70 watt sodium vapour lamp

Is the capacitor in series with the lamp? (i.e. current limiting) Or in parallel (i.e. noise suppression?)

Have you checked the supply voltage?if it is in parallel and on independent installation? Does it have an ignitor? What is the operating frequesncy of the ignitor?

all connection are in parallel. voltage is 240 vac. ignitor is for 50 HZ.

capacitor is in parallel.

Something is still missing. There must be a starting mechanism that allows a high open circuit voltage to initiate the arc. Perhaps the parallel capacitors from the 150 lamps are influencing your circuit. But, that would imply a poor power supply connection. Try removing the capacitor from the 75 watt circuit to see if it will start.

Once the lamp starts, the voltage necessary to sustain the arc increases as it heats up, reaching a maximum point. However, as a HPS lamp ages, this maximum voltage increases, eventually reaching a point where the ballast can no longer supply this maximum voltage. The result is once the lamp starts, it heats, the maximum voltage the ballast can supply is exceeded, the lamp goes out and cools, and then the whole cycle starts again. The lamp cycles on and off over a period of 10 to 15 minutes, and if the lamp is not changed, this cycling eventually kills the starter.

It sounds like your problem may be the result of a combination of two separate problems; an aged lamp and excessive voltage drop in your lighting system. Another possibility is the starter in the fixture may be malfunctioning.

It sounds like you have a voltage drop problem.

As you have told the Light Fittings are connected in Parallel to single source.The 70Watt Lamp connected along with other Lamps may not be getting required voltage to Ignite.Check the Voltage at Lamp terminals when connected along with other Lamps and Voltage at Lamp terminals when connected Indipendently.In case any drop in Voltage while connected along with other Fittings the Lamp may be tripping due to ageing.Change the Lamp and check.

Actually guys, if the supply voltage was low then there should be problems with the 150 watt lamps. I suspect that it is a weak igniter ckt or an old lamp. With all of the other capacitors effectively in parallel, the igniter ckt may not have enough voltage spike to initialize the arc. The capacitors will suppress some of the spike needed to start the lamp but if they are damaged or the ballast is weak, all bets are off for reliable starting.

Try swicthing the 70 w with a time delay ie after the 150 lamps start glowing.