Elements with several electrons in their valence band act like insulators. That is, they do not readily conduct current. This is not to say that they cannot conduct current under any condition. It actually means that it takes considerable energy (force) to dislodge electrons from the valence band (into what is often termed a conduction band) where electrons (current) can begin to flow. We say that the material has “broken down” when there is enough energy to get current flowing through it. Usually that means the energy has caused the material to change form.
A common example of this is wood. Wood is a very poor conductor of current and would normally be considered an insulator. But if enough energy is present—a lightning bolt, for example—current will flow through wood. The moisture content in the wood is a factor, and the wood changes form (burns) under this much energy. Nevertheless, this illustrates the point that in the presence of enough force or energy, an insulator can be made to conduct current.