Testing parallel resistances
Author: Jeff Jowett, Applications Engineer
True low resistance ohmmeters are mainly characterised by their use of high test currents and their ability to accurately measure very low values of resistance. In contrast, common handheld testers like multimeters, DMMs and the like, operate from on-board batteries – often AA size – and so can’t be expected to deliver much current. Accordingly, they typically use test currents in the milliamp range. This limits their accuracy and resolution when used for low resistance measurements. A very good DMM may measure to a resolution of 0.01 Ω, which is fine for many everyday applications. There is, however, a whole world of testing below this level and for this, a genuine low resistance ohmmeter is needed.
In this context, ‘low resistance’ is generally regarded as meaning below 1 Ω and, in this range, you need amps (not milliamps) to make good, reliable measurements. The industry standard is 10 A, but testers are available that use currents anywhere from one or two amps up to hundreds of amps. The very high current testers are most often used in the power industry for measuring the contact resistance of circuit breakers and relays, but they also have other specialised applications.
Low resistance ohmmeters, unlike DMMs and similar general-purpose instruments, use a design based on a four-terminal Kelvin bridge. Two current connections inject a large test current into the item under test (IUT) while two potential connections, which are placed between the current connections, measure the voltage drop between the potential connections. The tester then uses the measured current and voltage to calculate and display the resistance.
An important advantage of this test configuration is that it eliminates the effects of lead and contact resistance. These two extraneous parameters influence the current that flows in the IUT, but the resistance is calculated by measuring the voltage between the potential leads, which is unaffected by lead and contact resistance.
Clearly, this advantage is not shared by generic instruments like DMMs that use a two-wire connection. Measurements made with these are of course influenced by the resistance of the test leads and the resistance between the tips of the test probes and the IUT, but in the applications where they are normally used, this is not a great concern. However, in applications where maximum accuracy is necessary, the influence of lead and contact resistance cannot be tolerated. In these cases, it is essential to use the four-wire Kelvin bridge.
A low resistance ohmmeter with a four-wire Kelvin bridge design is all you need to make an isolated measurement between two discrete points. Examples include laboratory work and measurements made on electrically isolated joints and connections in electrical equipment. But in many cases, there are alternative parallel paths for the test current. This can even apply to a simple metallic test object on a laboratory bench, as we’ll see later. So, the first order of the day in performing a low resistance test is to ‘know your test object’.
It is often said – incorrectly – that current follows the path of least resistance. This statement can be somewhat misleading if it’s applied to low resistance testing, so let’s look at it more closely. In fact, current follows all possible paths and divides in strict proportion to their resistance. This has a critical bearing on the measurements you make with your tester because the tester measures all the current and then uses Ohm’s law (R = V/I) to calculate resistance.
Figure 1: With no parallel paths, the total resistance of the test object is evident.
If you connected a digital low resistance ohmmeter (DLRO) across the ends of an isolated section of wire, the current would flow between the two test points and nowhere else (see Figure 1). You would have an accurate reading of the resistance of that section of wire (which, by the way, should agree with the tabular resistance for that gauge). But if the wire was part of an installation (de-energised, of course) you would have to take into consideration the whole circuit and where else the current goes. If the test current flows through one or more other paths as well as through the wire itself, these parallel paths will influence the measurement you are making.
Parallel paths might exist when you’re carrying out a laboratory test on a metallic object, for example, or, if you’re testing the IUT in situ, they might result from its associated circuitry. So, don’t just randomly apply the leads across a particular joint or seam that may be of interest without considering the whole of the IUT.
Two points may have been welded together and then connected into a larger structure or apparatus. If any of those points of connection are metal-to-metal contacts, there may be an alternative current path that will influence the measurement (see Figure 2). This doesn’t mean you can’t test, nor does it necessarily mean that you have to break all other connections. What it does mean is that you need to take into consideration all the known and possible alternative current paths when you are interpreting your test results.
If an alternative path is present, your resistance measurement will be lower than the true resistance of the joint or seam. So what? It may be that a perfect measurement is not required. Nevertheless, a method that will get you much closer to the real result is to use separate current and potential leads and connect the potential leads as close as possible to the bond being measured. This will help to reduce the influence of those alternative current paths on your measurement (see Figure 3).
Figure 2: Parallel current paths can yield unexpected and inaccurate results.
Figure 3: Narrowing the measurement path by individual test leads reduces inaccuracies to minimal.
Figure 4: Non-uniform test current introduces a slight error to measurement.
The popular ‘duplex’ leads, where current and potential contacts are contained within a single probe, are easier to use because they require less physical manipulation around the IUT. However, the trade-off is that there is a small loss of accuracy because the current density is not uniform at the precise point where the potential probe is making its measurement. For maximum accuracy, the current density should be uniform. But it must first spread out through the IUT from the point at which it is injected and, with duplex leads, the potential probe is too close for this to happen (see Figure 4). The solution is to use separate leads.
An even better solution is to use the Megger DLRO100 low resistance ohmmeter with its current clamp. The current clamp was developed primarily to increase operator safety when utilities are testing circuit breakers, but it works well in any application where there is a need to remove the influence of parallel current paths on the resistance measurement. When the clamp is used with circuit breakers, they can be grounded on both sides during testing, which helps to ensure that the operator is safe even if a line fault occurs while the test is being carried out. The clamp removes the effect of the parallel ground path on the resistance measurement (see Figure 5).
Figure 5: Dual-Ground® test configuration allows safe grounding without distorting measurement.
Finally, when you perform a low resistance test in the presence of parallel current paths, it is essential to consider whether you need an absolute result, or whether a relative result will be sufficient. If you need to accurately measure the absolute resistance across the IUT, the parallel current paths must be removed. This could be in a research and development application, where the actual resistance of the IUT needs to be established or, as in the case of circuit breaker testing, where a lot is at stake. For many applications, however, a reasonably accurate comparative measurement is all that’s needed to tell the operator whether a connection is loose, a weld is poor, a bond is corroding, or one of many other problems has occurred that can readily be spotted with a trained eye. Operator judgment is critical here.
To summarise: Don’t act without preparation; consider your test item and your goals. If no parallel current paths exist, you can proceed with confidence. If parallel paths do exist, adjust your procedures and expectations accordingly.