a valve casing defining a chamber and a valve seat that divides the chamber into a upstream side in communication with a water supply and a downstream side in communication with a plurality of sprinklers,

a clapper valve arrangement comprising a clapper body adapted to engage the valve seat and being operable between a normally-closed position wherein fluid flow communication is prevented between the upstream and the downstream side, and a fully open position wherein fluid flow communication is permitted between the upstream and the downstream side, the chamber being in communication with a conduit defined by a stack, and

a probe arrangement operatively associated with the stack, the probe arrangement being positioned at a predetermined level along the stack relative to the clapper valve body, and including a probe; the probe including:

a conductive threaded portion in conductive communication with a conductive medium through the stack for introducing an electrical signal through the stack and into the conductive medium;

a conductive electrode portion for detecting the electrical signal in the conductive medium when the conductive medium contacts the probe arrangement at a predetermined level; and

an electrically insulating stand-off sleeve between the conductive electrode portion and the conductive threaded portion

wherein an alarm is activated when the conductive medium in the stack contacts the probe arrangement at the predetermined level.