Safety Issues
The space above the dropped ceiling is often used as a plenum space for ventilation systems, requiring only enclosed ducts that deliver fresh air into the room below. Return air enters the ceiling space through open grilles across the ceiling.
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 networking cable is the most common form of plenum cable, coaxial cable and telephone cable also needs to be plenum-rated for safety.
High-voltage electrical equipment (generally regarded as being over 50 volts) is not permitted to be exposed in the plenum space above a drop ceiling. High voltage wiring must be enclosed in conduit or raceways, and must 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 are not permitted inside the plenum space, though 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 so the risk of overheating and fire is limited.
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 added to keep the ceiling from bouncing vertically during an earthquake.
Lighting fixtures and other devices installed in a dropped ceiling are required to be firmly secured to the dropped ceiling framework. In the event of a fire above a dropped ceiling it is often necessary for firemen to have to pull down the ceiling in a hurry to quickly gain access to the conflagration. Loose fixtures merely resting in the framework by force of gravity can become unseated and swing down on their armorflex power cables to hit the firemen below. Binding the fixtures to the framework assures that if the framework must be pulled down the fixture will come down with it and not become a pendulous swinging hazard to the firemen.
Read more about this topic: Dropped Ceiling
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