A method of joining a first workpiece to a second workpiece is disclosed including securing a spark containment cap to the first workpiece, the cap comprising an electrically insulating washer and a cap body defining an air cavity. Once the cap has been secured, a tail of a fastener is inserted into the air cavity through the second workpiece, the first workpiece and the washer. Upon tightening the fastener, the tail of the fastener urges the washer against the first workpiece and the first and second workpiece are thereby clamped between the washer and a head of the fastener so as to form a joint. Advantageously, it has been found that forming a joint in this manner can help to better protect the joint from the effects of electrical arcing.