What is circuit resistance?
The total opposition to current around a series lighting loop — every load plus the cable, added together, setting the voltage the CCR must supply.
Circuit resistance is the total opposition to current in a series lighting circuit: the resistance of every load on the loop plus the resistance of the series cable and its joints. Because the loop is a single path, these simply add up. The CCR holds the current constant, so the voltage it supplies follows V = I × R — more resistance means more voltage, never more current.
Cable resistance rises with length, a smaller conductor and a higher temperature. A poor joint or corroded connector adds extra resistance at one point, seen as unexpected voltage drop and heat. Resistance is not insulation resistance, which is measured in megohms to earth, and it is only part of circuit impedance, which also includes transformer reactance.
- Isolate. De-energise and isolate the circuit before any measurement.
- Measure. Read the loop resistance end to end with a suitable low-resistance tester.
- Correct. Allow for cable temperature, since copper resistance rises as it warms.
- Compare. Check against the design value and the commissioning record.
- Investigate. A rise points to a bad joint, connector or termination; find it section by section.
- Record. Log the value so the next test has a baseline.
Frequently Asked Questions
Q1What is circuit resistance in a series lighting circuit?
The total opposition to current around the loop. It is the resistance of every load on the circuit plus the resistance of the series cable and its joints, and it determines how much voltage the CCR needs to push its set current through the loop.
Q2Why do the resistances simply add up?
A series circuit is one single path, so the same current passes through every part of it in turn. The resistance of each load, each length of cable and each joint is met one after another, so the total is the sum of them all.
Q3How does resistance affect the CCR?
The CCR holds the current fixed, so by V = I x R a higher circuit resistance makes it supply a higher output voltage, not more current. The power lost in the cable also rises with resistance, as I squared times R, and that is released as heat.
Q4What makes the resistance of a cable rise?
Greater length, a smaller conductor size and a higher temperature all raise it. A poor joint, loose connector or corroded termination adds extra resistance at one point, which shows up as unexpected voltage drop and local heating.
Q5How is circuit resistance different from insulation resistance?
Circuit resistance is the low resistance of the conductor path along the loop, typically ohms to tens of ohms depending on loop length and the number of transformers, and a rise in it is a warning. Insulation resistance is the very high resistance, in megohms, between the conductor and earth, and it is the value that should stay high.
Q6How is circuit resistance different from circuit impedance?
Resistance is the part of the opposition that turns power into heat. On an AC circuit the transformers also add reactance, and resistance and reactance together make up the circuit impedance, which is what the CCR actually sees.
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