Editing Wireline (cabling) (section)

===Ball-check valve===
If the wireline were to become severed from the tool, a ball check valve can seal the well off from the surface.  During wireline operations, a steel ball sits to the side of a confined area within the grease head while the cable runs in and out of the hole.  If the wireline exits that confined area under pressure, the pressure will force the steel ball up towards the hole where the wireline had been.  The ball's diameter is larger than that of the hole, so the ball effectively seals off pressure to the surface.