Making the right choice for low resistance

20 March 2018

Written by Tony Wills - Technical Support Group for Megger

Low resistance measurements have many and diverse applications, ranging from checking the condition of circuit breaker contacts to verifying the integrity of welded joints. Low resistance test sets are, however, available in an almost bewildering range of types; this article offers some useful advice on choosing the right test set for the application in hand.

Low resistance measurements are generally considered to be those that involve resistance values of less than one ohm. There are many reasons why such low values cannot be reliably measured with an ordinary ohmmeter – the resolution will almost certainly be too low to provide accurate results, the resistance of the test piece may well be swamped by the resistance of the test leads, the results will be distorted by the tiny emfs generated at the junctions of dissimilar metals in the test object, and so on.

To overcome these problems, special-purpose low resistance ohmmeters have been developed. There are many types and sizes, but most share certain features. These include the use of four-terminal connections, where the current to the test object is delivered by one set of cables, and the potential difference across the test object is measured with another set, an arrangement that eliminates the effect of test lead resistance.

Most types of low-resistance also have facilities for bipolar testing, which means, in effect, that two tests are performed with the polarity of the test current reversed between them. The readings are averaged to produce a result that is unaffected by thermal and contact voltages that may be present in the test object. And another feature that’s common to low resistance test sets is that their test currents are much higher than those of ordinary ohmmeters – currents from 10 A up to as much as 600 A are in common use. These high currents are needed because many types of test object behave differently with high and low test currents.

An example is the main contacts in a large circuit breaker. With a low test current, these sometimes show a comparatively high resistance, usually because of minor contamination, which can lead to the circuit breaker being judged as not fit for service. If, however, the contact resistance is measured at a higher current, comparable with the normal operating current of the breaker, the contamination will often break down and the contact resistance will be shown to be perfectly acceptable.

Having examined the features they share, let’s now look at how various types of modern low resistance test set differ. Traditionally, because of the high currents they need to deliver, these instruments have been large and comparatively heavy but, within the last couple of years, a new and very different type has become available in the form of the Megger MOM2 micro-ohmmeter.

The MOM2 is small, light handheld instrument that, despite its diminutive design, can deliver test currents as high as 220 A. The secret of this innovative device is that it incorporates an ultra-capacitor that has a capacity of several farads. Between tests, this is charged by a battery-operated inverter circuit and, during the tests it is discharged into the object under test. The test current and voltage are continuously and synchronously sampled throughout the test period, and the results automatically averaged to obtain the final value. The results are accurate and reliable, and this test method complies fully with IEEE and IEC standards for tests on a wide range of equipment.

The benefits of such a small and convenient instrument are clear – it can be taken and used where using a conventional instrument would be problematic, such as at the top of a ladder and tests can be carried out almost anywhere, as there’s no need for access to mains power.

This particular instrument also has another big benefit, in that it supports Dual Ground testing. Typically used when carrying out tests on circuit breakers in substations, Dual Ground testing allows the tests to be performed with both sides of the breaker grounded. This means that the potentially lethal hazards associated with voltages induced into ungrounded equipment are completely eliminated.

These compact new instruments are clearly invaluable in many applications, such as the testing of circuit breaker contacts, busbar joints and high-current connections, but are they the complete answer for every application? The answer has to be no. One thing that ultra-capacitor based instruments can’t do is to maintain the test current for more than a few seconds.

Often this doesn’t matter, but there are some applications where prolonged testing is needed, either because of the nature of the test or because that’s what the applicable test standards specify. An example is checking joints between the rails in railway systems, where low resistance is essential because of the very high traction currents drawn by electric trains. In this application, extended tests are always carried out with high currents.

In other applications, it may be useful for the application of the test current to be prolonged so that it heats the object under test. This is sometimes the case with busbar systems, where testing is best performed with the busbars heated to their normal operating temperature. It may also be useful to monitor how the resistance of the test object changes over a period of hours, in which case an instrument with continuous current capability is clearly essential.

For these types of applications, conventional test sets are needed, but even then the latest instruments are lighter and more compact than their predecessors. Versatile 200 A test sets are now available that weigh no more than 15 kg, making them relatively easy to handle although, admittedly, not at the top of a ladder.

The best of these instruments allow the test current to be adjusted to match the application, as the highest current is not always the most appropriate choice. Sometimes it may be desirable not to heat the test object unnecessarily, in which case a relatively modest test current will need to be used. Also, it may be useful to explore the behaviour of the test object with low currents as a preliminary to proceeding to higher current tests.

Another key feature in the best instruments is the availability of multiple testing modes. Continuous mode, as the name suggests, is ideal for users who need to monitor the resistance of the test object over a period of time, while with normal mode, after the test has been initiated, the current ramps up to the preset value, remains steady for a short time – typically two seconds – and then ramps down again to zero. This is well suited to everyday testing.

Some testers additionally offer a useful and time saving automatic mode. With this, after setting the desired test current, the user connects only the current leads and then presses the instrument’s “test” button. The test doesn’t actually start, however, until the potential leads are connected and it stops as soon as they are disconnected.

This mode is invaluable when testing multiple joints in a busbar as the current leads can be connected to the ends of the busbar and left in place, while the user applies the potential leads across each joint in turn. The instrument automatically carries out a test on each joint, and records the results in its internal memory.

As we have seen, there is no single low-resistance ohmmeter that is the ideal solution for every application, although many are very versatile. Hopefully, the guidance given in this article will help potential users to pick the instrument that’s best suited to their needs. Nevertheless, expert advice is always useful and is readily available from Megger, one of the few companies that can be truly impartial as its product range includes low-resistance test sets of almost every type.