Possibly using a latching relay on a device test circuit
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A possible solution.

How it works:


With 50 dots on a 42 V supply there is &lt; 1 V across each DOT. This is not enough to light the LEDs in the opto-isolators.
When a DOT fails 42 V will be available for the opto-isolator and its series resistor. 3.9 k&Omega; has been chosen to limit the current to about 10 mA.
The opto-isolators' transistors can be wired to the monitoring controller which, when a failure is detected, will switch on the associate relay coil, energising the bypass contact.
The circuit should be able to respond in tens of milliseconds so given that you are testing for hundreds of hours it's unlikely that multiple simultaneous failures would occur.