Electrical Component

In order for you to get a response you can use, you first need to provide more valid information.

How high a voltage?,

How low a voltage?

AC or DC?

Do you want a fixed time delay? If so how long? Do you need to vary the delay?

Do you want the unit to sound an alarm, or communicate the alarm with something else? How intelligent do you want the alarm to be?

Do you want an individual component, or a device capable of your need?

What is the budget for this device?

Is it for commercial purposes?

If all that you wish to do is detect a high and low DC voltage condition, then a window comparator circuit will do. If you have an AC signal then some rectification and envelope following low pass filtering followed by the window comparator will work. For the delay alarm circuitry, a simple retrigeable one shot circuit like a 555 will work fine. Now if instead your looking for regulation of incoming power, then purchasing a UPS will be much cheaper and more practical than designing your own version.

Try Basler, If medium or high voltage use a PT

Son, you go right ahead and buy yourself one.