Streetlighting is a vital asset to the community. Like all other assets, metal street lights are subject to degradations from a range of factors including corrosion and foundation issues. In fact, the past decade has seen the rate of failures increasing, including a high profile one on the Anzac Bridge in Sydney. To ensure the safety and viability of a steel or aluminium light pole, we recommend regular inspections of all poles in your network.
Traditional methods of inspections for light poles are being proven inefficient when compared with their new non-destructive testing (NDT) counterparts.
This blog will cover the importance of NDT in streetlight inspections, the three most common NDT inspection techniques for metal poles, and our top 3 street light inspection tips.
Why is NDT Important in Street Light Inspections?
As metal poles continue to be used in a range of lighting networks around Australia, it is imperative to regularly test and maintain them for safety and risk mitigation. As the age profile of poles rises, many owners are seeing significant increases in failure rates.
Street light inspections involve testing the asset with the goal of producing results that inform QA, safety issues, product integrity, accident prevention and more. NDT performs this crucial testing of methal street lights without causing damage to the pole. For more on why NDT is used, read our previous blog on the topic here.
We use a range of NDT tools and techniques to ensure a strong and complete NDT inspection process for street light assets. By employing a range of methods, a more comprehensive report can be developed, helping to improve accuracy in identifying defects.
Steel Pole Inspection Techniques
Depending on the desired outcomes, a range of test methods can be used in an NDT inspection. At Revo Group, our years of experience have helped us to narrow down our current methods/tools of choice from what is available to the market at present.
As the Australia/New Zealand/South-East Asia agents for Innerspec, we utilise their cutting edge Electro Magnetic Acoustic Transducer (EMAT) technology in our inspection systems. This test method creates electromagnetic sound waves in the asset and interprets reflected signals called echoes to quickly and accurately detect defects.
Ultrasonic Testing (UT) for Thickness
UT thickness tools are the most commonly used tools in the inspection of flange-based metal street lights (see Figure 1 below). As the pole corrodes, we can detect the loss in wall thickness and make an assessment as to the pole’s integrity. We use this information to determine if the pole failed, estimate its remaining life, and/or adjust inspection intervals to monitor the assets degradation.
Figure 1 – Typical location and result of corrosion for base plate mounted poles.
For direct-buried steel poles, the primary area of concern is at the ground line and below. While most corrosion usually occurs at ground line down to approximately 100mm below ground, there are increasing instances of corrosion failures more than 400mm below ground – well below the typical depth an inspector may dig to inspect a pole.
Figure 2 below shows some examples of the extent that corrosion can occur below ground. In cases like this, the UT thickness methods can be cumbersome, time consuming and inaccurate, or just plain inaccessible due to concrete or paving.
Figure 2 – Examples showing corrosion of steel poles can occur anywhere below ground.
See below for a video of the UT thickness tester in action. This tool is used in this video to alert the NDT Tech to a large patch of internal rust that was not fully visible on a visual inspection.
Pulsed Eddy Current (PEC)
Devices that use PEC measure the response of a section of steel to the induction of eddy currents. Mass loss can be derived when the response is measured against a baseline. The mass loss is then related to a strength reduction to assist in determining the risk of failure and priority for replacement.
PEC devices are limited in the poles they can test due to:
- Proximity and access issues.
- The potential to be significantly affected by cables, steel brackets and other steel sources in and around the pole.
- Detecting mass loss can mean that pitting corrosion, cracks and other localised defects can be overlooked.
Despite the drawbacks, these devices are commonly used during regular inspections.
Guided Wave Ultrasonics
Guided Wave UT uses ultrasonic sound waves travelling in the plane of the metal, guided by the inner and outer wall surfaces of the metal pipe or plate. The reflections from a change in cross section, such as corrosion, give an indication on the scan. The location of the defect relative to the probe can then be measured. This technology can also give an indication of the severity of a defect.
EMAT systems like the Innerspec Medium Range Ultrasonic Testing (MRUT) uses a Shear horizontal wave that commonly reaches the bottom of the buried pole. This system is unique in that the attenuation of the soil has little to no effect on the inspection, and inspectors can assess the entire pole below ground line without excavating.
We have used the EMAT MRUT system on flange-based poles as well. All flange-based poles must be installed with the flange above ground, yet we still see approximately 5 to 10% of flange-based poles with the flange buried beneath turf, gardens, paving and in extreme cases, concrete.
This practice creates significant issues by forming an environment that exacerbates corrosion of the pole, rag bolts and nuts. Soft turf and gardens can often be scraped away to expose the pole base and flange, to conduct visual and conventional UT thickness tests. However, if the pole base is not easily excavated, we use the EMAT MRUT to detect corrosion above the welded flange. We have discovered many poles with significant corrosion lurking beneath mall pavers and tennis court surfaces using this method.
Our Top 3 Inspection Tips
1. Get to Know Your Poles
As you learn more about each metal pole type, the easier and faster they are to assess. This is vital when searching for corrosion below ground line with a tool such as the EMAT MRUT. This system requires a high-level interpretation of test results for greatest inspection accuracy.
The greater the knowledge of each pole type, the faster each screen response can be identified. Whether it be a cable port, the base of the pole, or a welded earth lead tang inside the pole. Once each feature has been identified, any remaining responses on the test results that are unaccounted for will likely be from defects such as corrosion.
2. Expect the Unexpected
It is a regular occurrence that we find that not all the poles in the one street or park are the same. It is important to expect the unexpected. The moment you get used to all the poles in an area being flange mounted, but buried under a 100mm or so of garden, turf or paving, along comes a direct buried pole, identical from the ground up. Poles often look the same but require different inspection approaches.
How do you find out what is going on below the surface? The only way to work that out is to investigate each pole thoroughly, poke around with a scraper, even do an MRUT test and see if you get a strong signal from the flange you suspect is below the pavers. Or the test could show that the pole carries on for another 1400mm underground.
3. Beware of the Dog!
No, we don’t mean the angry ones that might bite you. That is a whole different problem that you will hopefully not run into! Beware of those territorial dogs that insist on peeing on every pole they see. When inspecting buried metal poles, it is important to pay particular attention to the side of the pole that is facing the path or common pedestrian traffic route. Time and again, we have found poles that are severely corroded at, and just below, ground line on the path side. Often, it is perfect new galvanised steel or powder coating on the opposite side.