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August 2009
Is your resistance low or are you getting hot?

Is your resistance low or are you getting hot?

06 August 2009

Clive Pink - Product manager

Low resistance measurement is a well-established technique that can be used almost anywhere where electrical conductivity is important – its applications range from checking the quality of earth bonds to verifying the density of graphite electrodes in aluminium refineries. Recently, however, thermal imaging has been proposed as a simple and effective solution in many of the same applications. But is it?

The real answer is that both low resistance testing and thermal imaging have their place. So, in order to decide which to use where, let’s take a look at the strengths and weaknesses of each.

A big benefit of low resistance testing is that it can detect problems even when there is no current (other than the test current) flowing in the object under test. This makes it very suitable for applications such as checking weld quality, verifying the performance of lightning protection bonds, confirming the integrity of aircraft structures and testing earth systems.

Low-resistance testing is also invaluable in manufacturing applications, particularly where it is necessary to test sub-assemblies rather than complete systems, and for checking new or modified electrical installations prior to energisation. Thermal imaging is unlikely to be suitable for any of these applications.

A further benefit of low-resistance testing is that it provides straightforward numerical results, which can easily be recorded and, even more useful, trended as part of a predictive maintenance programme.

Having said that, low-resistance testing does, of course, have its limitations. It can’t, for example, be used on live equipment. For equipment that’s in service, therefore, it’s necessary to arrange for the supply to be isolated before carrying out the test, which is not always convenient. In addition, if there are many connections to test, low-resistance testing can be time consuming.

Turning now to thermal imaging, it’s a good way of checking for overloads and unbalanced loads, which can’t be done with a low resistance tester. Thermal imagers also have non-electrical applications, such as finding the locations of heat loss from buildings, and detecting mechanical faults such as worn bearings in a motor, which heat up because of excessive friction.

 Thermal imaging also has the reputation of being easy to use, but that’s not always the case – the operator needs to understand what they are seeing and to be able to interpret the results. For example, is a transformer overheating, or is it at its normal operating temperature? What is the load on the equipment while the test is being carried out? At what point does the temperature rise become a problem?

In high-voltage environments, such as an electrical substation, a further complication is that it is often not safe to get close enough to the equipment to image it clearly. In addition, items such as fuses and circuit breakers are usually mounted in metal enclosures, and thermal imaging will not work through metal.