On reaching response temperature of fusible plugs in fluid couplings, fusible element melts and operating fluid vents out. Ventilation of operating fluid causes environmental damages in addition to making extra costs.  To  avoid  this,  Thermal  Switching  Pin  (TSP)  is  used  which  consists  of  a  pin  soldered  flush  into  a threaded plug, so the pin remains in its initial position. If the coupling heats up and the melting point of fusible alloy is reached, fusible alloy melts and the pin is pushed out of the plug by centrifugal force or spring load depending on TSP position on fluid coupling. The released pin actuates finger of a limit switch positioned  in  a  proper  location which  activates  an  alarm or  shuts  off the  drive.  However,  at  least  one Fusible plug should be used in fluid couplings and TSP is used as an additional safety equipment. 
TSP plug is available in brass, Aluminium, Stainless Steel, Steel, gun metal, etc. and custom made models are possible based on customer’s demand in addition to RATA standard models.
TSP  accessories  are  commonly  screwed  into  the  outer  wheel  of  fluid  couplings  and  are  available  for different response temperatures such as 70, 95, 110, 125, 140, 160  and 180  degrees of  Celsius (Other response temperatures are available on demand). Response temperature of TSP is identified by a color coding similar to that for Fusible Plugs. 
WARNING: 
TSP should be replaced when the pin is pushed out of the plug. 
In case of inner wheel drive and driven machine blockage, TSP is no longer useful.