Why so many specs for RCD vs US GFCI (or RCDs?)
There are many different classes of RCD's and GFCI's.   The 5mA 5 second test is the most common residential for bathroom grounded outlets or wherever secondary earth paths exist. ( see AC-2)

The selection criteria is based on location between source and load, country laws , and to avoid nuisance trips yet to prevent:


equipment failures (AC-4), 
fires  &amp; human injury(AC-3), 
human shock (AC-2)
higher risks for medical instrument use  (AC-1)




The typical HIPOT leakage test in America is &lt; 100uA using HVDC to eliminate the AC filter earth current but where 500uA is allowed per unit which comes from the EMI ground noise PI filter current.  Up to 2.5mA earth current is allowed per system that contains many PSU's.

Both RCD's and GFCI's measure the differences  in line current to magnetically activate a relay. The construction depends on the current rating and residual difference threshold and thus the acceleration which affects the response time.  Higher thresholds or time delays are selected to avoid nuisance trips such as from transient voltage suppressor currents to to earth.

Although a 5mA threshold device is more protective than 30 mA device, in residential areas with high humidity and dust, the device may false trip until the moisture or contaminants that cause the internal leakage to earth evaporate or are heated away.    

Residential RCD's with 30mA threshold may be common but in America  GFCI's now must have a 5mA/25ms threshold on outlets near moist connections to earth. Laundry room, unfinished concrete basement, outdoors, kitchen, bathroom.  Old houses with 2 wire outlets may use these in place or rewiring the house with 3 wire if labelled as "no equip. ground".