In what condition should you avoid using heat-shrink insulators?

Heat-shrink insulators should be avoided on wet surfaces primarily because moisture can prevent the heat-shrink material from adhering properly to the component being insulated. When the surface is wet, the heat-shrink tubing will not grip the object effectively, which compromises its insulating ability. This failure can lead to potential electrical shorts or faults since moisture can facilitate conductivity. For optimal performance, heat-shrink insulators are best applied to clean, dry surfaces to ensure they shrink uniformly and form a secure seal, thereby providing effective insulation and protection against environmental factors.

The other conditions mentioned may present challenges, but they do not directly undermine the physical attachment and sealing integrity of heat-shrink insulators in the same critical manner that wet surfaces do.