I need to monitor voltages of several DC circuits in a generator's contact box. Whenever the generator has an alarm, a LED is lit in the box and voltage on the line drops to zero. I successfully get readings using analogRead(), but whenever I connect a line it shorts to ground (which is shared between the contact box and the Arduino) and triggers the contact box's alarm. I need to devise a way to monitor voltages without triggering the alarm.
Each 5V line and ground from the generator has a bus bar termination point that connects to a point in the proprietary alarm box that houses the LEDs and buzzer. I connected Arduino ground to generator ground, then used A0-A7 on a Nano to connect to individual lines on the bus bar. Arduino VCC is connected to 5V regulated DC power.