What is the series circuit current path?
The single closed loop the current follows — out from the CCR, through every transformer in turn, and back again.
The current path of a series lighting circuit is one closed loop. Current leaves the CCR output terminal, runs through the primary cable and the primary winding of each isolation transformer in turn, and returns to the CCR return terminal. The lamps sit on the transformer secondaries and are supplied by induction, not by the loop current directly.
Because there is only one path, the same current flows at every point, and a break anywhere stops it everywhere. The earth is not part of the intended path; a connection to earth is a fault. The voltage across the loop is covered under series circuit voltage.
- Start. At the CCR output terminal.
- Follow. Trace the primary cable to the first isolation transformer.
- Continue. Pass through each transformer primary in turn, joint to joint.
- Return. Follow the cable back to the CCR return terminal.
- Check earth. Confirm by insulation-resistance test that the loop is insulated from earth along its length.
- Prove it. Test continuity around the whole loop with the circuit isolated.
Frequently Asked Questions
Q1What is the current path in a series lighting circuit?
A single closed loop. The current leaves the CCR output terminal, runs along the primary cable through the primary winding of each isolation transformer in turn, and returns along the cable to the CCR return terminal.
Q2Why does one break stop every fixture?
There is only one path, so a break anywhere in it stops the current everywhere on the loop. The fixtures further along do not have a separate way to receive current, which is why a break is an open-circuit fault for the whole circuit.
Q3Do the lamps carry the loop current?
No. The loop current passes through the primary windings of the isolation transformers. Each lamp is on the secondary side and is supplied by induction, so a failed lamp does not by itself break the primary path.
Q4Is the earth part of the current path?
No. The loop is insulated from earth and the current is meant to return through the cable. An unintended connection between the loop and earth is an earth fault. A single earth fault usually leaves the loop working, because the current still returns through the cable, but it removes the insulation margin. A second earth fault elsewhere can give the current a path through the ground that bypasses the section between the two faults.
Q5Is the current the same at every point on the loop?
Yes. In a single series path the same current flows through every part of it, and the CCR holds that current fixed. What changes from point to point is the voltage, covered under series circuit voltage.
Q6How is the current path checked?
With the circuit isolated, a continuity test is made around the whole loop to confirm the path is complete from the CCR output back to its return terminal. See circuit continuity for the method.
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