Measuring low resistance – which is usually taken to mean resistance of less than 1Ω – has many applications, from checking the condition of circuit breaker contacts to verifying the integrity of welds in metal structures. One very common question however is, “What’s the best test current to use?” As is usual with questions like this, there is no simple “one size fits all” answer, but here are some key points that will help you decide on the right test current for your application.
Noise currents – Low resistance ohmmeters measure the voltage drop produced by a test current. In electrical noisy environments, the voltage drop produced by induced noise may be similar in magnitude to the voltage drop produced by the test current. In these cases, increasing the test current will help as the voltage drop also increases in proportion, improving the signal-to-noise ratio and providing measurements that are more stable.
Heating of weaknesses – Higher test currents will heat weaknesses in the item under test, enabling them to be more easily detected. For example, if two metal components are in contact with each other but have surfaces that are not smooth, they will only touch at the high spots. Low current testing is unlikely to find this problem. Another example is a cable with broken strands. This will probably test OK at low currents, but with higher currents heating will cause a noticeable drift in the measured resistance.
Contamination of bonded surfaces – Lower test currents are better for detecting contamination of bonded surfaces, as high currents can temporarily “punch through” the contamination giving a misleadingly low reading. For this reason, it is often useful to compare the readings obtained with high and low test currents.
Simulation of working conditions – Sometimes high test currents are desirable as they more accurately reflect the conditions under which the equipment being tested will normally operate. In effect, the test will confirm the ability of the object under test to reliably carry the current it will experience when in service.
Higher accuracy – Higher test currents produce larger voltage drops that are easier to measure. In principle, therefore, results obtained at high test currents should be more accurate. This is, however, not a major consideration with modern test equipment.
Another factor that can affect the choice of test current – and, therefore, the choice of test instrument – is the general requirements of the market in which the instrument is going to be used.
In the USA, 200 A test sets like the Megger DLRO200 are the most popular choice as most low-resistance test sets are purchased for checking the resistance of the main contacts in large circuit breakers of the type typically used in substations. For this job, US standards specify a test current of at least 50 A, so a 200 A test set is more than adequate.
In Europe, low-resistance ohmmeters are commonly used for a wider range of applications, which boosts the popularity of 600 A test sets such as the Megger DLRO600. For example, London Underground – the company that operates London’s rapid-transit “tube” railway system – checks the resistance of large loops of railroad track with a test current of 70 A, but it uses a DLRO600 in case higher test currents are needed for fault finding.
It would be understandable to think that one of the key factors to consider when choosing the test current for a particular application would be the relevant standards. Unfortunately, in very many cases, there are no relevant standards! This leaves the choice of test current as a decision for individual engineers and technicians. Often they will simply do whatever has been done in the past and there is actually some justification for this, as it means that new measurements can be directly compared with historical results.
As was said at the outset, there’s no simple answer to the question of what test current to use for low resistance testing. Hopefully however, this short item has given a useful indication of factors that should be considered when making this important decision.
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