Discrimination - Am I bovvered?

[don't_touch_that!]

Hi All.

I'm new to the forum and this is my first proper post so please try to answer in words of one syllable or less.

I am installing 2.5mm SWA out to the garage to a small CU with RCD containing a 16A and a 6A MCB covering sockets and lights.

The 2.5mm SWA can carry enough current to feed anything I am likely to use for now (fridge and one power tool at a time and a couple of 35W fluorescent tubes) and it came at a very good price so I don't particularly want to install bigger cable at a higher price.

At the house end of the SWA it will be protected by a 20A MCB. There is no other load on the 20A MCB and the SWA feeds only the garage CU. I propose to put it on the RCD side of the house CU.

My question is - Should I care that there will not be discrimination between the 20A MCB in the house and the 16A MCB in the garage?

In other words am I right in thinking it is a matter of inconvenience rather than safety that the 20A could trip if there were a fault on the garage sockets?

 

[dingbat]

With the arrangement of 20A overload protection of the cable and 16A overload protection of the sockets you are achieving discrimination in the event of an overload. You may not achieve discrimination in the event of a short-circuit fault, depending on the magnitude of the fault current and the reaction times of the breakers. Don't worry about it.

Where your real problem is, is that in the event of a fault between either live conductor and earth you may not achieve discrimination between the RCD in your main consumer unit and the one in the garage. Best to protect your cable from short circuit and overload faults only - that is, put it on the non-RCD side of your main cu.

(It is a matter of both safety and convenience. Regulation 314-01-01)

If you really feel you need to RCD-protect the cable (there is no requirement) You'd need to employ an S-type RCD with built-in delay at the source... and they are not cheap.

 

[don't_touch_that!]

Thanks for that quick reply. Non-RCD side it is then!

I thought the RCD on the house end was really belt-and-braces but if it will cause more trouble than it's worth it has to go, cheers m8.