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Fluorescent Lighting

Fluorescent lighting is used in the saloons of some Mk1 coaches as it is more efficient than incandescent lighting.  As fluorescent tubes are high voltage devices, an inverter is required to convert 24v DC to a high AC voltage to run the tubes.  The standard inverter used by BR will drive either 1 x 20w tube, or 2 x 20w tubes, or 1 x 40w tube.  All of these combinations are found on the SRPS Railtour set.

Inverter Installation Schematics

Case of Single Tube (either 20w or 40w)

Single Tube Schematic

Case of 2 x 20w tubes

Double Tube Schematic

The fluorescent tube setup, when inverter driven, does not require a starter switch as you find with mains installations. The inverter powers the heater filaments at either end of the tube until the tube strikes. When the inverter "sees" current flowing through the tube, the filament supply is switched off automatically.

Failure of installations is usually due to tube failure, although occasionally an inverter ceases to function. Tube failure can be confirmed by substitution, of course. It is useful to be able to confirm tube failure before first taking the (sometimes lenghty) walk to get a replacement tube. The filaments can be easily checked with a multimeter. The filaments should have a resistance of a few ohms (up to 30 in some cases). Any tubes with open circuit filaments at either end should be replaced as they are very unlikely to operate.

Experience has also taught us that lighting circuits which blow coach fuses almost always have problems with insulation. This usually occurs near the inverter where the input 24v supply "flying" lead has been rubbing against something metallic. The other problem which can cause fuses to blow commonly is filament lamps with loose glass bulbs. The two wires from the filament to the base contacts can become shorted together if the loose glass bulb is twisted round far enough.