Instruments play an important role in installation work enabling the measurement of the current, voltage, resistance, power and power factor. The basic ammeter and voltmeter work on either the moving-iron or the moving-coil principle, whereas the modern digital instrument uses complex electronics. Using a test lamp to probe between line and earth for indication of a live supply on the line terminal has long been in practice. Some test lamps have LED indicators, and the internal circuitry of such test lamps limits the current to the earth to a level below which the residual current device can operate. When there are no drawings or items of information, especially on a large installation, there may be a degree of exploratory work to be carried out in order to ensure safety while inspecting and testing. As the client/customer is to receive the originals of any certification, it is important that all relevant details are completed correctly.