Avoiding catastrophes for 100+ years with condition-based maintenance programs

23 February 2022


It is no surprise to hear that when you think of Megger, you think of us as a supplier of high-quality portable test equipment. While this assessment is both fair and true; we are more than that. Megger is, and has been, a solution provider for companies (primarily power utilities) with condition-based maintenance programs for over 130 years.

Condition-based maintenance

So, what exactly is condition-based maintenance? By definition, condition-based maintenance (CBM) is a maintenance strategy that monitors the actual condition of an asset to decide what maintenance needs to be done. CBM dictates that maintenance should only be performed when certain indicators show signs of decreasing performance or upcoming failure.

In the electrical power industry, condition-based maintenance is performed on a piece of electrical hardware to find issues early and resolve them before they cause a catastrophic failure resulting in downtime. The goal of CBM is to improve grid reliability and to reduce customer losses and outages. CBM continues to grow within the electrical industry and has been dominating more and more of the discussion.

How condition-based maintenance began

According to a paper written by Ashok Prajapati, James Bechtel and Subramaniam Ganesan, entitled, “Condition based maintenance: A Survey” that appeared in the Journal of Quality in Maintenance Engineering; Branford (Vol. 18, Issue 4), the concept of CBM was first introduced by the Rio Grande Railway Company in late 1940s was called ‘predictive maintenance.’ The railway company used CBM techniques to detect coolant, oil and fuel leaks in the engine of a train by trending changes in temperature and pressure readings. The CBM monitoring techniques helped engineers learn where problems were beginning to arise and reduce the impact of unplanned failures by identifying when to fix a leak or replenish a coolant or oil sump.

Over time, the US Army caught on to CBM and later adopted it as a key maintenance strategy for supporting their military equipment.

CBM grew in popularity in industry through the 1950s, 1960s and 1970s, but really took off as a discipline with the advent of the ‘information age’ in the 1980s and 1990s. Computers allowed companies to use collected data to gain greater and deeper insight into the ‘health’ of a machine or piece of hardware.

Insulation testing

Sydney Evershed invented the insulation tester in 1889, at the dawn of the electrical industry. In 1895, Evershed and his assistant, E.B. Vignoles purchased Goolden and Trotter’s instrument department and renamed it Evershed and Vignoles, the foundation of what would become Megger. They key to the insulation tester was that, unlike other methods used to check insulation, it made the insulation measurements at a voltage comparable with normal working voltage of the unit being tested. This approach identified insulation issues that would occur when the circuit was in its normal working environment. The voltage was generated by a hand-crank on the instrument and the leakage through the insulation was measured.

Over several decades, as technology and research allowed for higher voltages to be generated and higher resistances to be measured, the instruments were refined.


In 1989, 100 years after Mr. Evershed invented the insulation tester, the ‘gold standard’ was the wooden box 5 kV S1 manufactured by Megger in Dover. This instrument generated up to 5 kV via its hand-crank and measured resistance up to 500 GΩ on an analog scale (accuracy depended on user interpretation). Maintenance managers used these units to double-check any questionable results that were found from using a unit with lower specifications. This instrument could be used to perform a spot test, a PI test or a step voltage test, although the latter two took significant user attention.

The 1990s brought a technological revolution that allowed Megger to elevate the capabilities of its insulation testers to levels never seen before. The hand-crank went away in favor of battery power. Voltage levels reached 15 kV, allowing more assets to be tested at working voltages. Measurement sensitivity reached to 35 TΩ, allowing the user to detect the beginnings of insulation failure far earlier in the process. The various diagnostic tests were now programed into the instrument and could be performed automatically. Results could be stored and then downloaded for later analysis.

Insulation testing and CBM come together

So, what exactly does insulation tester history have to do with CBM?

As insulation testing began to evolve, research found that trending results over time (if conditions were equal) could indicate that insulation was degrading and might eventually fail. The insulation test was not just a spot test that gave a pass or fail reading, it was also a view into the changing condition of the insulation.

Keeping track of the results of each test and looking at them over a period of time could provide information about the changing health of the insulation. Experience showed that falling resistance over time, whatever the values, indicated degrading insulation. That same experience showed that stable resistance over time, even at lower values, indicated more healthy insulation.


We began to provide test record cards with our insulation testers and sold additional packs as an accessory. A single test record card would stay with the item being tested. The results would be recorded on one side of the card each time a measurement was taken, and those results would be plotted over time on the other side. The graph of insulation resistance over time would provide an indication of the health of the asset being tested. Decisions about further maintenance would be influenced by the resistance graph.

Tracking and trending test results and making decisions about asset maintenance based on those trends is, at its core, condition-based maintenance. In an analog world, a CBM program uses paper, a pencil and end user experience. As technology evolved to the digital world we now live in, computer programs and electronic data capture and analysis sheets replaced the paper and pencil, but not the methodology. Capturing, storing and trending results in Excel, or with data analysis software, is the same thing that is done on a test record card.

Looking to the future

In the past, CBM programs (although they were not called such at the time) were supported by off-line test and measurement equipment. Making and trending periodic measurements was the only way to make estimates about asset health. The obvious next step was to find ways to measure and track indicators of asset health continuously with on-line test equipment. Digital technology made on-line testing a reality. On-line testing was and is expensive to implement, limiting it usage to specific assets. Off-line tests are still critical to any comprehensive CBM program.

The next step in the world of CBM is to digitize the user knowledge that evaluates trended results. Computer algorithms can take data and use it to make predictions about future asset health and performance.

Megger and CBM

Megger insulation testers, especially the higher voltage models, are a key part of an electrical CBM program. In fact, most of the off-line test equipment that Megger makes is used this way.

We have been in CBM since the early days of the electrical industry and are expanding into providing on-line solutions, in addition to the off-line solutions we have provided for over 130 years.