What is the primary cable?
The insulated cable that forms the series lighting loop — it leaves the CCR, runs through every isolation transformer primary and returns, carrying the same constant current all the way round.
The primary cable is the cable that makes up the series loop. It leaves one CCR output terminal, passes through the primary side of every isolation transformer in turn, and returns to the other terminal. Because it is one continuous loop, the same constant current flows through the whole length of it.
It is typically a single stranded copper conductor with an insulation layer and an outer jacket, made to a cable specification such as FAA L-824. The voltage at the CCR rises with the length of the loop and the number of lights, so the insulation has to be rated for the highest voltage the circuit can reach. Do not confuse it with the secondary cable, the short lead from one transformer to its own lamp. A break opens the loop and puts every lamp out, and treat the circuit as live until it has been isolated, locked out and proved dead.
- Specify. Choose conductor size and insulation class for the circuit current and voltage.
- Install. Lay the cable in ducts or direct-buried, with bedding and warning tape.
- Connect. Joint it to each transformer primary lead and terminate both ends at the CCR.
- Test. Continuity and insulation resistance on the closed loop.
- Record. Log the readings against the commissioning baseline.
Frequently Asked Questions
Q1What is the primary cable in an AGL system?
The primary cable is the insulated cable that forms the series loop. It carries the constant regulated current from the CCR out through the primary side of every isolation transformer and back to the CCR, so it is the main run of the circuit.
Q2What is the primary cable made of?
Typically a single stranded copper conductor with an insulation layer and an outer jacket, made to a cable specification such as FAA AC 150/5345-7 (L-824). Conductor size, insulation type and voltage class are set by the circuit design.
Q3How is the primary cable different from the secondary cable?
The primary cable is the long series loop from the CCR that feeds every transformer primary. A secondary cable is the short lead from one isolation transformer's secondary to its own lamp, so it carries lamp current for a single fixture, not the whole loop.
Q4Why does the primary cable need to be rated for a high voltage?
A series circuit holds a constant current, so the voltage at the CCR rises with the length of the loop and the number of lights. The cable insulation must be rated for the highest voltage the circuit can reach, not just the normal operating level.
Q5What happens if the primary cable is damaged?
A break opens the loop and every lamp on that circuit goes out. Damaged insulation can instead cause an earth fault or leakage to ground, which is found with insulation resistance testing and may not extinguish the lights at first.
Q6How is the primary cable checked?
With the circuit isolated, locked out and proved dead, by visual inspection and then continuity and insulation resistance tests on the loop, comparing the readings with the commissioning baseline.
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