"Let he who has ears ..... listen!"
This is the most basic of instruments and yet is so seldom used when tracing a power quality problem, that is the listening to a complaint and interpreting what is being said. One could say this is a universal instrument and could be applied to all fields of fault finding, making power quality problems no exception.
Ask leading questions but take care not to introduce "garden paths" or steer the plaintiff along avenues that don't exist, except in your mind. "When does it happen" is a good starting point. But if an answer such as "when my husband cuts the lawn" is presented, make sure it is only when hubby cuts the lawn that the lights trip, or whether it is merely these times that are recalled as the outbursts make them unforgettable.
Ask what also happens when hubby cuts the lawn. How about turning up the radio so that she can still listen to her favourite program? The fact that that wall socket could never be used, but for a small portable radio, has gone unnoticed as whenever something was plugged in the lights would trip. "Oh, except it also sometimes happens during the day when I'm trying to hear my program, but not always". Investigation of the wall socket reveals a Neutral-Earth swap that went undetected especially as the RCD was fitted much later. The portable radio did not take enough current to trip the RCD until, you got it, the volume was turned up!