Quick question

[Will2477]

Hi all, I have a quick question from a level 4 exam paper I'm stumped on.
Why would you test a rcd used as additional protection at 5 delta I before any other test on a periodic.
My possible answers
1 as it used as additional protection you are verifying it will trip within 40ms as required, if it doesn't then it's not functioning as additional protection
Any help will be appreciated
Thanks Carl

 

[Zerax]

The question mentions 'periodic'... so why would a test for correct RCD operation be more relevant with a periodic inspection rather than for initial verification ?