What is circuit impedance?
The total opposition to alternating current around a series lighting loop — resistance and reactance combined, setting the voltage the CCR must supply.
Circuit impedance is the total opposition to alternating current in a series lighting circuit. It combines resistance, which turns power into heat, with reactance, which comes mainly from the magnetic fields of the isolation transformers and the cable. The two are 90° apart, so they combine as Z = √(R² + X²), not by simple addition.
The CCR holds the current constant, so the voltage it supplies is V = I × Z: a higher impedance means a higher output voltage. Reactance also makes the CCR supply more apparent power (kVA) than the real power the lamps use. Impedance is the whole picture; circuit resistance is only its heat-producing part.
- List. Every transformer, fixture and length of cable on the loop.
- Add resistances. Cable conductor resistance plus the load resistance of each transformer and lamp.
- Add reactances. Transformer and cable inductive reactance from the data sheets.
- Combine. Z is the square root of total R squared plus total X squared, not R plus X.
- Find the voltage. Multiply Z by the circuit current to get the CCR output voltage.
- Check the CCR. Confirm voltage and kVA are within its rating, with margin.
Frequently Asked Questions
Q1What is circuit impedance in a series lighting circuit?
The total opposition to the alternating current in the loop. It combines the resistance of the loads and cable with the reactance of the isolation transformers and cable, and it is what the CCR actually sees when it drives its set current.
Q2How is impedance different from resistance?
Resistance is the part that turns power into heat and is effectively the same for DC and AC at mains frequency. Reactance comes from the magnetic fields of the transformers and cable and stores and returns energy instead. Impedance is the combination of the two.
Q3Why can't resistance and reactance just be added?
They are 90 degrees out of phase, so they combine like the sides of a right-angled triangle. All the resistances are added together, all the reactances are added together, and the two totals are combined as Z equals the square root of R squared plus X squared.
Q4What creates reactance in an AGL circuit?
Mainly the isolation transformers, through their magnetising and leakage inductance, and to a lesser extent the inductance of the long series cable. Cable capacitance is usually small by comparison, though it can matter on very long circuits.
Q5How does impedance affect the CCR?
The CCR holds the current fixed, so by V = I x Z a higher impedance makes it supply a higher output voltage. The reactive part also means the CCR supplies more apparent power than the real power used, which lowers the power factor.
Q6Can an ordinary ohmmeter measure circuit impedance?
No. A DC ohmmeter reads only resistance. Impedance depends on the AC frequency and has to be measured or calculated with AC methods, so the manufacturer's transformer data and the design calculation are used instead.
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