What is an auxiliary relay cabinet (L-841)?
A lockable cabinet of 20 pilot relays and a DC supply, so a low-voltage control signal can switch an airfield lighting control circuit.
L-841 is the FAA designation for an auxiliary relay cabinet. It is a lockable steel enclosure holding a DC power supply, control circuit protection and 20 plug-in pilot relays. A low-voltage control signal, a closed switch on a control cable, completes a DC loop through a relay coil, and the relay’s contacts then switch a 120 V AC control circuit at the cabinet. A small switch, far away, can therefore control equipment it could never switch itself. The specification is titled for pilot control of airport lighting circuits, where pilots switch the lights by radio, and a design may use the same relays behind other control points, such as an L-821 panel (Entry 11).
Each relay is lettered A to T on the cabinet, and a relay schedule inside the door records what every relay does. The cabinet is wired at the factory, so in the field only the external control circuits and the 120 V AC input are connected. An optional door interlock switch removes the AC input when the door is opened. Treat the inside as live: the 120 V AC input and the load circuits share the enclosure, so isolate before working. A relay left in the wrong state, a coil loop that is open or shorted, or a relay schedule that is blank turns a simple cabinet into a puzzle.
Powered by the cabinet’s 48 V DC supply and wired in No. 19 AWG. It is built to work over long control cables; the qualification test simulates a cable 3 miles (4.8 km) long.
Two sets of contacts per relay, rated for 10 A, wired in No. 12 AWG to the load circuit, with arc suppression across each pair of contacts.
- Command. A remote switch, such as a panel switch or a pilot-control receiver, closes.
- Coil loop. Current from the 48 V DC supply flows through the control cable and the relay coil.
- Pull in. The relay closes its contacts. When the loop opens, the relay drops out and they open again.
- Switch the load. The contacts close the 120 V AC control circuit, and the equipment responds.
- Confirm. Check that the right relay moved, using its letter and the relay schedule, and that the equipment responded.
- Record. Keep the relay schedule up to date, so that each letter still matches its job.
Frequently Asked Questions
Q1What is an auxiliary relay cabinet (L-841)?
L-841 is the FAA designation for an auxiliary relay cabinet. It is an enclosure holding a DC power supply, control circuit protection and 20 pilot relays, used to control airfield lighting circuits from a low-voltage control signal. It is built to FAA AC 150/5345-13.
Q2What does an auxiliary relay cabinet do?
It lets a small, low-voltage control switch, which may be far away, switch a 120 V AC control circuit. The switch closes a DC loop through a relay coil, and the relay's contacts then close the load circuit. The specification is titled for pilot control of airport lighting, and a design may use the same relays behind other control points.
Q3How are the relays identified?
Each of the 20 relays has a letter, A to T, painted on the cabinet beside it. The terminals follow the same letters: the coil of relay A connects to A+ and A-, and its contacts to A1 to A4. A relay schedule on the inside of the door, with 20 lines from A to T, records what each relay does and is filled in on site.
Q4What is the door interlock switch?
It is an optional switch, fitted if the user specifies it. It is designed to carry 5 A at 120 V AC and removes the 120 V AC input to the cabinet automatically when the door is opened. Even so, follow the site's isolation procedure before working inside, because circuits connected to the relay contacts may be fed from elsewhere.
Q5What goes wrong with an auxiliary relay cabinet?
A relay stuck or left in the wrong state, burnt contacts, an open or shorted coil loop, a control cable so long that the coil voltage drops too low, a failed DC supply or fuse, and a relay schedule that is blank or out of date. Check the DC supply, then the loop, then the relay and its contacts, and keep the schedule current.
Q6Is the L-841 only an FAA item?
Yes, the designation is FAA. L-841 is defined in FAA AC 150/5345-13B, and ICAO Annex 14 has no equivalent cabinet. Aerodromes elsewhere use auxiliary or interface relay panels set by the project specification or the national regulator. The principle is the same, a low-voltage DC loop switching a control circuit, but voltages, ratings and wire sizes follow the local supply (for example 230 V AC in India) and the manufacturer’s data.
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