Crushed rock is a standard earthing surface treatment solution, which is often used in industrial and utility installations to manage elevated step and touch voltage risks. AS/NZS 2067 references a standard crushed rock surface treatment layer with a resistivity of 3,000 Ω.m and a thickness of 100 mm for use within electrical substations. Further guidance as to the specification, preparation and maintenance of a crushed rock surface treatment layer used for earthing purposes is provided in ENA-EG0. This identifies that crushed rock with a gravel size of 30-50 mm would typically be specified as this provides a good balance between traffic-ability and sufficient minimum resistivity.
PowerEarth’s experience shows that the crushed rock surface treatment used at many existing Victorian electrical installations often comprises of small particle sizes (up to 15 mm diameter gravel size rather than 30-50 mm diameter gravel size) and often also includes fines and organic matter. The result is that the electrical resistivity of the crushed rock surface treatment may vary and may be significantly less than the 3000 Ω.m standard value referenced in earthing standards.
Figure 1 below provides an example of crushed rock surface treatments. The top half of the image shows a typical existing crushed rock surface layer at a substation installation, with small diameter gravel size, fines and organic material. The bottom half of the image shows a fresh section of crushed rock, with 30‑50 mm diameter gravel size and no fines or organic material.
Recent testing completed by PowerEarth shows that existing crushed rock surface layer treatments, with small diameter gravel size, fines and organic material, may only achieve typical resistivities of 1,000 to 1,500 Ω.m. This may mean that the surface treatments at existing installations are not as effective as they were designed to be in managing earthing safety risks.
Written by David HH – Senior Professional Engineer