Bushings are employed to carry conductors through grounded enclosures, often in transformers, high-voltage circuit breakers, or large switchgear. A bushing not only insulates the conductor from the grounded enclosure but also conducts current with less than a small amount of loss.

Bushing resistance is the resistance of the internal conductor passing through the bushing. In normal condition, it should be very low and consistent. But if the resistance starts to increase, it is usually an early indication of something amiss within the bushing—moisture intrusion, contamination, breakdown of insulation, or even internal corrosion.

In Australia’s substations and major switchgear installations, resistance testing of bushings is commonly performed at time of maintenance outage. The test can be a simple DC measurement of resistance, employing equipment that sends a constant current through the conductor of the bushing and measures the voltage drop to determine resistance.

Why this resistance is significant is because it can inform you of the integrity of the bushing. An increase in resistance with time might mean that the insulation is deteriorating or that the internal conductor is corroding—both of which would result in catastrophic failure if nothing is done about it.

Contact Resistance: The Silent Killer at Connection Points

At every location where two conductors intersect—be it at a breaker, a fuse holder, or a busbar joint—comes contact resistance. Contact resistance occurs because of microscopic irregularities on the surface of the conductors. Even if a joint appears to be tight, only part of the surface area may actually be carrying electricity.

Contact resistance is affected by a number of factors:

Surface oxidation or corrosion

Loose connections or lack of torque

Dirt, oil, or oxidation on mating surfaces

Misalignment or wear and tear

Why does it matter? Because contact resistance can lead to severe overheating. Current flowing through a bad connection causes I²R losses—that is, what seems to be very small resistance can generate dangerous levels of heat in high-current applications. For example, only 150 micro-ohms resistance in a splice carrying 800 A generates 96 watts of constant heat. That’s enough to rot insulation, melt metal, or warp into fire.

This is especially crucial in Australian switchboards where heavy-current applications such as chargers for electric vehicles, generators, or heavy commercial loads are increasingly found. Contact resistance must be monitored periodically with a micro-ohmmeter. These tools apply a known high DC current (usually 50–300 A) across the joint and measure the voltage drop, computing the resistance in units of micro-ohms. As they employ four-wire Kelvin sensing, they remove lead and contact wire resistance from the measurement.

On a properly torqued and well-installed switchboard or transformer, contact resistance should be much less than 100 micro-ohms. If it is greater than that—particularly if the readings are unbalanced across phases—maintenance needs to be done.

Bus Resistance: How the Backbone of the System Performs

Busbars are copper or aluminium solid conductors that transport heavy amounts of current inside a substation or switchboard. They serve as the backbone of a distribution system.

The resistance of a busbar is influenced by three parameters:

Material resistivity: Copper is less resistant than aluminium.

Cross-sectional area: Busbars of greater thickness lower the resistance.

Length: Longer busbar runs will inherently possess greater resistance.

The formula employed is simple:

R = ρ × L / A

Where R is resistance, ρ is material resistivity, L is length, and A is cross-sectional area.

Although this resistance is normally quite low in an initial installation, it can rise over a period of time with corrosion, oxidation in joints, or incorrect plating. Bus resistance is particularly important in high-current applications such as industrial switchboards or transformer panels. A small rise can lead to voltage drop and heating, especially at the point of joints where resistance is localized.

Similar to contact resistance, bus resistance is also tested by injecting a current and reading the voltage drop. It is common in many facilities to have these tested under their commissioning and maintenance programs. When it’s done regularly, it can indicate joints that are starting to deteriorate or weren’t correctly installed at all.

It should also be mentioned that mechanical busbar joint assembly has a significant effect on performance. Australian switchboard makers tend to silver-plate busbar contact surfaces to minimize resistance and guard against oxidation. Torque values upon installation matter equally—over-tightening can hurt components, and under-tightening raises resistance.

Testing Techniques and Field Experience in Australia

Throughout Australia’s commercial and industrial markets, periodic low-resistance testing is now an integral component of preventative maintenance programs. Whether a 22 kV distribution transformer in the Northern Territory or a commercial switchboard in the heart of Melbourne’s CBD, the same principles apply.

Up-to-date diagnostic equipment—micro-ohmmeters, thermal imaging cameras, and digital multimeters—facilitate early detection of problems. The preferred technique for bus and contact resistance testing is the four-terminal (Kelvin) method, which provides accurate measurements even when resistances are very small.

Thermal imaging is often used alongside resistance testing to validate results. A high-resistance joint will usually reveal itself as a hot spot under load conditions. This visual confirmation helps technicians prioritise urgent repairs and validate torque settings.

Practical Design and Installation Advice

If you’re designing or installing electrical equipment, the best way to avoid resistance issues is through good planning and correct assembly:

Utilize appropriately sized busbars according to AS/NZS 3008 recommendations and forecasted load current.

Make the choice of high-quality contact material and plating (such as silver or nickel on copper) to reduce surface oxidation.

Always use calibrated torque tools and follow recommended torque levels for bolted connections.

Clean all surfaces prior to connection—remove dust, oil, or oxide surfaces.

Implement a resistance test upon installation to create a baseline.