Testing voltage continuity using a beeper?
The simplest I can think of would use a relay, push-button switch and a buzzer.  If you can't find a suitable 48V relay and buzzer, you may need a voltage regulator.

Push SW1 to start the buzzer.  That will latch the relay on, unless the voltage drops out.  Then the buzzer will stop until you re-start it with SW1 again.







simulate this circuit &ndash; Schematic created using CircuitLab