Articles

Are You Putting the Cart Before the Horse with Your Earthing Testing?

 

Most aspects of our power systems are well understood technically and we have accurate and practical tests to assess their condition and performance. Testing of earthing systems remains one area in which simplistic tests are often carried out, by relatively untrained personnel, without any clear understanding of the suitability of the test method, nor of the limitations or risks of the test instruments being used. It is very common, particularly for HV asset owners in industry and mining, to receive test results from a contractor that indicate a “pass” but lack insight, and seem more of a box-ticking exercise.

In many cases, this issue stems from choosing the wrong instrument at the starting point. A web search for earthing test instruments quickly reveals a range of relatively low-cost instruments for testing earth resistance. The majority of these instruments may be quite effective for measuring the effective resistance of an isolated earth electrode in homogenous soil. Their low cost and simple operation make it easy for electrical contractors to add “earthing testing” to their capability statement and provide low-cost testing services. However, it is common that users then employ a simple instrument well beyond its limitations – in multi-element earthing systems, with interconnections and auxiliary paths, in complex soil conditions, in areas subject to AC and DC interference. The simple test instruments operate quickly and provide a number, but that number is often incorrect and is rarely sufficient information to effectively assess the state of the earthing system and its associated risks.

A better approach is to get the horse in front of the cart. Firstly, understand the system to be tested, and assess what failure modes it might experience and the impact they might have on the performance and safety of the system. Secondly, determine the best test method, or methods, to detect such failures as early as possible, and ideally without disruption to normal operations. Thirdly, choose a test instrument that is suitable for the test method, with an understanding of the test circumstances and whether the instrument will perform appropriately and provide suitable insight.

Step 1

What can go wrong?

Firstly, the design can be wrong; earthing design relies on many variables, some of which can’t be accurately known prior to installation, and so testing of earthing system performance is a critical step in the validation of a design.

Once commissioned successfully, the two most common things that can change future operation and safety are element deterioration/ failure (corrosion, damage, theft etc.), and configuration change either to the earthing system itself or the surrounding area (fences, pipes etc.). Changes to the supply network (e.g. fault level, protection settings) can also affect the magnitude of earthing-related hazards.

Step 2

What is the best test method?

For testing performance, there is only one method that fully assesses the effective operation of an earthing system. Injection testing, typically low-current, off-frequency, is used to simulate an actual earth fault and measure the resulting hazards. This test is necessarily complex, and usually requires sophisticated equipment and specialist personnel. In some very simple cases, a resistance measurement of the earthing system can be a sufficient test method, but only for very small, isolated earth grids and only where the measurement of touch voltages is deemed unnecessary.

Once commissioned, the best method to provide confidence in future performance is one that is simple and inexpensive enough to carry out more frequently (so as to detect changes as early as possible), and accurate enough to identify if something is not as it should be. Injection testing is expensive, and simple resistance testing is (for all but very simple cases) not sufficiently accurate enough to determine actual condition. The best method, which is now recommended by most relevant Australian standards including AS2067, is earthing integrity testing, which employs visual inspection and continuity testing. The objective of continuity testing, in this case, is to ensure that everything that should be bonded to the earth grid is effectively bonded and that nothing is bonded that should not be bonded.

Step 3

What is the best instrument for routine testing?

Measuring continuity seems simple, and in the right conditions may be simple. However, for the majority of earthing systems, testing is subject to a range of constraints that can substantially impact the effectiveness of generic instruments. Point-to-point continuity testing often requires measurements across large distances of up to 100m or more and is often subject to interference from AC and DC noise sources. The right instrument, therefore, needs to be able to accurately test regardless of lead length, and be immune to typical noise levels. The instrument’s test current magnitude needs to be only high enough to produce sufficiently accurate measurements. 1A is typically sufficient, provided its voltage measurement is sufficiently accurate and noise immune. Higher currents rarely yield better results, but can cause inadvertent damage or increased risks to test personnel. On the practical side, test equipment needs to be ergonomic, reliable, and off sufficient battery life to make routine testing acceptable.

It should be noted that a variation on point-to-point continuity testing is loop impedance testing, where a handheld clamp-on instrument is applied to an earthing conductor to measure the impedance of the loop of which that conductor forms a part. This instrument can be extremely effective at getting quick confidence that a conductor is continuous, or that an electrode is connected and functional, but it does have limitations that make it suitable for only a subset of installations.

 
 

What does best practice testing look like?

AS2067 and other modern standards recommend (or for major subs, mandate) a test plan along the lines of:

1. Commissioning Testing:
   Performance assessment (including touch voltage measurements) – typically, injection testing, using specialized injection and measurement equipment, but some small, isolated, high-resistance installations (e.g. single electrode) might be assessed by resistance test using generic ground tester.
   Condition assessment – Visual inspection and continuity testing, using a fit-for-purpose site continuity meter.
2. Routine Testing:
   Condition assessment, as above, typically carried out annually.
3. Occasional Testing:
   Performance assessment, every 5 to 10 years, or when triggered by suspicious condition testing results or system changes.

 
 

How can the limitations of test instruments lead to poor test methods and/or results?

Example: Wrong instrument leading to wrong test method
Annual testing at an 11kV padmount substation at a NSW surface mine was normally contracted to the local electrical service provider. They used a generic ground tester to test grid resistance, which provided a number that no one could determine acceptable or otherwise. Furthermore, they had not considered the highly interconnected nature of the site, which made that test method relatively ineffective. A more suitable approach was to do initial performance assessment by injection testing, and then routine continuity and loop impedance testing, which could be carried out easily by their own technicians.

Example: Wrong test method
At one Australian utility, it has been common practice to disconnect all earth risers from surface equipment to facilitate testing of their continuity of the risers to the grid. However, the biggest likelihood of failure was not reconnecting the bonds correctly. This method was high cost, high risk and could not be carried out as an ongoing routine test, and was principally derived due to the inability of the test instrument to identify poor connections whilst installed. A more fit-for-purpose instrument such as the Safearth CS3 allows for effective continuity testing on live assets without reconfiguration.

 
 

Example: Reasonable test method, wrong instrument
At another utility, it had been common to use a high-current instrument to test earth continuity. In one case, a set of HV VT stands had inadvertently become ‘islanded’ – disconnected from the surrounding earth grid – due to damage below ground. However, they were still bonded to the earth grid via their instrumentation ground wires. If continuity testing had proceeded using their normal high-current method, it would have likely damaged the instrumentation cables, probably triggering a power outage on that feeder. Fortunately, in this case, they were trialling Safearth’s low current CS3, which quickly identified the inadequate bonding without impacting system operations.

 
 

Summary

Much of the traditional testing of earthing systems has been designed around the available instruments, such as generic multifunction ground testers or other non-earthing-specific instruments. Consequently, many of the test methods employed are inadequate and produce limited benefit. A better approach is to put the horse in front of the cart: first, understand what type of testing is appropriate, and then choose the best instrument that has been designed for that test.