## 2 Experimental Arrangement
### Ground Electrode  
Figure 2 shows a small grid ground electrode, installed 30 cm below the ground's surface, connected to a rod electrode at each corner of the grid. Copper tapes, with a width of 25 mm and a thickness of 3 mm, were used to connect these copper rod electrodes  
Figure 1: Test arrangement used in this study.  
measuring 16 mm in diameter and 1.8 m in length. The ground electrodes can be categorised into three main parts: (i) a 2 m \(\times\) 2 m ground electrode installed in natural soil, denoted as GEN; (ii) 2 m \(\times\) 2 m ground electrodes installed in soil treated with EMs, denoted as GEM; and (iii) GEM ground electrodes poured with 10 L, 20 L, 50 L and 100 L of water, denoted as GEM_10L, GEM_20L, GEM_50L and GEM_100L, respectively. For the ground electrode (i), trenches with the size of 10 m width and matching the length of the grid were made, where the copper tapes were laid along the trench, backfilled with heap soil and gently tapped on these installations (see Figure 3), and the rod electrodes were driven into the soil by hammering all the way to a depth of 1.8 m. Ground resistance measurements at a low voltage and low current and impulse tests with increasing voltage magnitudes under both impulse polarities were then conducted on the ground electrode. Ground electrode (ii) can be divided into two parts: horizontal and vertical electrode installations. For the former, the soil in the trench was excavated. The copper tapes, which were installed earlier, were removed to facilitate the placement of the 2.5 cm thick EM underneath and on the copper tapes. The powdered EM was mixed with a specified percentage of water content, which turned it into a slurry mixture (see Figure 3a). This was then poured into the trenches. The mixture was prepared based on the manufacturer's requirements (10 L of water was mixed with 25 kg of EM, which turned the EM into a slurry mixture). This mixture was prepared first before it was placed on and under the copper tapes. It was stated in the manufacturer's data sheet that the EM complies with IEC62561-7 [21], with its composition comprising carbon-based graphite powder and Portland cement.  
The amount needed for the horizontal ground electrode was based on the 5 cm thickness and the lengths of the copper tapes (see Figure 3b), whereas for the vertical rod electrode, the amount needed was based on the 7 cm diameter of the hole, subtracting the 1.6 cm diameter of the ground electrode and considering the length of the rod electrode of 1.8 m (see Figure 3c).  
The copper tapes were laid back in the trenches, and another layer of the slurry form of EM, measuring 2.5 cm in thickness, was placed on the copper tapes. Heap soil was used to backfill the trenches and was tapped gently on the ground. To install the rod electrodes, the rod electrode that was installed earlier in (i) was pulled out to create a pre-hole, which was then filled with EM. A bore hole with a diameter of 7 cm was made, and the slurry form of EM was poured into the hole while ensuring the rod remained in the middle of this hole. Similar to (i), low voltage, low current and impulse tests were carried out on ground electrode (ii), GEM and ground electrode (iii) (GEM_10L, GEM_20L, GEM_50L and GEM_100L).  
Figure 2: Ground electrodes used in this study.