The best warning for a electrician of a hot power line is a high-voltage indicator. Other devices relying on secondary power sources or subject to protective measures such as fuses can give false positive readings because they are not working as a direct result of the electricity in the lines. A high-voltage indicator is very simple and very effective.
Most high-voltage indicators are simple circuits. They consist of a connector attached to a bus bar in the supply cabinet, and a light-emitting diode inside of a small housing with a window to view the light. The LED may or may not flash, but its illumination lets an electrician know the line is powered and must be shut off prior to work.
The warning light is highly reliable because it receives its power directly from the line and is not subject to external or secondary grounding. Other devices may be subject to user error or equipment failure. Voltmeters require the user to place metal pointers or connectors directly on wires; a false reading can occur if the connectors do not make full contact. Some battery-operated indicators do not receive power directly from the line and cannot work properly if the battery is old or damaged. Other devices have fuses to regulate power, but may give false negatives if a previous surge damaged the protector. Voltage indicators have a life expectancy of about seven years, at which time replacement is necessary.
With a high-voltage indicator installed directly at the source of the supply, an electrician can quickly assess what needs to be done prior to beginning work. No additional tools to check voltage are necessary. Some common uses for such indicators are in substation and transfer switches, high-resistance ground switches and electric motors for vehicles and equipment, according to Rockwell Automation, the parent company of electrical equipment manufacturer Allen-Bradley.
Consumers and electricians can purchase high-voltage indicators from home-improvement warehouses and electrical suppliers. Prices range up to $250 for a cabinet-ready device. As electricity can be fatal if handled improperly, all power must be shut off at the main breaker or disconnected altogether prior to the installation of a warning device.