Fuses blow when I put phase and ground wires in contact even when light switch is turned off

[daz01FR]

Hello to all, I stumbled upon a strange problem this morning. I wanted to change the light bulb socket in my bathroom, when I noticed that when the ground and phase wires come in contact,the fuse-box trips even though the light switch is turned off. So I took a pen voltage tester and tested all three wires. When the switch is off, the pen doesn't light up on any of the wires. When the switch is turned on, the pen lights up only on the red phase wire. Everything looks normal at this point so what could be causing this?

 

[Mike Johnson]

In fact you can buy 100M of flex plastic conduit/gain pre populated with three cores of 2.5mm for €89.90, the only extra you need are the glands: https://www.leroymerlin.fr/v3/p/produits/gaine-prefilee-d-installation-3-x-2-5-mm2-l-100-m-e29028
[automerge]1586357557[/automerge]
What I forgot to mention is that the empty gain comes with a draw wire already installed, its strong enough to pull 16mm through without a problem, comes in 8, 10,20,25 etc........

 

[Mike Johnson]

Opps and 16 and a few other size's as well, in various colours, red for underground electric and Blue and Green........