The ballast is a type of transformer. It raises the voltage to fire the fluorescent tube.
515 volts is 515 volts whichever way you look at it. So what meter do you use?
Why do you need to read the voltage after the ballast?
Often the ballast will say right on the label what the open circuit voltage is. Do you have a photo of the ballast?
Customer asked me to check power I checked the socket instead of taking it all apart there were no bulbs
Tell them there is power to the fixture, and that's all you can tell them without further work.
You are measuring the voltage with no load on the ballast, which is not what it would be with a tube in the circuit.
Can you replace a ballasted tube with an LED without removing the ballast?
If you are asking what I think you are asking, there is some hybrid lamps out there that will run off a ballast or directly connected to the line voltage and is very common to find LED lamps that only work when connected to the ballast .
True RMS meters can give you different readings particularly when dealing with distorted wave forms which can be expected with an electronic ballast. These ballasts don't necessarily have same voltage when a lamp is installed or at least after it has lit up. A high pulse can be needed with the fluorescent tubes to start the arc in the tube.
