ISOLATING TRANSFORMERS FOR AIRFIELD LIGHTING

PREFACE

There are two types of feed for lighting fixtures located along a runway - parallel or series feed. As a circuits of a runway are several kilometers long with the parallel feed, a progressive voltage drop occurs from one light to the other, even with wide cable ' sections. This results in expensive cable and a heterogeneous luminous intensity of lights. Additionally, low-voltage prefocus lamps still require a step-down transtormer.
A series feed requires the utilization of a constant current regulator, isolating transformers and lamps specially designed for rated current of 6.6 A or 20 A. All lighting fixtures are now designed for use these lamps. Figure 1 shows an alternate feed of runway lights by series circuits.

Sl.1

The transformers have a salient features:

• protect personnel by insulating the high voltage primary circuit from the secondary supplying light,
• high reliability thanks to systematic tests at all manufacturing stages,
• self-protected: operates indefinitely at full load, in short-circuit or in open circuit, at ambient temperatures ranging between -55°C and +65°C,
• the cables are an integral part of the transformer ensuring perfect watertightness,
• The coating is acid-, alkali-, and oil-resisting and also resists other chemical agents, as well as ozone and U.V. May be immersed, buried or exposed to the sun and to bad weather,
• maintaining the continuity of the primary circuit when the light is out of service
• simple and quick repairs through the easy disconnection of the doubtful cable sections
• operates either at 50 Hz or 60 Hz on series circuits rated up to 5000 V.

The secondary of each isolating transformer is normally connected to a single light by means of a secondary two-core cable or 2 single-core cables; but it possible to connect several lights when they are close to one another with a single transformer secondary. This arrangement, which is sometimes more economical, should be used with discrimination, for results in the use of more powerful transformers giving, in open circuit, a higher voltage, which may be dangerous for maintenance personnel.

Sl. 2

The transformer is completely moulded in a layer of high standard elastomere which covers the two primary single-core primary cables and the secondary two-core cable. Toroidal magnetic core made of low-loss, grain-oriented laminations, insulated by a nylon sheath.
The transformer complies with the FAA specification AC 150/534547.
The primary leads, one which ends with a factory moulded singlepole plug, FAA L-823, Style 2 and the other with a factory moulded single-pole receptacle, FAA L-823, Style 9.
The secondary lead, which incorporates a factory moulded twopole receptacle, FAA L-823, Style 7.

Sl 3.

The isolating transformers include segments of primary and secondary cables equipped with sealed impervious connectors. They can be buried near the light but it is preferable for them to be housed in an inspection man-hole such as the one in Fig. 3.
The primary cable is connected in segments from one transformer to another. It is buried in a trench joining the inspection manholes, between two layers of sand, out of reach of stones or any objects which could damage it.
The 3 or 5 kV insulation cable may or may not incorporate a concentric shield.
An unshielded cable will generally have to be protected from the effects of the weather by means of a bare equipotentional earth wire located about 10 cm, above it in the trench. The shield and equpotentional wire are connected with the regulator ear connection. On cables with a shield, the shield is insulated by the protective sheath, and should be connected with earth rods about every 300 m. The secondary cable connects the light to the transformer by means of a moulded plug or a KIT model to be mounted on the spot. This connection, running along a trench, in a wireway in the pavement or in a tube, can be made by means of a two-core cable or two segments of single-core cable joined, on the transformer side, to a KIT plug, Style 4. These secondary equipment are 1 kV isolated but the practical utilization voltages scarcely exceed 135 V.

MAIN CHARACTERISTICS

* Values for primary winding supplied by sine-wave voltage at nominal current.