Why measure low resistance?

Measuring low resistance is an important electrical test because it helps identify components where the resistance has exceeded acceptable values, threatening several potential negative outcomes. These could include overheating, energy loss, protective fault device malfunction, poor grounding and catastrophic failure during power surges. Test results can be used to identify changes caused by fatigue, corrosion, vibration, temperature or other conditions that may occur at the site.

Low-resistance measurements are typically below 1 Ω. At this level, it is important to use test equipment that minimises errors introduced by the test-lead resistance and/or contact resistance between the probe and the material being tested. These types of measurements require the unique characteristics of a low-resistance ohmmeter’s four-wire test method. Also, at this level, standing voltages across the item being measured (eg, thermal EMFs at junctions between different metals) may cause errors that need to be identified.

To allow a measurement to compensate for errors, a four-terminal measurement method is employed with a reversible test current and a suitable Kelvin Bridge meter. Low-resistance ohmmeters are designed specifically for these applications. The lower range on many low-resistance ohmmeters will resolve 0.1 µΩ. This level of measurement is required to perform a number of low-range resistance tests.

What low-resistance measurement identifies

For a given current, the rate at which electrical energy is converted to heat is directly proportional to the resistance. For example:

• 6000 A across a 100 µΩ bus = 3600 W.
• 6000 A across a 1 mΩ bus = 36,000 W, which results in overheating.

Low-resistance measurements indicate to the observant operator when degradation is taking place within an electrical device. Changes in the value of a low-resistance element are one of the best and quickest indications of degradation taking place between two contact points. These elements include ground bonds, circuit-breaker contacts, switches, transformer windings, bus-bar with cable joints and bond connections to ground beds. Often readings can be compared to ‘like’ test specimens.

These changes can occur from numerous influences including temperature cycling, chemical corrosion, vibration, loss of torque between mating surfaces, fatigue, improper handling and seasonal changes. Environmental and chemical attacks are relentless. Even air will oxidise organic materials while the ingress of moisture, oil and salt will degrade connections even more rapidly. Chemical corrosion can attack the cross-sectional area of an element, thereby increasing its resistance. Electrical stresses, particularly sustained overvoltages or impulses, can cause welds to loosen. Mechanical stress from vibration during operation can also degrade connections, causing resistance to rise.

Backup battery power supplies provide a good practical example of how degradation can occur under normal operating conditions. Changes in current flow cause expansion and contraction of terminal connections, causing them to loosen or corrode. Connections are also exposed to acid vapours, causing further degradation. These conditions decrease the surface-to-surface contact area with an associated increase in contact resistance, ultimately causing excessive heating at the connection.

Industries with significant resistance problems

Industries that consume vast amounts of electrical power must include low-resistance ohmmeter measurements in their maintenance operations. Not only can abnormally high resistance cause dangerous overheating, it also leads to costly energy losses.

Also, many industries require good earth-bond connections to ‘ground beds’. Poor connections reduce the effectiveness of the ground bed and can cause significant power-quality-related problems and/or catastrophic failure in the event of a major electrical surge.

In general manufacturing industries, motor windings, circuit breakers, bus-bar connections, coils, ground bonds, switches, lightning conductors, small transformers and resistive components all require low-resistance testing. Below are some typical applications:

Motor armature

Armature windings can be tested to identify shorting between adjacent coils or conductors. If motors appear to be losing power, low-resistance tests should be performed and repeated periodically.

Bar-to-bar testing on DC motor rotors is performed to identify open or shorted coils using spring-loaded hand probes. This is a dynamic method to determine the conditions of the windings and the soldered connections to the riser on the commutator segments.

Transformer testing

Transformer winding tests are performed in the factory and then periodically in the field. The factory test is performed at ambient temperature and also after continuous operation to allow for temperature rise.

Large transformers have ‘taps’ on both the primary and secondary windings. Their condition requires verification, since the secondary taps are operated daily and are exposed to excessive wear and vibration as the power-distribution system balances the load carried on the various circuits. The taps on the primary side are critical to major adjustments in the power distribution and should be tested to ensure that a low-resistance connection is available for the new power condition. Tap connections can corrode when not in use and may overheat.

UPS

UPS devices incorporate bolt-connected battery straps and welded cell-plate connections. Resistance measurements should be made between adjacent battery terminals and also between the plate welds. Because of the ‘float current’, two tests are made with opposite polarity and the results averaged. This is the only field application where operators make measurements on energised systems. Routine periodic tests should be made, given that high-resistance connections will reduce the available standby power and also increase fire risks.

Circuit breakers

Arcing at circuit-breaker pads causes carbonised layers to build up. For large oil circuit breakers, the best instrument is one that ramps up current, holds it for a period of time and then ramps down. This test method reduces magnetising, which would otherwise be created by the sudden switching on and off of the test current. This can result in inaccurate ‘CT’ measurements when systems are returned to normal AC operation.

Care should be taken when making measurements across CTs, as high DC currents may saturate the CTs, desensitising them to potential faults. Also, ripple on test currents may cause circuit breakers to trip. Careful positioning of probes should prevent this happening, and the ripple may be minimised by separating the test leads.

Cable-reel length calculation

Cutting a sample length of insulated cable will enable comparative measurements to be made with the whole reel, enabling the length to be easily calculated.

How to measure low resistance

The choice between two-, three- or four-wire DC measurements is generally determined by the resistance range to be measured.

• Two-wire measurement, as with a typical multimeter, is the simplest method and is valid if the probe contact-resistances are negligible compared to the circuit being tested. Two-wire testing is suitable for 10 Ω to a few megohms.
• Three-wire DC testing is reserved for very high resistance, typically above 10 MΩ.
• Four-wire testing is the most accurate method when measuring circuits below 10 Ω, as this method eliminates errors due to lead and contact resistances and is the technique used by low-resistance ohmmeters.

DC vs AC testing

The issue here is the selection of the correct type of test current. A DC instrument should be used when trying to measure the pure resistance of a circuit or device. An AC instrument is used for applications such as ground-bed testing or impedance testing. Ground beds use AC to prevent the test current polarising ions in the soil. The test frequency avoids the 50/60 Hz range to exclude measurement errors from AC mains ground currents.

How low-resistance ohmmeters operate

Low-resistance ohmmeters use two internal measuring circuits. The supply injects a current into the test sample through two leads and the magnitude of the current is measured. Concurrently, two probes measure the potential across the sample. The instrument then performs an internal calculation to determine the resistance of the sample.

High-current outputs are used in true low-resistance ohmmeters. Generic multimeters don’t supply enough current to give a reliable indication of the real current-carrying capabilities of joints, bonds, etc. Good instruments alert operators of open-circuit conditions on the test leads while a few models have automatic range selection.

Current selection

Depending on the instrument chosen, the current selection can be either manual or automatic. High current gives best accuracy. Above 10 A, care is required to minimise any heating of the sample that would itself cause the resistance of the sample to change.

Instruments designed for circuit-breaker testing have much higher current capabilities and operators must be careful when setting test current levels. Instruments designed specifically for transformer testing have high-voltage power levels at the beginning of the tests to saturate the windings. These units then switch to a lower ‘constant-current’ mode to measure the transformer windings.

Probe and lead selection

The potential and current leads are either connected separately or to probes. When probes are used, the potential connection is identified with a ‘P’. They are placed in contact with the sample so that the P-identified contacts or leads are positioned towards each other. The current contacts are then positioned outside or away from the potential connections, ensuring uniform current density across the sample.

Low-range testing

Errors inherent in high-sensitivity measurements including thermal EMFs are readily overcome by taking readings in forward and reverse polarity and then averaging them. Some models accomplish this with manually operated reversal switches, while others perform the two measurements automatically, then display the average reading. Another sophisticated technique automatically measures the magnitude and slope of thermal EMFs and subtracts them from the displayed reading.

Choosing testers

Milliohmmeters

Milliohmmeters are less sensitive than microohmmeters, with measurement capability in the milliohm rather than microohm range (minimum resolution of 0.01 mΩ). These instruments are normally used for general circuit and component verification. Milliohmmeters also tend to be less expensive, making them a good choice if measurement sensitivity and resolution aren’t critical. Maximum test current is typically less than 2 A and as low as 0.2 A.

10 A microohmmeters

Field portable microohmmeters with a 10 A maximum test current and resolution down to 0.1 µΩ cover most field applications.

100+ A microohmmeter

According to IEC62271-100, testing contact resistance of high-voltage AC circuit breakers calls for a test current with any convenient value between 50 A and the rated normal current. ANSI C37.09 specifies the test current be a minimum of 100 A.

Field portable microohmmeters are available that can deliver 100-600 A. By testing at 10 A and then at a higher current, operators get a better understanding of the maintenance requirements for the circuit breaker. In addition to circuit breakers, testing companies use higher current microohmmeters on other high-voltage apparatus, including cables, cable joints, bus-bars and switchgear.

Improved measurements at 100+ A at the same connection may indicate that the instrument is blasting away contaminants and that contact maintenance is required. For occasional industrial use, renting these high-power instruments may be a cost-effective option.

Transformer ohmmeter

Transformer ohmmeters are designed for problems found in measuring transformer windings and tap changers. Some instruments include dual meters with independent range controls, enabling primary (high-resistance) and secondary (low-resistance) windings of a transformer to be measured simultaneously.

Operation of the transformer ohmmeter is sometimes enhanced by connecting the test current through both windings with opposite polarity, thus providing the fastest test time (mutual inductance between windings is minimised). This current connection operation is used on ‘wye-to-wye’, ‘wye-to-delta’ and ‘delta-to-delta’ transformers. The power supply is often designed to deliver energy to saturate the winding and then provide a stable level of test current.

The test-set should also be able to test the windings and contact resistance on tap-changers with make-before-break contacts and voltage regulators. Tap-changers are the most vulnerable part of transformers and face more failures and outages than any other component. Frequent testing is required to ensure proper and reliable operation.

Test probes

Probes are available in several basic styles, each designed for specific situations:

• Fixed-point probes are economical and lightweight. The probes’ sharp points should leave a mark on specimens for future testing.
• Kelvin Clips feature spade lugs on the outboard end and alligator clips with insulated jaws.
• Linear Spring Points are designed with spring points that recess into the handle to allow for unevenness of the surface.
• Helical Spring Points have both potential and current probes in the same handle. Tips rotate and compress into the probe’s body, allowing them to break through any grease or surface film, ensuring accurate measurement. Additionally, these probes will leave a mark on test surfaces to identify the points where tests were performed. This arrangement provides a practical method when making repetitive measurements (eg, testing strap connections in UPS batteries). Care should be taken when using these probes if the surface being contacted is sensitive to pressure points.
• C-clamps: Current passes through the C-clamp and screw-thread while the potential passes through a four-point anvil insulated from the clamp metal.

‘P’ identification on probes identifies the position on the sample at which measurements were taken. Never connect potential clips to current clips, as this will cause measurement errors.

Temperature effects

Resistance measurements are dependent on temperature. If original data was read at one temperature but later tests were conducted at other temperatures, the temperature data is required to determine the suitability of the measurements. Aluminium, steel, copper and graphite all have different specific temperature coefficients that will affect the degree of changes that may take place with temperature at the site of the measurement.

This article is substantially based on the Megger publication ‘A Guide to Low Resistance Testing’