Abstract: For evaluating the condition of a first lightning arrester of the gapless metal-oxide varistor type that is connected across the high voltage winding of a transformer, I connect externally of the transformer a second arrester of the same type, but with means for selectively changing its breakdown voltage between a first value that is higher than the normal breakdown voltage of the first arrester and a second value that is lower than said normal breakdown voltage. Pulses are applied to the low voltage winding of the transformer while the second arrester has its breakdown voltage set at the first value and while said breakdown voltage is set at said second value. By determining whether or not the second arrester conducts in response to each of these pulses, it can be established whether the first arrester is then undamaged, damaged in a shorted mode, or damaged in an open mode.