Wiring and piping installed behind traditional plaster or wallboard ceilings is extremely difficult to modify once the finished ceiling is in place. Wires must either be fished through hollow spaces in the walls behind the finished ceiling, or the ceiling must be demolished in order for wiring or piping changes to be made.
One disadvantage with this ceiling system is reduced headroom. Clearance is required between the grid and any pipes or ductwork above to install the ceiling tiles and light fixtures. In general, a minimum clearance of 100 to 200 millimetres (4 to 8 in) is often needed between the lowest obstruction and the level of the ceiling grid. A direct-mount grid may work for those who want the convenience of a dropped ceiling, but have limited headroom. Stretch ceiling supports require less than one inch of vertical space, and no space is required for tiles to be lifted out with a stretch ceiling, but a greater clearance space may be chosen to allow room for MEC or for aesthetic reasons.
In earthquake prone areas (e.g., California) diagonal wire stays are often required by building codes in order to ensure the ceiling grid won't sway laterally during an earthquake, which can lead to partial or total collapse of the ceiling grid on the occupants below during a severe tremor. Compression posts may also be added to keep the ceiling from bouncing vertically during an earthquake.
In the event that the dropped ceiling is used as a plenum, low-voltage cables and wiring not installed inside conduit need to use a special low-smoke and low-toxicity wire insulation which will tend to char and stop burning on its own. This helps to protect building occupants so that they are not poisoned with toxic chemicals sucked through the ventilation system in the event of a fire, and helps to prevent fires from spreading inside the hidden plenum space. This special low-smoke cable is typically referred to as plenum cable or Low Smoke Zero Halogen (LSZH or LS0H) cable. While twisted pair cable for networking and telephone service is the most common form of plenum cable, coaxial cable also needs to be plenum-rated for safety.
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