Different transformers
CCR, fixture, sign and approach-light: four transformers that work on the same principle but differ in where they sit, what they feed and what happens when they fail.
An AGL series circuit uses transformers at both ends. At the power end the CCR transformer feeds the whole circuit. Out in the field, isolating transformers connect each light or sign to the loop. Three common field types are the fixture transformer, the sign transformer and the approach-light transformer. They differ in what they feed, where they sit, how they are rated, and what a fault does.
In every case the transformer keeps the load electrically apart from the loop. In the field types the primary is in series and carries the circuit current, and the secondary feeds one load through a short lead, so a lamp or sign fault normally leaves the loop closed. The exception to watch is an open primary, which can open the circuit. The CCR transformer is different in scale: it supplies the entire circuit, so its failure takes out the whole circuit.
One per circuit, at the power end. It isolates the field cable from the supply and builds the voltage the loop needs, rated in kW.
Many per circuit, out in the field. Each connects one load to the series loop and keeps it electrically apart, rated in watts.
| Aspect | CCR transformer | Fixture isolating | Sign transformer | Approach-light |
|---|---|---|---|---|
| Does | Isolates the field cable from the supply and builds the voltage the loop needs for its set current | Connects one light to the series loop and keeps its lamp or LED unit isolated | Connects one sign to the series loop and keeps the sign isolated | Connects one approach light to the series loop and keeps its lamp isolated |
| Where | Inside the CCR, at the power end of the circuit | Below ground in a base can or pit beside the light | At or near the sign, in its base or a nearby pit, as designed | Below ground in a can beside the base of the stem |
| Primary side | The supply, for example 230 V single-phase or 415 V three-phase | In series with the loop; carries the circuit current | In series with the loop; carries the circuit current | The series loop passes through the can into the primary |
| Secondary side | To the series circuit; a tap sets the most voltage it can develop | To the lamp or LED unit by a short lead | To one sign through a short cable | A short lead up the frangible stem to the lamp |
| Rating | In kW, set by the load on the circuit | A set size in watts, matched to the lamp or LED load | A set size in watts, covering the sign load | A set size in watts, matched to the lamp load |
| How many | One per CCR | Normally one per light | One per sign | One per light |
| If it fails | The whole circuit loses its supply | A lamp or secondary fault darkens that light; an open primary can open the circuit | A sign or secondary fault leaves the loop closed; an open primary can open the circuit | A failed lamp darkens only that light; an open primary can open the circuit |
| Watch points | Tap setting and headroom; never open a live series circuit | Rating, sealed connectors, insulation, water in the can or pit | Rating against the sign load, connectors, secondary cable | Rating, connectors, can seal and drainage, the stem lead |
What to check on transformers
Rating on the nameplate
Check the rating against the load it feeds and the primary against the circuit current, from the nameplate and the design.
Connectors and seals
Check the sealed connectors are intact, correctly mated and dry. A damaged or wet connector is a common cause of faults.
Insulation
Test insulation to the limits in the procedure and record the value, so a falling trend can be seen over the years.
Can or pit
Keep the can or pit dry, drained and sealed. Water in the can attacks connectors and leads long before the transformer fails.
Secondary lead and load
Check the secondary lead and the load match the design, and that the lead is undamaged where it runs up a stem or into a sign.
Isolate before work
Isolate and lock out the circuit first. Unplugging a primary connector opens the loop, and the secondary side should be treated as live.
Frequently Asked Questions
Q1What are the main transformers in an AGL series circuit?
Four common ones are the CCR transformer, the fixture isolating transformer, the sign transformer and the approach-light transformer. The CCR transformer sits at the power end and feeds the whole circuit. The other three are isolating transformers out in the field, each connecting one load to the series loop.
Q2What does the CCR transformer do?
It sits between the thyristor stage and the series circuit. It isolates the field cable from the supply and builds the voltage the loop needs to carry the set current. A secondary tap sets the most voltage the CCR can develop, so it also sets the headroom for the circuit.
Q3What does an isolating transformer do?
It connects one light or sign to the series loop. Its primary is in series with the loop and carries the circuit current, while its secondary feeds only that load through a short lead. The two sides are kept electrically apart, so a lamp or sign fault does not stop the rest of the circuit.
Q4How is a sign transformer different from a fixture transformer?
The principle is the same, since both are series isolating transformers. The difference is the load and the place. A sign transformer feeds one sign through a short cable and is rated for the sign load, which can be larger for a bigger or double-faced sign. A fixture transformer feeds one light, usually from a can or pit beside it.
Q5Why is the approach-light transformer arranged differently?
Approach lights stand on frangible stems, so the transformer is kept below ground in a can beside the stem. The loop passes through the can, and a short secondary lead runs up the stem to the lamp. This keeps the stem light and protects the transformer from weather and traffic.
Q6What limits and requirements apply?
Ratings, ratios, tolerances and insulation levels come from the standard, such as IEC 61823 or the FAA specifications, the airport's circuit design and the manufacturer's data sheet. Always use the requirements for your site rather than a generic list, and isolate and lock out the circuit before working on any transformer.
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