High-voltage electrical equipment (generally regarded as being over 50 V) is not permitted to be exposed in the plenum space above a drop ceiling but must be enclosed in conduit or raceways and be physically isolated from low-voltage wiring. High-voltage electrical devices similarly must be enclosed in a plenum space, inside a metallic container. Similarly, electrical outlets for domestic powered devices may not be inside the plenum space, but outlets can be installed on ceiling tiles inside electrical boxes, with the sockets exposed on the exterior bottom face of the drop ceiling. The purpose of these restrictions is to limit flame spread inside the unseen plenum space, in the event of high voltage equipment or wiring failure. Low-voltage cabling is permitted because current flow is typically negligible and so the risk of overheating and fire is limited.