If NEMA 4x or 6 (European IP 68 - submersible) is required, O-Rings or Seal Rings (ordered separately) should be used when mounting Strain Relief Fittings to a control panel, secured with a locking nut. The Straight Strain Relief fittings feature a molded O-Ring groove. For these fittings, the Buna-N O-Ring is the best solution. The Snap Elbows and some of the Conduit Fittings do not have a recess for an O-Ring. In that case, the Polyethylene Seal Ring would be the best choice, unless the hole can be beveled to accommodate an O-Ring.