Tree growth in polyethylene by applying dc and impulse voltage

For the test with the impulse voltage of 1/40μs only, the tree grows with increasing the impulse voltage and the temperature of samples in ranging from -50°C to 80°C. The relation between the tree-growths (ℓ+ and ℓ-) obtained by applying the positive and the negative impulse voltages is given by ℓ+ > ℓ-. For the test with the impulse superimposed on the dc voltage, the tree growth decreases when the impulse voltage is superimposed on the dc voltage of the same polarity, and increases when the impulse voltage is superimposed on the dc voltage of the opposite polarity, gut the tree-growth decreases when the positive impulse voltage is superimposed on the negative dc voltage, in spite of the application of the opposite polarity. For the dc prestressing test, the tree-growth decreases or increases in a similar manner as when an impulse is superimposed on a dc voltage of the same or opposite polarity.