## 2 Materials and Methods  
This research work is divided into two phases, finite element modeling and insulation strength testing. HFO-1336 mzz (E) and the mixture's dielectric breakdown strength were investigated under HVAC and HVDC conditions. The experiments were conducted following the IEC60270 standard, and standard atmospheric conditions were maintained in the lab (temperature t\({}_{0}\) = 20 \({}^{\circ}\)C, pressure b\({}_{0}\) = 0.1013 MPa, absolute humidity h\({}_{0}\) = 11 gm/m\({}^{3}\)) [22]. The TERCO test setup generates HVAC and HVDC test voltages for experiments. The test vessel is modified to meet the technical requirement of changing the electrode's air gap. For this purpose, a linear actuator is installed at the bottom layer of the test vessel. The test vessel is composed of aluminum, and plexiglass is used as the vessel window. The vessel height is 800 mm, the width is 90 mm, and the thickness of the vessel wall is 5 mm. Hence, based on the dimensions, the total volume of the test gas is approximately 5 L and can bear a pressure of 0.6 MPa. A constant electrode gap distance of 6 mm is maintained throughout the experimental and simulation analysis. Pressure and vacuum gauges are  
Figure 1: Chemical structure of C\({}_{4}\)H\({}_{2}\)F\({}_{6}\) or HFO-1336 mzz (E). installed to accurately measure the gas pressure inside the vessel. A one-way air valve is installed at the bottom flange to pump gas from the vessel in and out. Prior to conducting the experiments, the test chamber was vacuumed to 0.09 MPa through a vacuum pump and then filled with CO\({}_{2}\). This process is repeated 5 times to ensure no impurities are left inside the pressure vessel. Gas breakdown occurs as the critical field strength is achieved, and the externally applied voltage is recorded as the breakdown voltage. To investigate the basic insulation performance, i.e., the electric strength near its intrinsic strength, weakly non-uniform and uniform fields are used. These field distributions are formed by plane-plane and sphere-plane electrode configurations. The advantages of a sphere-plane arrangement are a simplified alignment, a relatively compact size, and fast conditioning due to the small surface area stressed by the highest field. The sphere-plane arrangement can create field utilization factors representing real GIS and representing real GIS components such as switches. A plane-plane electrode arrangement's main advantage is the uniformity of field distribution [23]. The electrode configurations are plane-plane and sphere-plane for uniform and slightly uniform electric fields. All the electrodes are cleansed with anhydrous alcohol and dried with cotton to remove the dust. The electrode surfaces are polished with 5000 mesh sandpaper to ensure that the surface roughness value is below 2.5 \(\upmu\)m; as a further precaution, 3 aging tests are conducted on the test electrode configuration before every test to remove the protuberances and tiny particles on the electrodes. The electrode schematic diagram can be seen in Figure 2. An AC voltage of known value is applied to the buffer gas CO\({}_{2}\) to calibrate and validate the measuring apparatus's accuracy. The test gas HFO1336-mzz (E) and buffer gases are injected into the pressure vessel following [PERSON]'s law of partial pressure. A standing time of 24 h is provided to ensure gas mixture uniformity. Considering the insulation medium's recovery time, the interval between two adjacent breakdowns was set to three minutes. A slow voltage increase rate avoids overestimations of the electric strength due to the statistical time lag but increases the experimental time. Therefore, a fast voltage ramping rate is utilized up to around 80% to 90% of the expected breakdown voltage, and then the voltage is increased in small steps of 0.5 kV/s compared to the initial breakdown value. Under each test condition, at least ten repeated tests were performed.