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A vision panel is a small window in a door which allows people to look through without opening the door. Vision panels are sometimes in walls, generally adjacent to a door. These have implications in safety, primarily to avoid opening doors on a person coming the other way, but also in case of fire to avoid opening a door onto a fire.
Mercury relays consist of a vertical (usually glass) tube containing liquid mercury. They have isolated contacts at the bottom of the tube and partway up, usually in a side arm of the glass. The relay works by displacement. A pool of mercury fills the lower portion of the tube, but is insufficient to bridge the contacts.
Each contact is a piece of electrically conductive material, typically metal. When a pair of contacts touch, they can pass an electrical current with a certain contact resistance, dependent on surface structure, surface chemistry and contact time; [2] when the pair is separated by an insulating gap, then the pair does not pass a current.
The door holders release, allowing the doors to close automatically using door closers. [1] [2] The electric power to keep the doors open is typically 12 VDC, 24 VDC (common), 24 VAC, 120 VAC, or 240 VAC. The closed doors may or may not mechanically latch or lock in the closed position.
Wood will expand and contract across the grain, and a wide panel made of solid wood could change width by a half of an inch, warping the door frame. By allowing the wood panel to float, it can expand and contract without damaging the door. A typical panel would be cut to allow 1/4" (5 mm) between itself and the bottom of the groove in the frame.
Every time the contacts of an electromechanical switch, relay or contactor are opened or closed, there is a certain amount of contact wear. If the contact is cycling without electricity (dry), the impact of the contact electrodes a slightly deformed by the resulting cold forging. [1]