Extending a non RCD existing installation

[Bazos]

Dear all, my house has a 15 year old installation with a Wylex consumer unit with MCB's but no RCD's within unit. I have a dedicated RCD for power shower and dedicated RCD socket for garden use. Everything works normally and I would prefer not to have to replace CU etc. I require a spur from downstairs ring to be installed to my detached garage with a RCD in garage for sockets. I intend to use local contractor, I would value you views on how my requirement fits regulations, Best Regards.

 

[ericmark]

Because the cables may also need protection I would consider supplying from a RCD FCU for example see http://www.screwfix.com/prods/14770/Electrical-Supplies/RCDs/Powerbreaker-13A-RCD-FCU this would then protect cables.
Also using one of these in the house means total draw can't exceed 13 amp so cable only needs to be rated for this load and not total 32 amp.
The one I have shown is a passive if the point on the ring where you are tapping on to is near centre of ring then an active type may be required. This is all down to volt drop under fault conditions as a RCD needs voltage to work and if a fault could pull the voltage to below the value at which it could work then we fit active RCD which will trip if voltage goes too low so fails safe. If a RCD was fitted in garage instead then it would be active type.
The electrician will measure earth loop impedance and from these reading he will select what types are required.
At the end of the day the electrician has to sign paperwork to say everything is OK and to show readings he has taken. It is up to him what he thinks is safe method and complies and does not really matter what we say on this it is all down to the man who signs the paperwork.
Eric