O

OHM
The unit of measurement for electrical resistance or opposition to current flow. Its symbol is the uppercase Greek letter omega.
It is named after the German physicist Georg Simon Ohm (1789-1854).
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OHM'S LAW
The all-important relationship between voltage (pressure), current (electron flow), and resistance. The current in an electrical circuit is directly proportional to the voltage and inversely proportional to the resistance. E=IR, or I=E/R, or R=E/I. Where E=voltage, I=current, and R=resistance.

OVERCURRENT
A current greater than the rating of a device or component. Protection is usually afforded by adding a fuse or circuit breaker to protect against overcurrent.
More info on Circuit Breakers.

OVERVOLTAGE
A voltage greater than the rating of a device or component. A serious situation that can cause catastrophic failure. An eventuality that is safeguarded against by fuses or circuit breakers.