RCDs are quite expensive, so its common in the UK for a single RCD to cover several circuits. The most common configuration is to have two RCDs each of which cover half of the circuits. Here's a typical consumer unit...

http://www.ctm.co.uk/gallery/Electric/Electric_photo_gallery...

RCDs detect the slightest earth leak, so they are far more likely to trip than the MCB or the fuse on the plug, which only detect a sustained over-current.