If its an RCD, not an RCCB then disconnect the earth from the 2.5 t&e at the garage consumer side so the earths are seperate. That way the supply cable is protected by an RCD to the garage point then your garage RCD will protect from there, that is so long as you have an adequate earth with your TT system that you have just made. (your Zs should read below 50 ohms as a rough guide, but can be greater if you know what your doing)
This system will work with only certain setups and RCD's though, any chance you can give us the BS no. of the primary RCD??
1. Introduction
It can be necessary for an electrical installation to include residual current devices (RCDs) connected in series or ‘cascade’, particularly where the installation forms part of a TT system. Where such RCDs are used for fault protection or otherwise to prevent danger, and discrimination in their operation is necessary to prevent danger, Regulation 531.2.9 of BS 7671 requires that their characteristics are such that discrimination is achieved. Discrimination is explained in item
2 of this topic.
The need for discrimination is also implicit in Regulation 314.2, which requires that a separate circuit be provided for each part of an installation which needs to be separately controlled so that it remains energized in the event of failure of any other circuit of the installation.
Furthermore, discrimination between protective devices (including RCDs) connected in series is implicit in Regulation 314.1 which requires that every installation be divided into circuits, as necessary, to avoid hazards and minimize inconvenience in the event of a fault.
Even where it is not strictly necessary to provide discrimination between RCDs to avoid danger or to minimize inconvenience, it is considered good practice to do so.
Where two or more RCDs are connected in series, discrimination will generally be achieved by selecting time-delayed characteristic(s) for the upstream device(s).
