Building A Voltage Threshold Indicator with Wide Voltage Window (0-600V)
The obvious answer is to use a resistor divider to scale down the voltage, then compare that to a proportionally lower threshold voltage.

You could use something like a TL431 as the threshold detector, or a comparator that compares to a threshold divided down from a regulated supply, or various other things.

Let's say you have a comparator that runs from 5 V.  The maximum of 600 V in should then result in about 5 V out after the divider.  Let's pick 2 M&Omega; for the top resistor.  That means the bottom resistor should be 16.8 k&Omega;.  The output signal from the divider will be the input voltage divided by 120.

60 V in will therefore yield 500 mV out of the divider.  The output of the divider goes into one input of a comparator, and a 500 mV reference voltage into the other.

Note that you have to consider the power dissipation of the top resistor of the divider.  Worst case, it will have 595 V across it.  That means this 2 M&Omega; resistor will dissipate 180 mW.  It should therefore be at least &frac14; W resistor.  It also needs to be rated to withstand 600 V.

Sometimes in cases like this it's easier to put multiple smaller resistors in series, both for the lower power dissipation and voltage for each individual resistor.