There are several types of kiln insulating materials, brick and ceramic fiber being the most common for use in firing metal clays. Most ceramic fiber kilns are called muffle kilns and this is the kind of kiln I have. Muffle kilns heat up faster than the brick type and can easily be moved from classroom to classroom without damaging the kiln. It's best to not move the brick type kiln because movement can cause the bricks to become loose since they are often held together with mortar.
Over time, the muffle can develop cracks. I've been told that the cracks shown here in my kiln were caused by opening the door while the kiln is very hot. I'm guilty of this, due to opening the door while enameling. These types of cracks are not harmful. As long as the muffle isn't breaking apart into pieces, and the elements are still secure inside the muffle. As the muffle heats up it expands and the cracks close.
It's also good to check the kiln's plug wires making sure they are not cracked or damaged. If they are, seek advice from a kiln repair company.
Until next time have fun claying around.