Design and construction of grounding system in high voltage laboratory at KMUTT

This paper proposes the design and construction of grounding system in high voltage laboratory at KMUTT. At first, the grounding system in high voltage laboratory at KMUTT was nine rods connected in grid connection. Each rod was 3 m long. The resistance of this grid connection was less than 5 ohms. The grounding of each equipment was connected to grid connected ground using copper strips placed on the cement floor. However, when the oscilloscope protected by isolated transformer was used to measure the waveform of the AC or impulse breakdown voltage, the 5A fuse of the isolated transformer had gone with AC voltage of magnitude about 140 kVp or impulse voltage of magnitude of about 170 kVp applied. The assumption for this problem was thought about the electric field at the ground wires placed on the cement floor was quite high. Therefore, the problem was solved by adding aluminium plates on the cement floor, then, connected the grounding of each equipment to them. Now, the oscilloscope can be used to measure up to the maximum AC/ impulse voltage, 290 kVp/ 320 kVp, generated in the laboratory. The solution can be explained by two reasons. The first one was the theory about the electric field configuration on conductor-conductor and conductor-plane gaps. The other one was the value of the characteristic impedances Z of the electrode configuration between the HV end and ground. With only copper strips on the cement floor, the value of Z was high while the value of Z was reduced when the aluminium plates were placed on the cement floor.