Sub main RCD or not?

[sythai]

Recently had to install additional fuse board for extension.

Spilt tails at source, henley blocks and installed 100A switched fuse to supply new board which is located other side of house.

25mm tails are 10m long, done calc all ok, and checked with local DNO all ok.

Got the tails up the cavity through to the first floor joists along and down the cavity on other side, but still not sure if should change switched fuse to RCD. Thought switched fuse was the correct way ? And presumed you'd only need RCD if less than 50mm deep?

Any thoughts please?

Thank you,

Sy

 

[johnboy6083]

if you put the sub main on an rcd, then wouldnt this mean that you would have 2 RCD's in series? i thought that this reduces their sensitivity, and you lose discrimination.

in hind sight, wouldn't it have been better to run the sub main in 3c armoured, and then have an RCD in your new CU?

I am aware that it is easy to suggest things once the job has been done, and im not on site under any pressure!

 

[Spudnik]

No need for RCD on tails.

And as the extension will have its own circuits, then you could use an RCD main switch CU.

 

[sythai]

Would normally have done in SWA, but unfortunately wasnt practical in this situation.

Didnt think I would need RCD, just checking...

But what gets me is how is that cable protected back to the incomer by the RCD at the new CU, I thought it would only work one way and just protect everything from the new CU outwards ? Just say if you had to run some tails up a wall and they were less than 50mm deep would you put a RCD in that circumstance?

Many thanks,

Sy

 

[johnboy6083]

i thought that you had to provide RCD protection to cables if they are ran in walls and :a, are not more than 50mm deep, b, not protected by an earthed metal covering (ie conduit, SWA, pyro ect) or c, under the supervision of suitably skilled person.

as it would be difficult to obtain conduit big enough to proctect 25mm tails, i would opt for the SWA.

Then you would not need RCD protection on the submain.You would still require RCD proctection for the circuits leaving the submain CU.

Please correct me if i am wrong, as i am here to learn.

john

 

[davelerave]

you should try avoid the rcd on the sub-main -by using armour if neccessary

 

[Des 56]

Would normally have done in SWA, but unfortunately wasnt practical in this situation.

Didnt think I would need RCD, just checking...

But what gets me is how is that cable protected back to the incomer by the RCD at the new CU, I thought it would only work one way and just protect everything from the new CU outwards ? Just say if you had to run some tails up a wall and they were less than 50mm deep would you put a RCD in that circumstance?

Many thanks,

Sy

The cable back to the incomer is not protected by the Rcd its protected by the fuse
You havn't buried the tails in a wall so they dont need Rcd


If you ran tails up a wall less than 50mm deep and not mechanically protected you would have to Rcd those tails

 

[sythai]

If you ran tails up a wall less than 50mm deep and not mechanically protected you would have to Rcd those tails

I see thank you... just presuming your using 2x30mA 80/100A RCDs within the, CU what RCD would you then use on the tails ? Would it just be 30mA also ?