What is a circuit selector switch (L-847)?
A switch that connects one constant current regulator to the series lighting loop you select — locally or remotely — so one regulator can serve several loops.
L-847 is the FAA designation for a circuit selector switch. It connects the output of a constant current regulator to one or more series lighting loops, chosen locally at the switch or remotely. One regulator can then serve several loops, for example loops for PAPI, stop bars or taxiway centreline lighting, and the same switch can be the interface that brings a standby regulator onto a loop.
The switch carries series-circuit current at high voltage, so the selection is changed with the regulator de-energised, following the manufacturer’s manual and the site procedure. In FAA AC 150/5345-5B the type number shows how many circuits it controls, L-847-1 to L-847-4 for one to four, with positions marked S1 to S4 as appropriate, and an optional door interlock switch is covered. A wrong position left selected, a worn contact or a bypassed interlock can leave a loop dark or a person exposed.
- Plan. Decide which loop the regulator is to feed, and confirm that the regulator and the loop suit each other.
- De-energise. Switch the regulator off before the selection is changed, as the procedure requires.
- Select. Operate the handle locally, or send the command remotely, to the wanted position, S1, S2 and so on.
- Verify. Check that the position shown matches the loop wanted, then re-energise the regulator.
- Check the output. Confirm the output current and that the selected loop lights as expected.
- Record. Log the position and the change, and tag the switch if a loop is isolated for work.
Frequently Asked Questions
Q1What is a circuit selector switch (L-847)?
L-847 is the FAA designation for a circuit selector switch. It connects the output of a constant current regulator to one or more series lighting loops, selected locally at the switch or remotely, so that one regulator can serve several loops.
Q2Why use a circuit selector switch?
It lets one regulator feed different series loops, for example loops for PAPI, stop bars or taxiway centreline lighting, without a separate regulator for each. It can also be the interface that brings a standby regulator onto a loop when an operating regulator is out of service.
Q3What do L-847-1 to L-847-4 mean?
In FAA AC 150/5345-5B the type number shows how many circuits the switch controls: L-847-1 controls one circuit, L-847-2 two, L-847-3 three and L-847-4 four. The positions are marked S1 to S4 as appropriate.
Q4Can the selection be changed while the regulator is on?
Not as a normal practice. The switch carries series-circuit current at high voltage, so the regulator should be de-energised before the selection is changed, following the manufacturer's manual and your site's isolation and permit-to-work procedure.
Q5What goes wrong with a circuit selector switch?
The wrong position is left selected after maintenance, contacts wear or burn, a door interlock is bypassed, a loop is not properly isolated for work, or the selection is changed while the regulator is energised. Check the position and the loop output after every change and record it.
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