The RGU-10 B is the type B member of the RGU-10 family, and it is specified for circuits fed through variable-frequency drives. 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. It works with an external toroidal transformer of the WG-TB range, ordered separately. Sensitivity and time delay are both adjustable. 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. A variable-frequency drive is the textbook case: its rectifier front end can put a smooth DC component into any earth fault downstream, and a type A relay watching that circuit can be left saturated and silent by exactly the fault it was fitted to catch. Manufactured by Circutor (Spain) and supplied under the Function brand; the Circutor catalogue reference is RGU-10 B. Supplied by cBallast on quote-to-invoice terms; lead time and freight are quoted per RFQ against the destination Incoterm.