DIYbanter - View Single Post

ARW

On 30/06/2018 11:42, wrote:
On Saturday, 30 June 2018 08:15:22 UTC+1, ARW wrote:
On 29/06/2018 21:51, John Rumm wrote:
On 29/06/2018 00:19, tabbypurr wrote:
On Monday, 25 June 2018 01:08:08 UTC+1, tabbyÂ* wrote:

Current fusebox has several fuseways each feeding a meter then a 2
way fusebox which powers each rented room. Plan is a new CU & new
secondary 2 way CUs, the meters will be retained. The question is how
may RCD protection be implemented. If done in the main CU with an
RCBO, a trip results in a room losing all power including lighting.
If done in the secondary small CU, the cable from main CU to meter to
secondary would not be RCD protected. This wiring is surface clipped
direct. Neither of those options strike me as fully satisfactory.
How's good to do this?

NT

OK, some questions.
Does there need to be separate rcd protection for lights & sockets so
lighting isn't lost when sockets trip?

Ideally, yes

Since SP RCBOs don't disconnect N-E faults, I presume the whole
install still requires 2x DP RCDs. Can these be 300mA time delayed or
must they be 30mA?

If there are additional downstream 30mA trip devices, then they can in
theory be 100mA type S devices. However you won't then be able to
achieve discrimination with the RCD protecting the submain feeds to the
meters. So you are no nearer a workable solution.

So that leaves you needing either 2 DP RCBOs per room, or two
conventional 30mA RCDs and two MCBs (likely 6 ways in total per room).

The cheapest option may be a small CU for the head end - Type S RCD on
its incomer, then 4 x regular MCBs for your submain feeds.

It would probably be a a lot cheaper and easier to use a hi integrity
dual split load CU with a 100mA main switch, 4 MCBs for the rooms fed
from the mains switch and to use the 30mA RCD supplies for the communal
areas and just fit a 2 way garage CU in each room.

Unless I'm mistaken the surface wiring to & from the meters must be DP RCDed on this TT system, either at 30mA or 100mA time delayed. I think it would be ok to feed all 4 meters from the same RCD.

It would have to be 100mA S type for the meters

I take it discrimination between lights & sockets in each room is not required, meaning both can run off the same RCD. That certainly simplifies things, as you say a garage CU would do the job. I was looking at a single larger secondary CU with SP RCBOs because that would be cheaper & neater, but if SP RCBOs are no good then so be it.

I am trying to think of a way to simplify it.

--
Adam