An enclosure (10) that contains a switch (40, 42) and that is surrounded by an explosive or flammable environment that could be ignited by a spark from the switch, includes a body (52) that guides a flat cable (16) through a body bore (56) to the inside of the vessel. A tight and strong seal is formed around the flat cable by a tubular seal (60) that lies in the body bore, that is formed of Teflon, and that has a tapered seal section (84) that lies in a tapered section (74) of the bore. A nut (62) at the rear of the body can be turned to push the tubular seal forward within the body bore and thereby compress it against the outside of the flat cable to form an explosion-proof seal.