The birth of insulation resistance testing

Nick Hilditch, editor, Electrical Tester

Insulation testing, in all its many guises, is something that is almost taken for granted today, at least in everyday applications. But it wasn’t always so. Back in the early decades of the twentieth century, insulation testing was new technology and a lot of work was being done to try to understand how insulating materials behaved and how test results should be interpreted.

It will be no surprise that one of the leading workers in this field was Sidney Evershed, the inventor of the Megger, which was the world’s first practical insulation tester. After years of painstaking research, Evershed presented some of his findings in an important paper that he read before the Institution of Electrical Engineers (the IEE, now the IET) in London on 27th November 1913.

His paper was no brief summary – with the appendices and discussion, it runs to 32 closely printed pages and contains 30 figures! Despite its age, the paper still makes interesting reading. On the very first page, for example, Evershed makes clear his disdain for flash testing with a footnote that reads “The flash test as applied to some costly piece of electrical apparatus must have been inspired originally by something akin to the heroism of the savage.”

The paper also makes clear the deficiencies of the insulating materials that were in common use in the early years of the last century. In the introductory part of the paper, Evershed provides some test results for “lighting circuits” – no further details are specified, but this could easily have been a domestic lighting installation.

These results (see Figure 1) show that on a dry day, the insulation resistance of the installation when tested at 500 V was almost exactly 1 MΩ. This in itself is worrying enough, but on a wet day the tester couldn’t deliver enough current to raise the output voltage above 200 V and the insulation resistance had fallen to a truly alarming value of just under 400 kΩ! Maybe in that era there was something to be said for gas lighting, after all.

In spite of the shockingly low resistance values obtained in this and other tests, the main thrust of Evershed’s argument throughout the paper is that the insulating materials themselves are almost perfect.

For example, when discussing a sample of micanite cloth – a material that was then widely used as an insulator for windings – which had been very thoroughly dried, he quotes an initial insulation resistance after drying of 1,300,000 MΩ (1.3 TΩ). After 24 hours of exposure to the air this had already fallen to 200,000 MΩ and after 90 days had dropped to 1,000 MΩ. That’s still not bad, but it does represent a decrease in resistance by a factor of 1,300!

As an aside, it’s interesting to note that more than a century ago, Evershed was already able to make measurements in the teraohm range, with test voltages of just 500 V. And he mentions the use of a guard wire. Teraohm measurements and guard terminals are clearly not just the province of modern instruments.

But if the insulators were almost perfect, why was the insulation resistance measured in practical situations often so low? Evershed’s conclusion was that moisture was the culprit. When carrying out an insulation test, he concluded, it was the resistance of the moisture on and in the test object that was being measured – the intrinsic resistance of the insulator itself made no significant contribution to the results obtained.

To support this conclusion, he carried out tests on a wide range of materials from drawing paper to engine oil and demonstrated that, in every case, if the materials were well dried, they exhibited very high insulation resistance and obeyed Ohm’s law, showing little variation in resistance with applied voltage. If even the slightest trace of moisture was introduced, however, the insulation resistance dropped dramatically and became non-ohmic, falling as the test voltage increased.

There’s much more of interest in this seminal paper and we may well revisit it in future issues of Electrical Tester. In the meantime, let’s finish with another perhaps unintentionally amusing comment from Evershed. Speaking of his attempts to devise a formula describing how the measured resistance of a damp insulator varies with the applied voltage, he notes that “nature generally declines the Procrustean bed of formula”. So much more interesting, I think you’ll agree, than simply admitting he couldn’t quite get it right!