Before you use it you must test it each time and make sure it lights up from a known source. The tip will light up momentarily from static electricity but it takes a live AC source over 90 Volts to make it light continuously. I use this for AC troubleshooting. If something isn't working I start back as far as I have to go to detect AC present and then I keep working forward until I find what the problem is. It will read voltage through the insulation of a wire, even a shore power cable.
I also use it every time I open up an AC panel for inspection. I know I unplugged the boat and disconnected the inverter but maybe there is something I forgot to do. Before I go touching anything I make sure its not live.
This particular one is by Fluke and has an on/off switch. I have an older one without a switch, its powered by a AA battery which seems to last forever. I have seen many similar devices on sale, in hardware stores, Home Depot etc. They range from $15-25 and are well worth the money. I think every boat with shore power should carry one.
We don't sell these at PKYS since they are so widely available but I though I would share this handy tip with you anyway. For more on AC electrical supplies visit our section on Shore Power.