Amy Lyons - Application specialist
Yet another test? That’s probably the last thing any plant operator wants. But wait a minute – surely what they really want is efficient, reliable functioning of their plant with no breakdowns and no accidents. If a little additional testing can deliver all this, isn’t it certainly worth carrying out? That’s what low-resistance testing is all about, prevention rather than cure.
This looks good on paper, of course, but seeing low-resistance testing as a preventative measure can make it hard to justify. After all, if it ain’t broke, don’t fix it. Why test equipment that seems to be in perfect working order? As we’ll see, the answer to this question is that there are many reasons why the peace of mind – think of it as insurance – that low-resistance testing provides is very good to have.
In spite of its benefits, low-resistance testing is all too often neglected. No major item of electrical equipment would ever be put into service without an insulation resistance test to make sure there are no immediately apparent faults likely to cause immediate or even short-term failure. But what about hidden faults that will allow the equipment to operate normally for a while, but will nevertheless lead to inefficient operation or premature failure, probably at the most inconvenient possible time?
It would be unrealistic to suggest that low-resistance testing can uncover all of these insidious and far from obvious faults, but it can certainly reveal many of them. Examples are poorly made busbar joints, terminals that have been insufficiently tightened and defective plug-in connectors. Faults like these can lead to overheating, which not only wastes energy but invariably accelerates equipment failure. Excessive resistance can also limit fault currents, making it impossible for protective equipment to operate correctly, which creates a major safety hazard.
For these reasons, Megger recommends that low-resistance tests be performed as part of the standard commissioning procedure on all major items of electrical plant, and that the results are carefully recorded. The tests can then be repeated throughout the life of the plant, and the new results compared with the old, as is routinely done with insulation resistance test results. In this way, any slow – but potentially significant – changes will be revealed, allowing them to be investigated before they progress to become a full-blown failure.
Fault finding methods
If low-resistance testing has so many benefits, why is it not used more widely? Part of the answer is the common misconception that a continuity test will do the same job. That’s just not true! Continuity tests certainly have their place but in reality they’re limited to doing what their name suggests – confirming continuity. To uncover problems, low-resistance testing often needs to be performed with high test currents – 2 A to 200 A or more – and the equipment used to carry out the test needs to be able to measure accurately down to microohms. These requirements simply cannot be met with an ordinary multimeter of the type typically used for continuity testing.
A dedicated low-resistance test set is needed and, as well as offering high test currents and good measurement resolution for small values of resistance, this will provide another essential feature: support for four-wire measurement.
With ordinary two-wire resistance measurements of the type provided by a multimeter, the resistance of the test leads, any probes or clips used as well as variation in contact resistance will unavoidably affect the reading. When looking for results accurate to a few microohms or even a few milliohms, such changes are very significant.
The solution to the problem of test lead resistance affecting measurements is four-wire testing. Essentially, one pair of leads – the current leads – is used to supply the test current to the item under test, while a second pair of leads – the potential leads – is used to measure the resulting volt drop across the test object. The arrangement is shown in Figure 1. Crucially, with four-wire testing the voltage is measured actually at the test object, so the resistance of the test leads has no effect on the results obtained.
Why make low-resistance measurements?
The justifications for making low-resistance measurements have already been discussed in general terms, but a more detailed look will be beneficial. In general, the justifications can be considered under three headings: safety, reliability, and efficiency.
Safety – In the event of a fault, the path with the lowest resistance will always carry the largest proportion of the fault current. It is important that the lowest resistance path is the one intended by the equipment designer, and that the current doesn’t flow some other path, possibly leading to a fire or electric shock. The resistance of the intended fault current path must also be low enough to ensure that sufficient current flows for protective devices such as circuit breakers to operate properly. Low resistance is also essential for lightning protection and static dissipation systems. Testing is the only reliable way of verifying that current paths are of the required low resistance.
Reliability – Joints and connections with abnormally high resistance – and high resistance in these cases can mean just a few milliohms – lead to excessive heat generation and this, in turn, accelerates insulation degradation that ultimately leads to premature failure. Low-resistance testing can reveal poorly made joints and connections before they lead to failures and is, therefore, a valuable tool for predictive maintenance and for maximising operational reliability.
Efficiency – A few microohms of unexpected resistance in a circuit doesn’t sound much, but in high current circuits the results can be frightening. Imagine, for example, a busbar joint with a resistance of 1 µΩ carrying 6000 A will dissipate 36 Watts. If the resistance was to rise to just 1 mΩ this would equate to 36 kW resulting in excessive heating and wasted energy. If the system has several joints like this, the power losses start to add up and significantly impact overall efficiency. Many times, these increases would go unnoticed until it’s too late.
When all of these factors are taken into consideration, it becomes clear that a very good case can be made for low-resistance testing not only of new equipment, but also routinely for in-service equipment.
Figure 2 shows a cable connected to a busbar – something that’s very commonly seen, particularly in industrial installations. How is it possible to find out if there’s a problem with this connection? If it’s carrying current and there’s a problem, the result will usually be heating which might be detected using thermal imaging, although this will often fail to reveal the precise location of the fault. If, however, the joint is part of an earthing system and doesn’t normally carry current, thermal imaging will be no help at all.
On the other hand, low resistance testing will provide a complete picture of the performance of the connection. In this example, it shows that the resistance from the busbar to the lug is acceptably low, at just 28 µΩ, but the resistance between the busbar and the cable jumps to 1.8 mΩ. Clearly, there’s something wrong with the crimping of the lug. A fault like this would certainly not be detected by a continuity test using a multimeter, but it would certainly be enough to promote further deterioration, possibly leading to a total failure or even a fire.
Appropriate corrective action would be to re-crimp the lug and then test again with a low-resistance ohmmeter. This would confirm that the problem had been rectified before the joint was put back into service – something that’s impossible to do with thermal imaging. Performing tests of this kind and remedying any problems revealed during planned shutdowns can save a lot of money, not only by preventing costly breakdowns but also by reducing energy losses.
Low-resistance testing also provides a dependable way of checking that bolted connections have been assembled with the correct torque. It is a fallacy that, when making connections of this type, tighter is always better. In reality, if terminal lugs or lapped joints in busbars are overtightened, the material around the joint can become distorted so that the surface area of the joint is reduced and the resistance increased, as shown in Figure 3. Low-resistance tests will quickly reveal problems like this and can also be used to determine the optimum tightening torque for connections of a particular type.
We have highlighted a number of areas where thermal imaging is less useful than low-resistance testing. Nevertheless, thermal imaging should not be seen as competing with low-resistance testing. The technologies are complementary and there are some applications where thermal imaging offers benefits. These include, for example, carrying out a rapid survey on equipment while it remains energised.
Insulation resistance testing is performed routinely on new and most in-service equipment, and rightly so. Low-resistance testing is, however, frequently seen as an unnecessary and even troublesome extra. This is unfortunate because, as we’ve seen, low-resistance testing can be just as useful as insulation testing in providing advance warning of problems that could, if left uncorrected, develop into costly and disruptive failures. In reality it is very desirable to make low-resistance testing part of the routine test regime for all power assets. After all, if doing so prevents just one major breakdown, the cost of testing will be recouped many times over.
Much more information about low-resistance testing and its benefits is available in the Megger Guide to Low-Resistance Testing, which is available as a free download from this link.