With significant industry disruption, interstate and regional travel dramatically limited, and in some cases reduced capacity for revenue generation, asset owners are under more pressure than ever to reduce costs. In the case of critical infrastructure like power systems, it’s imperative that routine maintenance and inspections are not disrupted, and so smarter and more efficient approaches need to be adopted.
In its 2016 revision, the principal standard for high voltage (HV) electrical assets in Australia, AS 2067, introduced substantial improvements to the way HV earthing systems are maintained.
This standard is now well established, and compliance is a legal obligation in most instances. If deferring maintenance is not an option, businesses need to discover ways to improve efficiency.
Section 8.7 of AS 2067:2016 requires a maintenance schedule to be implemented for earthing assets, which should consider (amongst other things) which test methods are most appropriate. The different test methods tend to fit into one of two categories:
Performance tests are those that test the effective operation of the earthing system.
The principal performance test is an earthing injection test, where the power line is configured to be representative of an earth fault (e.g. a single phase to earth fault) and a test current injected in that circuit.
In this case, a ‘real’ (though small scale) earth fault is established, allowing measurements of touch and step voltages and current distribution to be taken.
Satisfactory results on a performance test give the stakeholders confidence that an actual full-scale fault would not produce touch and step voltages that constitute an unacceptable risk, and would not exceed insulation and current ratings on equipment.
Performance tests are the only measure of risk associated with an asset. Since they are relatively expensive and require specialist expertise and equipment, they are usually scheduled infrequently -typically every five to ten years.
However, if an earthing system experiences a change that might negatively impact performance, it is unlikely to be detected until the next performance test, and therefore there could be a latent risk issue- the voltages produced during an earth fault might be unacceptably high.
Condition tests are those that ascertain whether there has been any change in the condition of the earthing system, such as deterioration, inadvertent damage, or configuration change.
Condition testing is less expensive and might be carried out every one to two years. Whilst it doesn’t measure system performance, and cannot on its own indicate the safety compliance of an earthing system, it is often considered the most valuable testing, for two main reasons:
- No change in condition implies no change in performance. If the previous performance test indicated safety compliance, consistently positive routine condition test results provide confidence that the earthing system will perform effectively in the event of an earth fault.
- Condition testing will identify risks more quickly. Because they are less expensive, the time between condition tests (and therefore maximum time that a latent issue might go undetected) can be much shorter than performance tests.
Condition testing also has the advantage of being less complex. In fact, many asset owners are now having their in-house teams trained and equipped to carry out routine earthing condition testing themselves, which overcomes some of today’s cost and travel challenges.
Similarly, some electrical service providers are developing their capability in order to add to their existing service offerings.
What does earthing condition testing look like? There are two aspects, often carried out together: Visual inspection and integrity testing.
Visual inspection requires a trained eye and looks for signs of physical change in the earthing system elements.
Integrity Testing assesses the electrical continuity of the earthing system elements, ensuring that everything that should be bonded is bonded effectively and that anything that should be separated is in fact separated.
The test requires a test instrument suitable for the environment, typically a 4-wire, switched polarity DC continuity meter with sufficient AC and DC noise immunity, robustness, portability, and battery life.
Such instruments are readily available, and in-house or online training courses in the test method are available.
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