Any advantage to using RCD spur outlet off RCD-protected ring-main?

Question

I'm fitting a security light to my exterior wall, and need to add another socket 'gang' to power its internal switch.

I CAN'T convert the existing nearby single socket to a double due to lack of space at its location, so need to take a spur from it. (I've tested and confirmed that this single socket is on the ring-main.)

I have a two-year-old Wylex fuse box with 30mA RCD-protection on all the mains sockets.
Would there be any safety advantage at all if I got an RCD socket or RCD fused spur unit for this outlet? These cost several times more than a basic fused spur unit. The only advantage I can think of is if the RCD on the spur had a faster trip time (don't know what it is on the fuse box).

I intend to use a 3A fuse in the spur unit, as it is only to power the light.

(Please do not advise me to get an electrician out - I have successfully done plenty of sockets / light switches before - my question here is about whether I should use an RCD FCU, or a normal FCU)

Answer 1

No advantage in adding another RCD, and not required anyway as the light is not a portable item, you could feed it from the lighting circuit, as long as the existing earthing meets requirements. Use a FCU either way.

Answer 2

Simply being that an RCD is there to provide protection outside and earthing-equipotential zone. IE that that is not in your home, nearby a plce of earthing, CPC's And Supplimentary Bonding (Pipes Etc...).

The 30mA Trip Is there to Ensure that in the event of a fault outside these zones, that it doesn't rely on the Fuse/MCB Trip Current/Time factors to disconnect the circuit. Unless the Trip time for the 30mA RCD Is above 200ms, then Short answer is:

No, There is not a lot of point. Its not really necessary as the RCD For the Ring Should cover this. However You can never be too careful... ;-)

If you find the RCD trip time at 1x Rated current is >200ms Then its time for a new RCD.

But to Be honest its NOT really good practice to put on a lighting circuit to the ring, even tho you are placing a 3A FCU to spur off it. Ideally it should be on the lighting radial.

Answer 3

i would say the RCD in the fuse box is sufficient. its just basically like having two fuses in series.

Answer 4

external lighting requires part P qualification, you are not allowed to install this unless you are a part P registered contractor, or you employ one to do the job and produce the necessary certification, its the law.