The D0900 is a detecting relay which can monitor up to 16 arc detectors in parallel through junction box D0400.
In case of an arcing fault, the relay generates a fast tripping pulse, less than 1 mSec., to the circuit breaker(s) supplying the installation. The arcing time is thus reduced to the mechanical opening time of the circuit breaker. The relay is solid state with complete isolation between input and output. The relay has energy stored in a storage capacitor charged with a DC voltage to trip the circuit breaker.

The TEST position on the relay is used to test the tripping without opening the circuit breaker.