Master of Science in Engineering - Engineering / An investigation was conducted on the failures of 88kV line surge arrestors at an Eskom Distribution substation. The surge arrestor failed when one phase of the transmission line was open circuited and all three phases were not isolated by the protection relays. The transformer was lightly loaded and a surge arrestor blocked the neutral path to the ground. The investigation used a modified version of an equivalent circuit proposed by Marti [1]. Transformer magnetising losses, the non-linear resistance of the neutral surge arrestor and the transformer magnetising reactance of the closed-circuit phases were all included. The investigation revealed that the failure of the arrestor was due to the combined volt drops across the open circuited phase of the transformer magnetising reactance and the neutral surge arrestor. A resonance condition lowered the magnetising reactance of the open circuited phase of the transformer and forced the transformer into saturation. This resonance is characteristic of the fundamental frequency ferroresonance condition [2].