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I know that’s why I don’t think it’s a dead short and the reading is from the USB points, live to earth IR test, goes down the earth which is also connected to the neutral at the supply TNCS will go through this neutral back around all socket and through the line conductor hence giving me a short circuit, but when line and neutral connected together all clear ??
If you know there are usb sockets in the circuit, testing N-E insulation is a bad idea.
you can't say there is a dead short if testing with an insulation resistance tester.

I would only consider something to be a dead short if measured below 1 ohm.
But regarding the USB sockets. If I have not disconnected the neutrals from neutral bar and tested between live and earth and I am getting low reading basically zero near enough, this should be because the USB points are bringing the reading down as I will be going through the earth and being TNCS will be linked to neutral so will be testing the USB point. As when I link out live and neutrals at the consumer unit and go to a socket then test live to earth insulation resistance it comes out clear. >999mega ohms. Now if that was a damaged cable that would be picked up, does that make sense ?
The USB installation blurb differs between brands. I fitted a LAP usb socket and it states that you can do installation test at 500 V but others state that they must be disconnected.

first thing mentioned on my insulation test is that dimmers should be disconnected for the test.

Short circuit between live conductors after the RCD will not ensure it’s tripping ( RCD guide Eaton.EU) may apply and example shown is N to E.

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