A power supply system for operating fluorescent lamps in a hazardous atmosphere comprise a 1 kHz to 500 kHz constant current power supply (24 to 28, 31, 32) which feeds primary windings of transformers (3, 4, 5- - -N) which are connected together in series. Fluorescent lamps (11,12,13---Z) are connected across the transformer secondary windings. A voltage control circuit (29, 30) is connected to the power supply output to ensure that the amount of electrical energy supplied to the transformers cannot ignite the atmosphere under any normal or fault condition. Capacitors (6, 7, 8- - -X) are connected in series with the transformer primary windings to reduce the circuit reactance so that, for a given level of supply voltage, an increased number of lamps can be fed with starting voltage. Due to the substantially constant current which is fed througt the transformer primary windings, the situation cannot occur that the firing of one lamp prevents the others from firing. The transformers preferably include windings to feed heaters of the lamps. The lamps may be disconnected from the supply by removal of the transformer cores or by separation of the windings.