The RGU-100B is a single-channel type B earth leakage monitoring and protection relay, working with an external toroidal transformer of the Circutor WGB range which is ordered separately. An earth leakage relay does not break the circuit itself. It measures the residual current — the difference between the current flowing out to a load and the current returning from it, which is only non-zero when some of it is escaping to earth — and signals a separate switching device to open. That split is what allows it to protect circuits far larger than any self-contained RCD can handle, and to be set for sensitivity and time delay independently of the breaker it drives. Sensitivity is adjustable from 30 to 3000 mA. It provides two output relays — alarm and pre-alarm — on pluggable terminals, and shares the three-module case and 195–265 V AC supply range of the CBS-400B alongside which it is normally specified: the RGU-100B where one circuit needs type B monitoring, the CBS-400B where four do. It is a type B device: it responds to all forms of residual current — AC, pulsating DC and smooth DC. Smooth DC leakage is the case that matters, because it magnetically saturates the core of a type A or type AC device and blinds it to any further fault. Type B is therefore the correct and often the mandatory choice downstream of frequency converters, rectifiers, EV chargers and photovoltaic inverters. Manufactured by Circutor (Spain) and supplied under the Function brand; the Circutor catalogue reference is RGU-100B. Supplied by cBallast on quote-to-invoice terms; lead time and freight are quoted per RFQ against the destination Incoterm.