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Spark gaps can be easily added to a PCB design, which can provide a certain level of ESD protection. While it's not the best approach, its basically free and can be good enough for many applications. I generally suggest in this order, TVS diodes, gas tubes, MOV's, then spark gaps. Depending on the expected surge energy, the gas tubes may jump to the front, as they can typically handle larger amounts of energy. Any how, here's a link about how to make and use a spark gap on a PCB. http://www.carousel-design.com/SparkGap.html

eevblog's longer winded version here