How is a PAPI transformer arranged?
The isolating transformer that feeds each PAPI unit — one per unit, its primary in the series circuit, placed in the base can, the unit or a nearby pit, and joined to the lamps by a short secondary lead.
On a series-circuit PAPI each unit is fed through its own isolating transformer. The transformer's primary is connected in the series circuit and carries the circuit current, while a short secondary lead feeds the unit's lamps, isolated from the high-voltage series path.
The arrangement decides what a fault does. A lamp or secondary fault normally stays with its own unit, but an open primary opens the whole series circuit and puts every light on it out, including all four units. Rate each transformer to its lamp load, keep to the unit order in the design, bond, seal and identify it, and prove it before the pit is closed. Treat the circuit as live until it has been isolated, locked out and proved dead.
- Match. Check the transformer rating against the unit’s lamp load and the design, from the rating plate and the manufacturer’s data.
- One per unit. Fit a transformer to each unit as designed and keep each unit on its own place in the series path.
- Place. Install it in the base can, the unit or the nearby pit the type and drawings require, supported and dry.
- Connect. Connect the primary into the series cable and the short secondary lead to the lamps, with the specified connectors.
- Bond. Bond the transformer case, can and unit to the earthing system as the design requires.
- Identify and record. Label each unit and transformer, seal the connectors, prove insulation and continuity and record the arrangement.
Frequently Asked Questions
Q1What is a PAPI transformer arrangement?
It is how the isolating transformers that feed the PAPI units are chosen, placed and connected. On a series-circuit PAPI each unit normally has its own transformer: the primary is connected in the series circuit and a short secondary lead feeds that unit’s lamps.
Q2Why does each PAPI unit have its own isolating transformer?
The series circuit carries a constant current at high voltage, so the lamps are fed through a transformer that isolates them from it. Each unit then has its own supply, and within the transformer’s limits the series circuit stays closed if a lamp fails, so one lamp fault does not put the whole circuit out.
Q3Where is the PAPI transformer located?
It depends on the PAPI type and the design. It may sit in the base can under the unit, inside the unit housing, or in a nearby pit with a short secondary lead to the unit. Follow the design drawings and the manufacturer’s data.
Q4How is the transformer rating chosen?
From the lamp load of the unit and the manufacturer’s data. An overloaded transformer runs hot and shortens its life. The circuit load the CCR must carry is the sum of every transformer and cable on that circuit, so the PAPI transformers are counted in the circuit design.
Q5What happens if a lamp or a transformer fails?
A lamp or secondary fault normally affects only its own unit. An open primary winding, or a poor primary connection, can open the whole series circuit and put out every light on it, including all four units. Some designs split the units between circuits for that reason, so follow the design for the site.
Q6What is checked on the transformers before energising?
That the rating plate matches the design, each transformer is connected to the correct unit, the primary and secondary connectors are mated and sealed, the case and can are bonded, and insulation and continuity are proved. Treat the circuit as live until it has been isolated, locked out and proved dead.
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