in lieu of the set screw the plate 10 is fitted to the coupling 4 by inserting the mounting feet 11 in the grooves 8 following which the ribs 9 are deformed to engage and interlock with the feet the passage 6 normally is sealed by a valve or plug 16 of such size as to be accommodated in the passage an o ring 17 carried by the valve 16 forms a seal the valve 16 also includes a flange 18 which normally overlies and rests upon the face 7 of the fitting 4 although the valve 16 normally seals the passage 6 it is capable of being unseated by the pressure of water in the line 1 and in the passage 6 it is necessary therefore to provide means for holding the valve in its passage sealing position until such time as the passage should be opened such holding means comprises a strut assembly 20 composed of a pair of members 21 and 22 arranged end to end but spaced by a gap g in which is sandwiched a wafer 23 formed of eutectic material the members 21 and 22 are formed of thermally insulating material such as bakelite epoxy and the like whereas the wafer 23 is formed of a eutectic material having excellent heat transfer properties such as an alloy of bismuth tin and lead the material from which the wafer is made has a predetermined critical temperature at which it liquifies typically such wafers liquify at temperatures of between 135 u00b0 f and 500 u00b0 f the member 21 has a reduced diameter shank 24 which forms a shoulder 25 encircling the insulating member 22 the shank 24 and spanning the gap g is a heat collecting sleeve 26 formed.