1.  For AC ramp breakdown testing a Phenix AC Dielectric Test Set, Type 600C was used with a custom built test cell.  The test cell used mushroom electrodes held horizontally covered in silicone oil to prevent flashover. The electrodes were checked frequently for signs of pitting.  12 breakdown sites were then chosen for each thickness material with the voltage at breakdown recorded and processed using the Reliasoft Weibull 7++ software package. Figure 3. The Weibull plot for absolute Figure 4. The Weibull plot for relative breakdown voltages for all samples breakdown voltages for all samples Figure 5. A plot of absolute breakdown voltage Figure 6. A plot of relative breakdown against thickness with a fitted exponential voltage against thickness with a fitted rise to maximum curve (α values) exponential decay curve (α values)  Observations  Figure 3 shows that increasing the thickness of the samples increases the absolute breakdown voltage, as expected.  Figure 5 shows that this increase is not linear, and instead follows an exponential rise to maximum.  Figure 4 shows that the relative breakdown voltage drops with increasing thickness.  Figure 6 shows the decrease in relative breakdown voltage follows an exponential decay.  For an ideal material one may expect the relative breakdown strength to be directly proportional to the thickness of the material, so by doubling the thickness you would double the breakdown strength however this is clearly not the case. These exponential trends could therefore be due to the increased probability of the breakdown path meeting a defect within the material, or that the ‘surface’ or ‘initiation’ of the material has more control over the breakdown strength resulting in addition of further bulk material having little effect on the overall behaviour. The effect of sample thickness on the relative breakdown strength of epoxy systems M Reading *, Z Xu, A S Vaughan and P L Lewin University of Southampton, Southampton, UK Sample Production and Materials AC Electrical Breakdown Results Introduction Conclusions M Reading, mdr@ecs.soton.ac.uk University of Southampton, Highfield, Southampton, SO17 1BJ, UK Contact details :  The breakdown strength dependence of a polymeric insulating material with respect to sample thickness was investigated.  The absolute breakdown strength of the epoxy increased with thickness, but fall below a linear relationship, being well described by an exponential rise to maximum function.  The relative breakdown voltage of the samples was seen to decrease in an exponential decay, suggesting that addition of further material was proving less effective at increasing the breakdown strength.  DER 332 epoxy resin cured with Jeffamine D-230 was chosen due to the large amount of interest in such thermosetting materials of late.  Samples were produced with a stoichiometric rate of 1000 resin to 344 hardener and cured at 100 0 C for 4 hours followed by gradual cooling for 10 hours.  Samples were produced using a gravity fed pre-made mould technique established previously, shown in Figure 1 with a 1 mm aluminium spacer and sample. A QZ13 release agent was used to aid in removal of the polymer film from the mould.  Sample thickness was varied using Melinex spacers obtained from DuPont to produce the samples listed in Table 1. Example spacers are shown in Figure 2. Figure 1. Pre-made mould produced Figure 2. Melinex spacers 1 mm thick sample Table 1. Samples produced and Melinex spacers used  To analyse electrical breakdown data it is a standard that Weibull statistics are employed. One, two or three parameter Weibull equations are often considered. The one and three parameter equations are shown below in equation 1. (1)  The 1 parameter is often found to provide an unacceptable fit to data and the 3 parameter is considered by many to “over-paramaterise”, therefore a 2 parameter Weibull equation is often preferred, shown in equation 2. (2) Here, P f (x) is the cumulative probability of failure at time x, x t is a threshold time under which no failures can occur, α represents the location parameter and β the shape parameter.  When assessing materials based upon data analysed using Weibull, it is often beneficial to observe the α and β parameters, as these give an indication to the materials insulating performance and uniformity respectively. Since it is difficult to safely generate the very high voltages requird to give breakdown data on final-geometry samples, especially without causing excessive damage to equipment, smaller samples are often produced for testing to give an indication of the material’s properties. In recent work, 100 mm thick samples were created to provide breakdown strength data for a range of epoxy-based systems; the quantitative effect of scaling up from the limited sample thickness to technologically realistic values needs to be considered. Volume and area effects are intrinsic to Weibull analysis since they affect the probability of a defect or impurity being in the breakdown path. This investigation aims to analyse the effect of sample dimensions on the experimental breakdown strength of epoxy systems with varying sample thickness. Using a proven sample production technique, thin epoxy films with thicknesses varying from 50 um up to 1 mm have been produced. These samples were then electrically tested using a specialised electrical breakdown instrument and data processed using Weibull statistics. This paper analyses the breakdown characteristics of the samples relative to their thicknesses in order to (a) test the validity of the Weibull distribution and (b) to provide estimates of the optimum sample dimensions for different material formulations. SampleThickness of spacer / μm SampleThickness of spacer / μm A50F300 B70G350 C120H500 D190I1000 E250 Weibull Analysis