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Discussion Starter · #1 ·
I'm trying to get a Pod Point Solo installed and they've identified a couple of things that need correcting before continuing with the installation.

I'm trying to get the charger installed on a detached garage which has a room above (used mainly for storage and as a winter resort for mice). The garage is about 25m away from the house, connected by a chunky 16mm SWA cable, and has its own CU.

Pod Point have asked for changes so that:
  • Supply to the sub-board is either non RCD protected or protected by a Type A RCD.
  • A Spare 32A or 40A MCB available on sub-board which is either non RCD protected or is protected by a Type A RCD.
Currently the garage has a type AC RCD and no spare MCB (though there is space for one). I can get both of these issues sorted. The problem is that the garage is fed from a 30ma type AC RCD in the house which is acting as the main switch for both the garage and the house.

From what I understand, to comply with the first of Pod Point's requirements, that needs changing to a type A? From what I also understand, having two like RCDs in series is not ideal for discrimination and which one trips when a problem occurs.

If using an RCD, it could be a 100ma type S ... which still doesn't meet Pod Point's first requirement. It also isn't an option since the main switch RCD is also the only RCD for my whole house and type S doesn't offer the required protection.

Or if I get the RCD in the garage swapped for a type A, should Pod Point be happy even though there's an upstream AC which could be blinded? That is, the garage RCD is providing all required protection.

Diagram to try and explain things attached.
 

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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.
 

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Discussion Starter · #3 ·
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.
 

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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.
Maybe replace the MCBs in the house board with RCBOs and change the main house RCD with a switch?
 
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