I understand your concern and I appreciate your warning. However, please note that it is not my intention to "mess with the design": The interconnect wire is the interface exposed by manufacturers to interface with the alarms, I am merely looking for information that will allow me to correctly interface. Please also note that my question is directly motivated by safety: In my region it is increasingly popular to directly wire fire alarms into the home automation system, I believe that interfacing with stand-alone fire alarms is safer. Finally, please note that I am also a qualified electrician.