Very odd goings on. Can you solve it?

[Doc Hudson]

“Failed to test?” It’s perfectly fine to check before testing. You’re just being pedantic. 

https://www.hse.gov.uk/pubns/indg231.pdf

Any person thinking the correct way to test an electrical installation, alteration or addition by just switching the power on, is about as sensible as checking if you can walk across a road blindfolded, with earplugs in. Pedantic NO. Sensible YES.  The situation you describe is not very odd at all, rather simple and easy to diagnose by any competent person with basic test equipment, who has connected numerous RCD's and/or other protective devices during their career.  You could possibly learn a lot by taking on board various comments from this thread.    

Doc H.