insulation has a pinhole that arcs to the machine frame, a high-pot test will register a failure. If, however, moisture has entered the machine and lowered the insulation resistance to a level just above the trip setting, the high-pot tester will indicate a pass, but the machine is likely to fail as soon as it is next exposed to moisture.

In addition, high-pot testers have high output currents that can sustain an arc. The testing itself can, therefore, make existing problems worse. A failure that could originally have been remedied by drying out the machine may, for example, turn into a rewind job after high-pot testing.

Insulation testers, or “Meggers” as they are incorrectly known – although this is a registered trademark – overcome these problems. Their output current is limited so that they can’t cause further damage to weak insulation, and they also provide an accurate reading of insulation resistance rather than a simple pass/fail indication.

This gives invaluable advance warning of problems. If the insulation resistance is just above the minimum acceptable value, it’s a clear indication that action is needed if failure is to be avoided. Often, simple cleaning and drying is enough to restore equipment to a condition where it will function reliably.

From what’s been said so far, it’s easy to see why high-pot testing wasn’t helping GM EMD to eliminate locomotive failures, but why wasn’t the insulation tester providing the information needed to predict these failures? The answer lies in the test voltage.

Insulation tests are usually performed at 1 kV and, indeed, this is adequate for many applications. There are, however, problems that a 1 kV tester may not reveal. Depending on their position in relation to the machine frame, pinholes and cuts in insulation, and even carbon tracks may ‘pass’ a 1 kV insulation test. Nevertheless, such weak spots are likely to deteriorate rapidly in service, resulting in breakdown and failure.

To address this issue, GM EMD carried out an extensive study of the results from insulation
tests carried out at between 1 kV and 5 kV. The company found that each increase in test voltage revealed additional problems. Fortunately, by the time 5 kV had been reached, close to 100% success had been achieved in detecting problems. As a result, GM EMD now recommends that its products are insulation tested at 5 kV.

Choosing the right test voltage is an important step in insulation testing. For most applications, voltages up to 1 kV are perfectly satisfactory. However, with bigger equipment and in more demanding applications, higher voltages are needed.

Fortunately, as the work carried out by GM EMD confirms, enormously high voltages are unnecessary; 5 kV has been proved in practice to be an excellent compromise between what’s theoretically possible and what it’s reasonable to strive for in practice.

For more information on Megger’s insulation testers click here