What is a short circuit?
An unintended shortcut between the loop conductors — current takes it, and everything beyond it is bypassed and goes dark.
A short circuit is an unintended low-resistance path between the two conductors of the series loop, usually from failed insulation, a damaged cable or a wet connector. Current takes the shortcut instead of travelling round the rest of the loop, so the section beyond the fault is bypassed.
Fixtures between the CCR and the short usually stay lit, while those in the bypassed section, between the two points the short joins, go dark, and the CCR sees a much smaller load and holds its current at a lower voltage. That separates it from an open circuit (no current, every lamp out) and from an earth fault (a path to ground rather than between conductors). The fault point can overheat and damage the cable, so the circuit is isolated and proved dead before it is touched.
- Fault. Insulation fails or a connector floods, bridging the two conductors.
- Bypass. Current takes the shortcut and skips the rest of the loop.
- Partial outage. Fixtures in the bypassed section go dark; those on the CCR side of the short stay lit.
- CCR sees less load. It holds its current at a lower voltage and may alarm or trip.
- Isolate. CCR off, locked out and tagged; confirm the circuit is dead and earth the cable ends as the site procedure requires.
- Locate & repair. Split the loop, test each section, repair, then re-test.
Frequently Asked Questions
Q1What is a short circuit in airfield lighting?
An unintended low-resistance path between the two conductors of the series loop, usually from failed insulation, a damaged cable or a wet connector. Current takes the shortcut instead of travelling round the rest of the loop.
Q2What happens to the lights when the circuit is shorted?
Fixtures in the bypassed section, between the two points the short joins, go dark because no current reaches their transformers. Fixtures between the CCR and the short usually stay lit, so a partly lit circuit with a dark section is a typical sign of a short rather than an open circuit.
Q3How is a short circuit different from an open circuit?
An open circuit breaks the loop and stops current everywhere. A short circuit keeps current flowing but cuts the loop short, so only the section beyond the fault is lost. Resistance reads far lower than the baseline, not over-range.
Q4How is a short circuit different from an earth fault?
A short circuit is a path between the loop conductors themselves. An earth fault is a path from a conductor to earth, and the loop can keep working with one. Both start with the same cause, damaged insulation, and are separated by insulation and continuity testing.
Q5How does the CCR respond to a short circuit?
The CCR tries to hold its set current into a much smaller load, so output voltage falls. Depending on the equipment, the change shows as a low-load or load-change indication, or an over-current or protection trip. Alarm behaviour is set by the CCR design and its settings.
Q6How is a short circuit found?
With the loop isolated, locked out and proved dead, the loop is split at transformer pits and connectors and each section is tested. The section reading abnormally low resistance, or failing insulation resistance, holds the fault, and splitting it again narrows the location.
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