Fault signal switch 1 NO + 1 NC for circuit breakers according to UL 489 Fault signal switch SENTRON 5ST3 as an additional component for residual current protective devices and circuit breakers. These additional components can be mounted universally. They are easy and convenient to install and connect. Thanks to the standard touch protection, safety during installation is reliably guaranteed. Optimum technology for your safety - only from Siemens.