What is the series circuit fault path?
Where the current actually goes once a fault changes the loop — nowhere, through a shortcut, or round the loop with a leak to earth.
A healthy series circuit has exactly one current path: out from the CCR, through the primary cable and every isolation transformer primary in turn, and back. The fault path is the route current takes once a fault changes that loop.
An open circuit removes the path, so no current flows and every lamp goes out. A short circuit adds a shortcut that bypasses the rest of the loop, so fixtures beyond it go dark. An earth fault leaves the loop intact with a leak to earth, so lights usually stay on, until a second fault gives current a path through the ground. The circuit is isolated and proved dead before it is touched.
- Normal path. One closed loop through every transformer primary in series.
- Fault appears. A break, a bridge between conductors, or a leak to earth.
- Current re-routes. None, via the shortcut, or round the loop with a leak.
- Read the symptoms. All dark, partly dark, or all lit.
- Isolate and test. CCR off, locked out and tagged, proved dead and earthed at the work point (see Circuit Isolation); then continuity and insulation tests.
- Locate & repair. Split the loop, test each section, repair, then re-test.
Frequently Asked Questions
Q1What is a fault path in a series lighting circuit?
The route current actually takes once the normal loop has been changed by a fault. A healthy circuit has one route, out from the CCR through every transformer primary in turn and back. A fault removes it, shortens it or adds a leak to earth.
Q2What is the normal current path?
A single closed loop. The CCR drives a regulated current through the primary cable and every isolation transformer primary in series and back to the CCR. The same current flows through every part of the loop.
Q3Where does current go in an open circuit?
Nowhere. The break leaves no complete path, so current stops in the whole circuit, every lamp goes out and the CCR's open-circuit protection acts.
Q4Where does current go in a short circuit?
Through the shortcut. The fault bridges the loop conductors, so current skips the section beyond it. Fixtures before the short usually stay lit, and fixtures beyond it are bypassed and dark.
Q5Where does current go in an earth fault?
The loop stays intact and the circuit current keeps flowing round it, with a small leak to earth at the fault. Lights usually stay on. A second earth fault elsewhere can give current a path through the ground that bypasses the section between the two faults.
Q6How do you tell the fault paths apart in the field?
From the symptoms and then the tests. All lights out with an over-range continuity reading points to an open circuit. A dark section with very low resistance points to a short circuit. Lights still on with poor insulation resistance to earth points to an earth fault. The circuit is isolated and proved dead before any test.
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