work area, because of the inherent limits of vertical-only movement. The contraction of the scissor action can be hydraulic, pneumatic or mechanical (via a leadscrew or rack and pinion system). Depending on the power system employed on the lift, it may require no power to descend, able to do so with a simple release of hydraulic or pneumatic pressure. This is the main reason that these methods of powering the lifts are preferred, as it allows a fail-safe option of returning the platform to the ground by release of a manual valve. Apart from the height and width variables, there are a few considerations required when choosing a scissor lift. Electric scissor lifts have smaller tyres and can be charged by a standard power point. These machines usually suit level ground surfaces and have zero or minimal fuel emissions. Diesel scissor lifts have larger rough terrain tyres with high ground clearance for uneven outdoor surface conditions. Many machines contain outriggers that can be deployed to stabilise the machine for operation. Hotel lift There are a number of smaller lifts that use mechanical devices to extend, such as rack and pinion or screw threads. These often have juxtaposed sections that move past each other in order to facilitate movement, usually in a vertical direction only. These lifts usually have limited capability in terms of weight and extension, and are most often used for internal maintenance tasks, such as changing light bulbs. Motive mechanisms AWPs, by their nature, are designed for temporary works and therefore frequently require transportation between sites, or simply around a single site (often as part of the same job). For this reason, they are almost all designed for easy movement and the ability to ride up and down truck ramps. Unpowered These usually smaller units have no motive drive and require external force to move them. Dependent on size and whether they are wheeled or otherwise supported, this may be possible by hand, or may require a vehicle for towing or transport. Small non-powered AWPs can be light enough to be transported in a pickup truck bed, and can usually be moved through a standard doorway. Self-propelled These units are able to drive themselves (on wheels or tracks) around a site (they usually require to be transported to a site, for reasons of safety and economy). In some instances, these units will be able to move whilst the