There are a few issues here.

First off is trying to ensure that any chain of RCDs/RCBOs has selectivity. That generally means ensuring that the downstream RCD/RCBO will always trip before the upstream one. Simplest way to do this is fit a 100ma RCD at the upstream end and then 30mA RCDs at the downstream end, as long as that doesn't leave any final circuits at the downstream end only protected by the 100mA RCD. The alternative if 30mA RCD protection is needed upstream is to use a time delay RCD upstream if allowable. In your case neither of these options is acceptable, as the upstream RCD is the only thing providing RCD protection to the house. As the garage is also fed from this then it's will trip the whole supply if there's a fault in a garage, which isn't great.

The serious risk is that any DC earth leakage from the charge point installation may well make the whole installation unprotected, by blinding the main RCD on the incoming supply. Replacing the main RCD at the incoming supply with a Type A would very slightly help, but isn't really a good fix, and it still leaves the selectivity problem with the downstream RCD in the garage.

Best bet would be to re-jig the garage supply so that the switch fuse connects directly to the incoming supply, via a couple of Henleys. The existing RCD can then continue to protect the house (it would be better if it was a Type A). The switch fuse will protect the 16mm² SWA to the garage and the garage RCD can be swapped for a Type A. The charge point can then connect to the spare MCB in the garage CU.

 

The serious risk is that any DC earth leakage from the charge point installation may well make the whole installation unprotected, by blinding the main RCD on the incoming supply. Replacing the main RCD at the incoming supply with a Type A would very slightly help, but isn't really a good fix, and it still leaves the selectivity problem with the downstream RCD in the garage.

Thanks for the reply Jeremy. I wasn't sure whether having a type A on the garage would negate the need to change anything at the house. However, you're obviously correct that despite having a type A at the garage, a fault could still blind the 'main' RCD. That would not be ideal. Perhaps I take this opportunity to go wild and have more than one RCD for the whole house CU.

 

Why not just move the Henley Block to before the RCD Main Switch? The existing RCD protects the house as before and now the garage has its own desicated non-RCD protected supply. No rcd needed as in SWA, but the fuse switch is handy if you ever wanted to isolate the garage, in case you ever noticed that it was on fire for example and also protects this 16mm cable from over-currennt and short-circuit. Then, just replace the type AC RCD in the garage CU with a type A, or even better a type B!? Only need to buy one thing and move a few wires...! Adam.

 

This thread is over 2 years old, hopefully the OP has sorted his CU.