With you all the way for low-resistance testing!

Electrical Tester – 1 November 2018

Author: Amy Lyons, Application specialist

Low-resistance testing can save you time, money and trouble, whether you work in electrical power, the rail transport industry, airframe manufacturing or a whole host of other sectors of industry and commerce. With it you can, for example, quickly check the integrity of electrical joints in busbars, contact performance in circuit breakers and switches, continuity of connections between rails, and the quality of welds in airframes and other fabricated structures.

If you’re an engineer or a technician, low-resistance testing is an invaluable technique and you probably know already that the convenient and dependable instruments in Megger’s current DLRO range make it all the more useful and accessible. What you may not know, however, is that these instruments have a pedigree that stretches all the way back to the very first practical low-resistance test sets.

At the end of the nineteenth century, Sidney Evershed, a talented and largely self-educated engineer, had realised that there was a need for a practical and reliable method of insulation testing. To meet this need, he developed the world’s first practical high-voltage insulation tester and, in 1903, he registered the well-known Megger trademark, which we still use and strongly defend, for the instrument.

With the success and popularity of Megger insulation test sets assured, Evershed turned to another challenge his customers were facing – how to measure low resistances easily and accurately. This was important in the power sector then as now, because excessive resistance in the wrong places leads to heating, premature failures and sometimes fire. Evershed’s solution was alow-resistance test set for which he registered the name Ducter. Like Megger, this trademark is still registered and we still defend it, although we no longer use it for new equipment.

The early Ducter test sets – the first of which were produced in 1908 – were a logical progression from the Megger insulation testers of the day as they used the same type of meter movement, with two coils rigidly fixed together in a magnetic field. In the Ducter, one coil carries a current (I) proportional to the current flowing in the object under test, while, the other carries a current proportional to the voltage drop (IR) across the object. The deflection depends, therefore, on the ratio of these two currents (IR/I) and it will be seen that this ratio is unaffected by either the test current or the applied voltage.

By the 1920s Ducter test sets were being widely used in the field and the original analogue design continued to evolve until 1976 when the James G Biddle Company – another of the companies that ultimately became Megger – developed a digital low-resistance ohmmeter. This was named the DLRO, and this designation is once again a trademark of Megger.

Over the intervening four decades, we have drawn on our more than a century of experience in the design and development of low-resistance ohmmeters to continually update our product range and bring you all the benefits made possible by the latest developments in technology. These benefits include, for example, data storage and downloading capabilities, new test modes, reduced weight and extended battery life.

The present-day DLRO range includes readily portable instruments that deliver test currents up to 10 A, larger instruments that can supply test currents up to 600 A, and many other models in between. All support four-wire testing to eliminate the effect of test lead resistance, and they offer invaluable time-saving features that include bipolar testing, which automatically reverses the test current to ensure that your results are unaffected by thermal and contact voltages.

One of the most recent additions to the range is the DLRO10HD, which has been specifically developed to meet the growing need for a versatile, easy-to-use instrument that combines the highest standards of safety with suitability for use in laboratories, factories and on-site. It offers 0.1 microohm resolution, 0.2% basic accuracy, and a dedicated mode for use with inductive loads. It also gives you a choice of high and low power ranges, an innovation that greatly extends its usefulness.

The high power (up to 25 W) ranges are suitable for measuring up to 250 milliohm at 10 A or 2.5 ohm at 1 A and you’ll find them ideal for heating a weakness or charging inductive loads. The low power ranges, which are limited to 250 mW, are suitable for applications where you need to avoid heating the item under test.

The DLRO10HD has five test modes. In normal mode, you press the “test” button to start the test. The instrument verifies continuity of all four test connections, and then applies the test current in both forward and reverse directions before displaying the result.

In automatic mode, the test starts as soon as your probes make contact with the item under test. Automatic unidirectional mode is the same as automatic mode, but the test current is applied in one direction only. This reduces the testing time, but thermal EMFs from contact between dissimilar metals may reduce the accuracy of your results. Continuous mode makes repeated measurements at three-second intervals. Inductive mode is, as the name suggests, optimised for tests on motors, transformers and other inductive loads, supplying a full 10 A for up to 60 seconds.

To help keep you safe, the DLRO10HD is fully protected against accidental connection to live supplies and, when used with appropriate insulated test leads, it meets the requirements of IEC 61010 for CATIII 600V use. As is required by IEC 61010, this category rating has been confirmed by testing.

At Megger we have a longer history in the development, design and manufacture of low-resistance ohmmeters and micro-ohmmeters than any other company in the world. And, as you can see, we’ve never been content to rest on our laurels – our record is one of continuous innovation to provide you with an ever-increasing range of benefits and facilities. Choose Megger test equipment for your vital low-resistance tests and you can be sure you’re getting the best that our industry has to offer.