North American distribution boards are generally housed in enclosures, with the positioned in two columns operable from the front. Some panelboards are provided with a door covering the breaker switch handles, but all are constructed with a dead front; that is to say the front of the enclosure (whether it has a door or not) prevents the operator of the circuit breakers from contacting live electrical parts within. carry the current from incoming line (hot) conductors to the breakers.
Glass optical fibers are almost always made from, but some other materials, such as,, and as well as crystalline materials like, are used for longer-wavelength infrared or other specialized applications. Silica and fluoride glasses usually have refractive indices of about 1.5, but some materials such as the can have indices as high as 3. Typically th.