Abstract: A portable tester is disclosed for determining in the field the condition of a lightning arrester connected in parallel with a high voltage winding of a transformer. The tester comprises a battery; an inverter connected to the battery for generating an AC output; at least one transformer and a rectifier for transforming the output of the inverter to a high voltage DC having a magnitude greater than the breakdown voltage of a lightning arrester connected in paralled to the high voltage winding of a transformer being tested; at least one capacitor connected across the rectifier for being charged to the high voltage DC; an overvoltage spark gap switch for discharging the high voltage charge of the capacitor across the high voltage winding of the transformer being tested; and a storage oscilloscope for detecting the response of the winding to the high voltage pulse. A normally functioning lightning arrester will clamp the voltage across the transformer winding at the arrester's known breakdown point.