Non-powered test lamps are useful in determining the live part of a circuit; however, make sure that the circuit voltage you are testing is not higher than the test light is rated for, otherwise the test light could be damaged.
Most test lights are rated for 6- or 12-volt systems, and using them in a 24-volt system will usually blow out the bulb.
You should use a test light to test SRS (Supplemental Restraint Systems), as unintended deployment of the airbags could result – a very dangerous and costly mistake. In addition, using a test light on a computer circuit designed for very small amounts of current flow can damage the circuit.
Reads correctly in the book..