How is a PAPI cable connected?
How the supply cable reaches the four units of a PAPI wing bar — taken in and out of each unit in turn, joined through the isolating transformer to the lamps, earthed, sealed and proved before the circuit is energised.
A PAPI is normally fed from the airfield series lighting circuit. The primary cable is brought to the wing bar and taken in and out of each unit in turn: cable in, through the unit's connection to its isolating transformer, cable out to the next unit and on to the rest of the circuit. The transformer then feeds the unit's lamps through a short secondary lead, and every unit is bonded to the earthing system.
Because the units share one series path, a poor primary connection can open the whole series circuit and put out every light on it, not just one unit, and a unit connected in the wrong place or order gives pilots a wrong slope cue. Seal every entry and connector, identify the cable and each unit, and prove the work before closing the pit. Treat the circuit as live until it has been isolated, locked out and proved dead.
- Isolate. Isolate the circuit, lock it out and prove it dead, and work to the airport's permit-to-work.
- Route. Bring the cable by duct to each unit's base can or pit through a sealed entry, with a slack loop inside.
- Connect. Connect cable in and cable out at each unit in turn with the specified connectors, keeping the unit order and identity as designed.
- Secondary. Join the isolating transformer to the unit's lamp circuit through the secondary lead and connector.
- Earth. Bond the unit, its mounting and the base can to the earthing system as the design requires.
- Seal, prove and record. Seal the connectors, prove insulation and continuity, mark each unit and record the connection.
Frequently Asked Questions
Q1What is PAPI cable connection?
It is the way the supply cable is joined to the units of a Precision Approach Path Indicator. The cable is brought by duct to each unit's base can or pit through a sealed entry, connected in and out of the unit in turn, joined through the isolating transformer to the lamps, earthed, sealed and proved before the circuit is energised.
Q2How are the four PAPI units connected electrically?
On a series-circuit PAPI the primary cable is taken to each unit in turn: cable in, through that unit's isolating transformer connection, then cable out to the next unit and on to the rest of the circuit. Some PAPI types are fed from a low-voltage supply instead, so the exact arrangement follows the design drawings and the manufacturer's data.
Q3Why does a PAPI unit have an isolating transformer in its connection?
The transformer separates the lamp circuit from the high-voltage series circuit. It also lets the series circuit stay closed, within the transformer's limits, if a lamp fails, so one lamp fault does not put the whole circuit out.
Q4Why is earthing part of connecting a PAPI unit?
The unit, its mounting and the base can are bonded to the airfield earthing system as the design requires, so fault current has a defined path and exposed metal does not stay at a dangerous potential. The bond is made and checked at the same time as the cable connection.
Q5What commonly goes wrong at a PAPI connection?
Water in a poorly sealed connector, a connector that is not fully mated, a cut or crushed sheath at the entry, a missing or loose earth bond, and units connected in the wrong order or to the wrong transformer. Many of these only show later as insulation faults or a wrong colour pattern.
Q6What is checked before a PAPI is energised?
That the entries and connectors are sealed, the cable is identified, the earth bond is in place, and insulation resistance and continuity are proved to the project's acceptance values. After energisation all four units are checked to be lit and the circuit current is verified. Treat the circuit as live until it has been isolated, locked out and proved dead.
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