manipulates several levers and pedals to control each function of the crane. Safety When a tower crane is used in proximity to buildings, roads, power lines, or other tower cranes, a tower crane anti-collision system is used. This operator support system reduces the risk of a dangerous interaction occurring between a tower crane and another structure. In some countries, such as France, tower crane anti-collision systems are mandatory. Self-erecting tower cranes Generally a type of pedestrian operated tower crane, self-erecting tower cranes are transported as a single unit and can be assembled by a qualified technician without the assistance of a larger mobile crane. They are bottom slewing cranes that stand on outriggers, have no counter jib, have their counterweights and ballast at the base of the mast, cannot climb themselves, have a reduced capacity compared to standard tower cranes, and seldom have an operator's cabin. In some cases, smaller self-erecting tower cranes may have axles permanently fitted to the tower section to make maneuvering the crane onsite easier. Tower cranes can also use a hydraulic-powered jack frame to raise themselves to add new tower sections without any additional other cranes assisting beyond the initial assembly stage. This is how it can grow to nearly any height needed to build the tallest skyscrapers when tied to a building as the building rises. The maximum unsupported height of a tower crane is around 265 ft. For a video of a crane getting taller, see "Crane Building Itself" on YouTube. For another animation of such a crane in use, see "SAS Tower Construction Simulation" on YouTube. Here, the crane is used to erect a scaffold, which, in turn, contains a gantry to lift sections of a bridge spire. Telescopic A telescopic crane has a boom that consists of a number of tubes fitted one inside the other. A hydraulic cylinder or other powered mechanism extends or retracts the tubes to increase or decrease the total length of the boom. These types of booms are often used for short term construction projects, rescue jobs, lifting boats in and out of the water, etc. The relative compactness of telescopic booms makes them adaptable for many mobile applications. Though not all telescopic cranes are mobile cranes, many of them are truck-mounted. A telescopic tower crane has a telescopic mast and often a superstructure (jib) on top so that it functions as a tower crane. Some