Is Your Resistance Low or Are You Getting Hot?
Low resistance measurement is a well-established technique that can be used almost anywhere electrical conductivity is important – its applications range from checking the quality of earth bonds to verifying the density of graphite electrodes in aluminum 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 subassemblies 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 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.
It is often unsafe to remove covers or open doors with the supply switched on but, by the time the supply is isolated and the covers removed, the equipment will have cooled significantly, making the thermal imaging data of dubious value.
It can also be difficult to accurately relate the thermal image to the equipment being evaluated, and it is sometimes necessary to take a normal digital photograph and then use a PC to overlay this with the thermal data. Finally, trending thermal images to identify changes over time is not particularly straightforward.
Thermal imaging is, as we have seen, a very useful technique but it complements rather than replaces low-resistance testing. And there are many applications where nothing but a low-resistance test will do. It does, however, pay to take a little care in selecting a low-resistance test set if it is to offer maximum versatility and convenience.
For example, it is all too easy to make an accidental connection to a live supply when attempting to carry out low-resistance tests, particularly when testing busbar bonds and battery straps in UPS installations. It is important, therefore, that the instrument is suitably protected.
In many test sets, this protection is provided by a fuse, but this is not particularly convenient as, if a suitable replacement is not to hand, the instrument is not useable until a replacement can be obtained. Better low-resistance testers, such as those in the Megger DLRO10 family, are intrinsically protected against connection to live supplies. With these instruments, it’s possible to carry on testing normally as soon as the errant supply has been properly isolated.
It is also important to select an instrument that can supply a test current appropriate to the application – ideally, it should offer a choice of test currents covering a wide range. This is because high test currents can, in some cases, cause unwanted heating of the test piece, while in other cases the heating caused by high currents is actually desirable, as it can help to reveal weaknesses such as broken strands in a multi-core cable.
Similarly, the usefulness of low test currents is also dependent on the application. Low currents may be a problem in some circumstances, as they make not break through the contamination in bonds. In other circumstances, however, this may be a benefit, because the same situation can provide a useful indication that contamination is present!
In addition, a low test current combined with test current reversal may eliminate the need for temperature compensation of the results, and it also has the benefit of extending battery life in portable instruments.
Finally, ease of use is a crucial factor. For maximum convenience in day-to-day use, the test set should have an intuitive user interface, and it should perform tests quickly and efficiently, otherwise it will rapidly become a constant source of irritation rather than a useful tool.
In conclusion, it’s clear that both thermal imaging and low-resistance testing are invaluable techniques and the ideal situation is to have access to test equipment for both. Only then can you be absolutely certain of providing a definitive answer to the question that we’ve all, at one time or another, asked – is your resistance low, or are you getting hot?
Megger DLRO10HD Low Resistance Ohmmeter – Megger Model# 1000-348 Catalog# DLRO10HD
Dual power 10 A low resistance ohmmeter
- High or low output power selection for condition diagnosis
- Rechargeable battery or line power supply, continuous operation, even with dead battery
- 10 A for 60 seconds, less time waiting to cool, great for charging inductance
- High input protection to 600 V, inadvertent connection to line or UPS voltage will not blow a fuse
- Heavy duty case: IP 65 lid closed, IP54 operational (battery operation only)
- Rotary switch selects one of five test modes, including auto start on connection, giving ease of use
Equally at home in the laboratory, the workshop or in the field, on the bench or on the ground, the new heavy duty Megger DLRO10HD low resistance ohmmeter combines rugged construction with accuracy and ease of use. It features an internal rechargeable battery and can also operate from a mains supply, even if the battery is completely flat.
- Model: DLRO10HD
- Manufactured by: Megger
- Megger Test Equipment Page
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