Retarded time accounts for the finite speed of light in electromagnetic calculations. When measuring a field at a specific distance from a source, the field observed right now was actually generated at the retarded time in the past. It is a fundamental parameter in calculating retarded potentials for antennas, radar, and long-distance transmission, ensuring that cause always precedes effect in the wave equations.
You can't ignore the retarded time delay when analyzing the transient response of a transmission line that long.
In power systems operating at 50 or 60 Hz, the wavelength is so long that retarded time effects are usually negligible, but they become critical in high-frequency switching or lightning transient analysis.
Related terms
Advanced time
A theoretical time coordinate used in electromagnetic field calculations to describe field states in the future.
Electric field
A region of space where an electric charge experiences a force.
Space permeability
The physical constant that defines how easily a magnetic field can form in a classical vacuum.