High Quality Content by WIKIPEDIA articles! A Safe Load Indicator (SLI) is a device fitted to a crane designed to alert the operator if the lift is exceeding the safe operating range of the machinery. In some cases, the device will physically lock the machinery in circumstances it determines to be unsafe. SLI systems are usually composed of a microprocessor connected to various sensors on the crane itself. The SLI measures the angle and extension of the boom along with the load weight and compares this with the manufacturer`s specifications to determine if the lift is safe.