What does НГ mean in the names of some tubes?


These Russian letters mean that they are flame retardant.
The tubes with this kind of index in the name are self-extinguishing.
Among KBT range they are ТУТнг, ТТК(3:1), ТТК(4:1). There are antypirenes in the composite of polymers, they are responsible for flame retardance. Antypirens prevent oxygen supply, that makes burning impossible.
Recently fire safety requirements have become more strict- insulation of cables is required to be flame retardant, so the heat shrinkable tubing as well.