Impedance, Z, is the complex ratio of voltage to current in an AC circuit, made up of resistance R and reactance X from inductance and capacitance, with a magnitude equal to the square root of R squared plus X squared. It changes with frequency, so a cable has a characteristic impedance for signals and a series impedance for power flow. Engineers quote transformer impedance as a percentage, use source impedance to judge fault levels, and match impedances in RF and data links.
That coax is 75 ohm characteristic impedance, plug it straight into a 50 ohm analyser without a matching pad and the readings are junk.
Also heard as
- Z
Related terms
X/R ratio
Ratio of reactance to resistance in the fault path, which sets how large and slow the DC offset in fault current is.
Earth fault loop impedance
Total impedance of the path a fault current takes from the source, through an earth fault and back again.
Skin effect
Tendency of alternating current to crowd toward the surface of a conductor as frequency rises.
Per-unit system
Shorthand that states voltages, currents and impedances relative to chosen base values, so 1.0 means nominal.