High voltage warning
Voltage is the differential in electrical potential that makes electric charges move, and which was measured in volts. High voltage or megavoltage was the upper and extreme upper limits of the voltage scale.
While reviewing the new computer chip brought to him by his contractor, Jim, in 1996 for the new HyperPro PC, Henry Starling noted that the new design presented to him was "crap" because, among other things, "its voltage variance was out of spec." (VOY: "Future's End")
Signage featuring the warning "high voltage" was found aboard Earth starships such as in corridors aboard the ECS Fortunate and in the torpedo bay and outside of the shuttlebay aboard Enterprise NX-01. (ENT: "Civilization", "Fortunate Son")
In 2267, Spock described the Companion's attack on him as it having "imparted to me a rather quaint, old-fashioned electric shock of respectable voltage". (TOS: "Metamorphosis")