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The results of tree-initiation tests using impulse voltages show that the time required for the build-up of space charge at 25C is 30Â¿s~80Â¿s. The results of prestressing tests with DC and superimposing impulse voltages show that the space charges are homo-charges. An investigation of tree-initiation in polyethylene was carried out using dc and impulse voltages, the latter having a wave-front duration ranging from 1 Â¿s to 700 ps. The relationship between the tree-initiation voltage and the duration of the wave-front of the impulse voltage shows that the times required for the build-up oif the space charge at the tip of a needle electrode are 30Â¿s~140Â¿s at 40C for both polarities and greater than 700 Â¿s for the positive needle and 80Â¿s~l40Â¿s for the negative needle, respectively at 60C. The tree i Ltiation voltage decreases when the impulse voltage is superimposed on a dc voltage of the opposite polarity and increases when the impulse voltage and the dc voltage are of the same polarity. Some of the samples were prestressed with dc voltage for 10 minutes, and were then subsequently tested by applying an impulse voltage with a wavefront duration of 1 Â¿s after a waiting period of 2, 30 and 60 minutes, respectively. The tree-initiation voltage was reduced in a similar manner as when an impulse was superimposed on a dc voltage of opposite polarity.