This is one step of RCD testing. So you can prove that your box is really an RCD and not an counterfeit. Change resistors to find an triggering threshold. Change the plug polarity if device does not trigger (it will trigger only when you connect the resistor to L wire). This method is valid for testing 10 mA RCD if you have 30 mA RCD installed in your house. To test a 30 mA RCD you should find a line not protected by RCD. For 10 mA RCD try a number of resistors between 11 k and 40 k.
And of course, BE CAREFUL!!!
