Why would a bulb need a 30mA RCD for the bathroom?
An RCD is different from a fuse.  An RCD (residual current device) is known as a GFI (ground fault interrupter) or GFCI (ground fault current interrupting) device in the US.  

In non-technical terms, if the RCD device sees current "missing" on a circuit - more leaving than returning - it shuts down power, based on the idea that that missing current may be starting a fire or shocking someone.  

A 30ma RCD trips when 30ma of current is "missing."  In some locations, a 30ma trip is the maximum allowable for personal protection, prevention of electrical shock injuries.  

RCD / GFCI are especially important in bathrooms because people are in contact with water and pipes, which increases electrical hazards.