Why heat-shrinkable terminations for voltage up to 1 kV are divided into two types: for internal and for external use?

The main reason for that is tracking. Tracking is the creation of carbon paths on the surface of heat-shrinkable terminations as a result of electrical tension, humidity and dirt. This all tell about soon loss of functional capacity of the termination. That is why terminations for 10 kV are divided into type types: for internal and external use. Terminations for external use have additional resistance to tracking. Tracking of the external terminations is proved to occur when the voltage is more than 3 kV. So terminations for 1 KV are not divide into these two types. The most practical choice is termination up to 1 kV.