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April 2009
Resistance revelations

Resistance revelations

01 April 2009

Paul Swinerd  - Product manager
Accurate measurement of low resistance is an invaluable diagnostic aid in a wide variety of applications. It can, for example, reveal defective welds, poor joins in metallic structures such as aircraft skins, and substandard electrical connections that can generate heat and lead to premature equipment failure.

The importance of ensuring good electrical connection should never be underestimated. The consequence of undetected poor electrical connections can be more than just wasted energy due to heating. There can be far more serious repercussions such as ineffective circuit and personnel protection, subsequent faults developing into major equipment (and very expensive) equipment damage.

For a good example look no further than the recent problems experienced by CERN, the European Organisation for Nuclear Research. For 20 years the best physicists and engineers from 80 countries have been developing and building the largest and most complex scientific instrument ever built.

The Large Hadron Collider (LHC), a particle accelerator to you and me, consists of a 27km long ring of superconducting magnets. There are 9300 magnets in total, each cooled to -271.3ºC by liquid helium in operation. Two beams of particles are accelerated to 99.9% the speed of light, travelling in opposite direction. The beams then collide resulting in 600 million particle collisions in a second. The results of experiments are then recorded by the most powerful supercomputer system in the world providing information on the basic building blocks of the universe itself.

In September 2008 all this was brought to an abrupt stop, magnets were damaged and liquid helium leaked out. The result was a very expensive repair that will see the LHC out of action until June 2009. The initial cause? Quite simply a faulty electrical connection between two of the accelerator’s magnets.

Measuring low resistances – typically less than 1 ohm and sometimes down to a few µ½ – with any degree of precision is, however, not quite as easy as it may at first seem. Conventional ohmmeters, which use two terminal connections for the test piece, are not well suited for this type of application. They are unlikely to have low enough measuring ranges and the resistance of the test leads – which may be many times more than that of the test piece itself – has a big influence on the results.

Dedicated low-resistance ohmmeters, such as those in Megger’s DLRO10 family, use four terminal connections. With separate connections for current and voltage, the influence of test lead resistance is eliminated. This is not, however, the only factor that complicates the accurate measurement of low resistances.
Thermal EMFs produced at junctions between dissimilar metals can adversely influence results, as can electrical noise picked up from external sources and even the heating of the item under test caused by the passage of the test current. Inductive test pieces, such as transformer and motor windings, have their own special problems.

The best of modern instruments take all of these factors into account. The effect of thermal EMFs is minimised by arranging for tests to be carried out automatically with the DC test current flowing first in one direction and then in the other. The test instrument averages the measurements obtained to provide an accurate result.
Electrical noise can, to a certain extent, be compensated by the instrument but, above a certain level, this becomes impossible. The instrument can, however, provide an indication that this level has been reached and prevent further tests being performed under these conditions, as the results produced would not be valid.
Heating in the test piece can be greatly reduced by using low test currents, but it is sometimes desirable – when long test leads must be used, for example – for the instrument to have a high current capability.

The best solution, therefore, is for the instrument to offer the option of testing at high and low currents. For inductive loads, the major problem is that it is necessary to wait for the test voltage to stabilise as the inductive element is charged.

A good instrument will, therefore, make provision for a continuous test current to be applied in one direction only, and will provide a frequently updated display of resistance. As the voltage stabilises, the resistance shown will gradually decrease, allowing the user to decide when an accurate endpoint has been reached.

As well as being physically rugged a good instrument will well protected against inadvertent connection to voltages. Testing battery strap connections on a UPS for example can easily result in accidental connection to voltages up to 600 V. Ideally first line protection on an instrument should not involve blowing a fuse either, not unless the user carries plenty of spares.

While appropriate electrical characteristics are of course essential for a low resistance ohmmeter, they are not on their own enough to make an instrument convenient to use. Rugged construction is another highly desirable feature, for example, as it is frequently necessary to use instruments of this type in the field.

Simplicity of operation is also important, as is a choice of testing methods. Bidirectional testing, for instance eliminates the effects of thermal EMFs, as has already been mentioned. Unidirectional testing, however provides faster results in applications where thermal EMFs are not present.

Modern test instruments are available that meet all of the criteria discussed in this short item. Their ruggedness, simplicity, versatility and accuracy make them the perfect solution for measuring low resistances in the laboratory, on the production line and in the field.

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