You can use a Relay, to connect power to the circuit. But the Relay only connects, if you apply the correct Voltage. Pretty simple way to do that is to use a Microcontroler or so. The simple plan:
You need some resistors, to split up the voltage. You ever heared of 20 Led Voltage Meter? You can use this circuit, changed just a bit, to detect, whether you exceed the voltage or not. 1 Led goes on means, you only drive 25 Volts into the resistors for example. 2 Led goes on means, you exceed the Voltage a bit, so do not allow current to flow. This is VERY simple and works for 100%. I will give you a simple circuit for the LED Meter, you can sure do something with it and change it for you to match perfect.
>Clicking right here<Just remember: Led 2 turns on, is a signal of 1, so if you want to use a Relay, you have 2 ways: Use one, that can switch between on1 and on2 or use a NOT-Gate, to turn the Relay on, if power is off.
The relay is inductive load, so make sure, you won't damage triggering components by overvoltage. Also, it has a delay, so it will not work, if you get a Squarewave HV Spike, cause it CANNOT trigger so fast. Than the Relay becomes useless. Maybe a HV and High Speed MOSFET etc. can do it better and react fast enough. But don't ask me about MOSFET's.
(Together with a current limitting resistor on the inpot and a Capacitor, you can smooth out the spike a bit and it propably won't be so bad for the circuit - this is, how I fixed the triggering spike of my 50$ Microphone - Resistor+Capacitor over the power in and it does not spike anymore.)
This are enough ideas for you, I think. Right?