A thin, fragile pipe loop wraps the house and stays full of pressurized water. That pressure holds a spring-loaded gate up, out of the service line. When fire melts the loop or the house collapses onto it, the pressure is gone. The spring drives the gate down and water to the house stops. No power, no electronics in the shutoff path.
The loop is a dead end fed from the service line, so it sits at line pressure. That pressure on the piston beats the spring and holds the gate up.
Fire softens and bursts the thin loop, or the collapsing house crushes it. A small trickle escapes, faster than the feed orifice can refill, so pressure falls.
Chamber pressure falls below the spring's trip point. The stored spring drives the piston and blade down across the service line.
Cap or repair the loop, then pull the reset handle. It lifts the piston off the feed port so pressure can rebuild and re-cock the spring. Let go: water pressure now holds the gate open.
Hosing the house does not trip it: the loop has no outlet, so hose flow never drains it. Only a breach in the loop does.
Tube / Tube_2service lineboxgate housing on the lineTube_1_2pressure chamber (cut in half)COIL_SPRINGclosing springCylinderpistonCube_1gate bladeSweep + Tube_1feed tap from the lineTube_3line out to the house loop—reset handle: shown in the schematic, not yet in CAD v3Animation 0–2 s: pressure lost, spring extends, blade drops. 2–4 s: reset, pressure lifts the piston back. Drag to orbit, scroll to zoom.