RCD tripping before rcbo

[jpadie]

I have a garden room with a cu fed from a time delayed RCD in the main consumer unit.

 

The garden room CU is populated with rcbos.

 

I was installing an isolator for a vmc this evening and when I came to turn the local rcbo back on (all circuits were turned off but the main isolator switch was not) I found that the RCD in the main CU had tripped.  

 

Any thoughts on why this happens?  

 

I've also had a recent experience where the RCD in the main unit tripped whilst none of the rcbos did. Related to a brief short to earth on a circuit in the garden room.  

 

All rcbos have tested good and earth is good, albeit the rod is much closer to the main house than the garden room.  

 

I'm swapping the main RCD for an mcb shortly so this is just trying to understand what's happening rather than looking for a solution