A rcd has to be tested if present to make sure it disconnects in the required time a press of the button does not determine this using a rcd tester does by carrying out the rcd test sequence
and nearly always done directly at rcd or rcbo in ccu only time you dont have to is for a socket circuit in a convienient location ie near the ccu so you can switch rcd back on every time it trips and these should be done before pressing the test button to check mechanical parts of the rcd
and its carried out with all loads removed ie
lights switched off
tvs, computers, removed or switched off
cooker switched off at the dp cooker switch
but its still a live test has to be or the rcd wouldnt work anyway when tested
if done at ccu isolate all breakers then load is isolated but the main switch is on and the rcd and is tested on the load side
if done on sockets then everything is on but unplug everything to remove the loads and test as close to the ccu as you can to aid resetting the rcd after each test
once all done and correct then press the test button for the functional test
