On Saturday, 30 June 2018 15:57:00 UTC+1, John Rumm wrote:
On 30/06/2018 11:42, tabbypurr 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.

Yup, if you look carefully at what Adam wrote "100mA" main switch rather
than the more common "100A" main switch, I think he was implying that
the main switch would be a 100mA trip type S device.

yes, I misread that

I think it would be ok to feed all 4 meters from the same RCD.

I would be happy with that since the chances of a fault on the submains
is likely to be rare enough for it to be an acceptable risk /
inconvenience.

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.

This is one of the cases where the designer will need to make a
judgement call based on the circumstances.

No discrimination between circuits in the room is less than the perfect
ideal, but may be acceptable since the risks are low - i.e. no stairs,
no rotating machinery etc, and its only one room etc.

yes, all power to a room being cut off in the event of credit running out is standard practice.

A similar situation will exist with a 17th edition style split load unit
- one RCD trip will affect several circuits (although you normally
arrange them such that the power circuit would not cause a trip on the
same lighting circuit that serves the power circuit)

You could also mitigate with a £15 non maintained emergency light in the
room if you wanted.

If discrimination against N-E faults is required, why are single pole RCBOs sold? There's something I'm not clear on yet here.

For TN-C-S installs there is unlikely to be any significant voltage
difference between N & E at the origin of the installation (since they
are joined there). So once the loads are disconnected there is unlikely
to be any significant current flow in the CPC. What there is, will be
conducted by the suppliers PEN conductor which is designed to be a
current carrying conductor in normal operation (i.e. it is a "live" wire)

With TT, the neutral at the point of supply could be at a different
potential to that of the local earth spike. So a persistent N E fault
could drive significant current through the local earth spike (which in
turn could cause heating, loss of moisture and rising external earth
impedance).

So on balance a SP RCBO will still result in a significant reduction in
serious injury risk for most installations - its just not as suitable
for TT installs

gotcha
Presumably for the same reason it wouldn't be well suited to TN-S either, only for PME supplies.


(although I would highlight that I have a 16th edition style TT system
here, with 100mA trip type S incomer, and then a DP 30mA normal RCD for
power circuits. I also have a SP RCBO on my kitchen circuit (since that
was nuisance tripping the 30mA RCD). I decided it was preferable to
accept that a N&E fault on that one circuit could result in tripping the
incomer RCD as well, and be rid of the nuisance trip issue that was
there before). (I do have several non maintained emergency lights as
well though)

I've also been reading about the 18th edition, such as it is so far. Fixing of cables in escape routes has come up. I presume the usual plastic cable clips will no longer be permissible, are we going to go back to metal buckle clips? Is their lack of insulation ok today?

Yup this is a significant (and long overdue) change especially for
commercial / office installs.

(The main issue being that in many office / commercial fires things like
suspended ceilings would be covered is a mass of wiring. The moment the
flimsy ali frames fail in the heat, this is dropped onto the people
below - typically fire fighters - where it will snag on their breathing
apparatus and risks trapping them).

For the commercial installs, better use of cable trays, metal trunking
etc will solve most of the problems.

For smaller installs and domestic situations, I would expect there will
emerge a new range of non combustible cable restraint systems.

In this case the submain wiring is unsupported & there's no possible way to clip the cables using any known cable clip product. Insulated copper cable & wafer head screws should meet the requirements.


NT