The test button only proves the RCD is mechanically operating.
RCD's are faster when the fault occurs in one half cycle than when it occurs in the other. For example if a fault occurs just at the time the AC wave is going positive, the RCD may not 'see' the fault until several milliseconds later when the wave starts to go into the negative.
Because of this it's important to test the RCD speed twice, once with a simulated fault using a waveform that starts on the positive half cycle and a second identical test using a waveform that starts on the negative half cycle. The slowest disconnection time from both the tests is the worst case scenario and is the figure that should be used when deciding if the RCD is tripping within the time limits.
It's worth noting that an RCD that hasn't been tripped for a long time might be mechanically slow, the internal mechanism can get stiff over time. It's always a good idea to test it once or twice using the test button before you do the ramp/speed tests. This is also a reason that it's important for the customer/property owner to test the RCD periodically.
