As far as I can tell you are installing an RCD at the origin of the installation to ensure that on an earth fault the supply will be disconnected within the required time stated in table 41.1.
Since this is a TT installation and assumed to be at nominal 230V the disconnection times would be 0.2s (200ms) for final circuits and 1s (1000ms) for distribution and >32A final circuits (63A if sockets present).
Your tester should be able to measure a time of up to 500ms on a 1 x I∆n test for a type S RCD, or 300ms if not time delayed).
If all your final circuits have 30mA RCD protection then a 300ms time delay would be acceptable, if any final circuits <=32A (<=63A with sockets) are not protected by a 30mA RCD then you would need to reduce the total trip time for the upstream RCD to less than 200ms.
However in order to ensure that 30mA RCDs tripped in preference to the main RCD the time delay should not be reduced below 40ms (100ms is a good compromise)
The RCD tester should be correctly set up so the test parameters are for a type S RCD, (there will be a 30s delay in testing when set to type S) the RCD should ideally not be connected to any outgoing loads at the time of testing and there should be no other circuits running on the same supply that may introduce mains transients (motors and switching power supplies,etc.).