I need some help from an expert as I have been researching and cant find the answers online.

Recently the switch pull cord snapped when trying to turn on my power shower. Its fairly old and the string had just worn. I bought a replacement switch and decided to change the switch myself (im not an electrician but DIY'er)  as i thought it would be fairly simple. I then made a stupid mistake - i turned the electricity off at the main consumer unit and began work and then one of the wires sparked and pop and burnt slightly. It was then I realised that the shower must run off the seperate RCD unit which I didnt switch off

So I then obviously switched the seperate RCD unit off and completed the work and seems to be working on. My question is:

Why did the RCD for the shower not trip? I thought the whole point of the RCD was to trip and switch the power off? I could have had a very nasty shock

Appreciate your help