Another distinguishing feature is the difference between indoor and outdoor construction. The performance characteristics of the two constructions are essentially the same, but the physical appearance and hardware are different. The outdoor unit must be protected for possible contaminated environments while indoor units are protected due to their being mounted in an enclosure of some kind. Thus most outdoor units will have larger spacing between line and ground, which is achieved by the addition of skirts on the design. This provides larger surface creepage distances from the primaryterminals (at line potentials) to the secondary terminals and the base plate (at ground potentials). For outdoor types the hardware must be of the non-corrosive type and the insulation must be of the non-arc-tracking type. One other feature that differentiates the indoor from the outdoor is the orientation of the primary terminals. The indoor types must be compatible for connection to bus type electrical construction as opposed to the outdoor types that are normally on the pole-top installations.