Electricity can cause serious injury or even death which is why safety must always come first when working with electricity or electrical devices. Prior to starting work on an electrical box, for example, (such as an AC mains switch-board) or a power supply, you must first verify there is no AC voltage. If you can't completely isolate your device from the supply wires, how can you be sure that there's no voltage remaining? There could be residual current due to faults such as a short circuit.
Situations like this are where a non-contact AC voltage detector comes in handy. There are several options available on the market and they range in price. In true DIY fashion, you can quickly and easily create your own non-contact AC voltage detector in less than an hour.