The Siemens SENTRON 5SM2121-0 is a residual-current device (RCD) unit designed as a plug-in accessory for the 5SY series miniature circuit breakers. It provides earth leakage protection for single-phase AC circuits at 230 V, rated 16 A with a 10 mA trip sensitivity — a high-sensitivity threshold suited for protecting individual circuits against fault currents that could otherwise go undetected by a standard MCB. As an instantaneous (type AC) device, it responds to sinusoidal AC residual currents without intentional delay, which means it clears ground faults as they occur but does not discriminate against nuisance tripping from transient surges on the line.
It clips directly onto the right-hand side of a compatible 5SY MCB — the fastening method is REG (rear-edge guide), so the combination forms a single 2-pole assembly that shares the busbar connection. The IP20 rating applies once the distribution board is installed with conductors connected; the assembly is not sealable, so it remains accessible for periodic testing via the built-in test circuit (minimum supply voltage 195 V for the test equipment).