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  #1   Report Post  
Kalico
 
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Default Did we ever agree if an RCD for the whole installation was allowed?

I remember this being discussed some time ago, but not sure if we
reached a conclusion.

The situation where an RCD is used to protect all circuits in a
consumer unit, rather than just sockets etc in a split-load
arrangement.

I have to replace an old Wylex fuse-board that has only 5 circuits:-
1x30A Ring main (it's only a studio flat)
1x30A Fan heaters
1x30A Shower
1x15A Not sure yet as not labelled
1x5A lights

My understanding is that the shower circuit should have an RCD fitted,
although this is not clear from the OSG.

The problem is that the space available for a replacement CU rules out
the usual split-load units, though I guess I could make one up (though
this can be very expensive compared to the kits available).

Anyway, I'll wait to hear your thoughts.

Rob


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  #2   Report Post  
Ed Sirett
 
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On Mon, 25 Oct 2004 19:26:06 +0100, Kalico wrote:

I remember this being discussed some time ago, but not sure if we
reached a conclusion.

The situation where an RCD is used to protect all circuits in a
consumer unit, rather than just sockets etc in a split-load
arrangement.

I have to replace an old Wylex fuse-board that has only 5 circuits:-
1x30A Ring main (it's only a studio flat)
1x30A Fan heaters
1x30A Shower
1x15A Not sure yet as not labelled
1x5A lights

My understanding is that the shower circuit should have an RCD fitted,
although this is not clear from the OSG.

The problem is that the space available for a replacement CU rules out
the usual split-load units, though I guess I could make one up (though
this can be very expensive compared to the kits available).

Anyway, I'll wait to hear your thoughts.


AIUI you don't absolutely need to have an RCD if you can show the circuit
+ MCB + size of Earth wire would meet the required disconection time of
0.2s. However it is simpler to fit an RCD. This could be done in a
separate unit outside of the main CU.
If none of the sockets could reasonably be expected to power outdoor
equipment then non RCD is needed for the Ring Main either.
Given that the space heating is electric is the hob/cooker gas or only 1
or 2 plates (13A)?
I expect the other end of the 15A supply will be found in what was or is
the airing cupboard?

--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html


  #3   Report Post  
Andrew Gabriel
 
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In article ,
Kalico writes:
I remember this being discussed some time ago, but not sure if we
reached a conclusion.


I don't have the regs or On-Site guide on me, but I think
a single RCD protecting a whole installation has to be at least
100mA, which means it's unsuitable to provide protection against
electrocution for appliances used outdoors and in some cases in
a room containing a bath or shower.

The situation where an RCD is used to protect all circuits in a
consumer unit, rather than just sockets etc in a split-load
arrangement.

I have to replace an old Wylex fuse-board that has only 5 circuits:-
1x30A Ring main (it's only a studio flat)
1x30A Fan heaters
1x30A Shower
1x15A Not sure yet as not labelled


Immersion heater?

1x5A lights

My understanding is that the shower circuit should have an RCD fitted,
although this is not clear from the OSG.


First we need to know what earthing system your installation uses.

There are two reasons for using RCDs:

1) Protection against electrocution, which is required for sockets
which might be used to power outdoor appliances, and some cases
of appliances in show/bathrooms (not the shower though).
RCD protection in this case must be no more than 30mA.
It is a bad idea to have lighting on a 30mA RCD shared with
anything else (and they don't normally merit a dedicated one).

2) Protection against high earth fault loop impedance, which normally
applies to TT systems (own earth rod). In this case, the protection
is normally a minimum of 100mA.

So the shower would only possibly require the 2) catagory, in
which case all of your installation would anyway.

If you don't need protection in the 2) catagory, then a CU with
no RCD protection and an RCBO for the ring main would be one
way. RCBO's are combined MCB and RCD in one module, and are available
for many CU's in same width module as an MCB (check before you buy
though as for some CU's they are double width modules, or not
available at all).

--
Andrew Gabriel
  #4   Report Post  
Owain
 
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Default

"Kalico" wrote
| I remember this being discussed some time ago, but not sure
| if we reached a conclusion.
| The situation where an RCD is used to protect all circuits
| in a consumer unit, rather than just sockets etc in a split-
| load arrangement.

I have suggested that a whole-house RCD contravenes the following regs:

130-01-01 Good workmanship and materials shall be used.

130-02-01 All equipment shall be ...installed .. so as to prevent danger as
far as is reasonably practicable.

314-01-01 Every installation shall be divided into circuits as necessary to:
(i) avoid danger in the event of a fault, and (ii) facilitate safe
operation, testing and maintenance.

314-01-02 A separate circuit shall be provided for each part of the
installation which needs to be separately controlled for compliance with the
Regulations *or otherwise* to prevent danger, so that such circuits remain
energised in the event of failure of any other circuit of the installation,
and *due account shall be taken of the consequences of the operation of any
single protective device*.

from and copyright IEE Wiring Regulations Sixteenth Edition 1991. [* my
emphasis *]

The above taken in conjunction with Peter Parry's comments on the number of
deaths through falls possibly linked to sudden loss of light on staircases
suggests to me that a whole-house RCD is not only in breach of the Regs, but
incompetent verging on negligent (unless there are other provisions eg
emergency lighting).

You could fit an emergency lighting unit which would help mitigate the risks
arising through loss of discrimination, if you decide you need an RCD.

| I have to replace an old Wylex fuse-board that has only 5 circuits:-
| 1x30A Ring main (it's only a studio flat)
| 1x30A Fan heaters

These would usually be on 15A radials. If its a radial circuit at 30A it
should be in 4mm cable not 2.5mm (which is for 30A ring or 20A radial
circuits)

| 1x30A Shower

If rewiring, I would suggest provisioning a new circuit at 45A. You could
use an RCBO instead of an MCB - most are two-module, but it would save space
compared to a 3-module main switch plus 3-module split-load RCD.

| 1x15A Not sure yet as not labelled

As Ed Sirett says, probably immersion heater

| 1x5A lights

| My understanding is that the shower circuit should have an RCD fitted,
| although this is not clear from the OSG.

As Ed says.

Owain





  #5   Report Post  
Christian McArdle
 
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I have to replace an old Wylex fuse-board that has only 5 circuits:-
1x30A Ring main (it's only a studio flat)
1x30A Fan heaters
1x30A Shower
1x15A Not sure yet as not labelled
1x5A lights


You can get single width RCBOs which replace MCBs and provide RCD protection
for that circuit only. Personally, I would like one on the socket circuit as
well, although I would also prefer the fridge/freezer on a non-RCD circuit,
which would require another circuit (or some careful calculation and
possible MCB upsizing on another circuit so that it can share).

The 15A is probably the water heater.

Christian.




  #6   Report Post  
Pete C
 
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On Mon, 25 Oct 2004 19:26:06 +0100, Kalico wrote:

I remember this being discussed some time ago, but not sure if we
reached a conclusion.

The situation where an RCD is used to protect all circuits in a
consumer unit, rather than just sockets etc in a split-load
arrangement.

I have to replace an old Wylex fuse-board that has only 5 circuits:-
1x30A Ring main (it's only a studio flat)
1x30A Fan heaters
1x30A Shower
1x15A Not sure yet as not labelled
1x5A lights

My understanding is that the shower circuit should have an RCD fitted,
although this is not clear from the OSG.

The problem is that the space available for a replacement CU rules out
the usual split-load units, though I guess I could make one up (though
this can be very expensive compared to the kits available).

Anyway, I'll wait to hear your thoughts.


Hi,

The usual objections are freezers defrosting and falling to your death
down dark stairs, but with a studio flat there may not be space for a
freezer and falling down stairs is unlikely in any case.

I'm sure there are fatalities due to falling down in the dark when the
RCD trips, but there are also fatalities due to faulty wiring and/or
householders putting screws or nails in the wrong place, so it's a
balance of risks.

I wonder if it's permitted to put a 6/16A RCD garage consumer unit in
as well, for lights and fridge/freezer, then the rest of the property
could be on a normal RCD protected consumer unit.

I'd be very suprised if a whole house RCD is forbidden for _any_
installation.

Personally I'd fit a whole house RCD in this case unless expressly
forbidden or there is a risk of defrosting a freezer with 100 worth
of food, then I'd add a separate unit for it as above if that was
permitted.

cheers,
Pete.
  #7   Report Post  
Kalico
 
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Default

Well, thanks to all for the advice.

Seems that the 15A circuit will supply the fan heating and the three
30A supply the ring main, shower and cooker.

The shower is run in 6mm and I am reluctant to replace the cable so
will replace the shower with a 8.5KW which is within limits.

Regarding the new CU, I have ordered a small 9way Wylex split load CU
which should do the job 'properly' and I can put the shower and
sockets on the RCD.

Given it is a tiny flat on the first floor I am minded to think it
unlikely that the sockets will supply kit to be used outdoors but I
like to see an RCD on sockets anyway, freezer or not.

As an aside, I have some rented houses that have a 100mA RCD on the
whole of the CU. Must have been a common way to re-wire them when
they were done approximately 10 years ago. Should I now re-wire them
split load. I hear the arguements about falling down stairs but think
it more likely a tenant will put a nail through a wire or similar.

A good example was the tenant who went to replace a light rose/pendant
with a new fitting his wife had bought at B&Q. Of course, he made no
notes of what was connected where and couldn't understand why there
were 'so many wires' coming through the hole in the ceiling when the
new fitting only had a two way bit of chocolate block!

Let me know if you have any more thoughts and once again, thanks for
the help.

Rob


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  #8   Report Post  
Shabs
 
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Default

Talking about MCBs, i have just had a split load cu installed and was
going to use the split load to power my outdoor circuits in my
outbuilding. i plan to have a 6a mcb on the rcd side taking power to
3 lights, and a 32a mcb taking power to 4 sockets on a ring main. Is
this the correct way to do it? as i have read that a 16a mcb should be
used for outdoor socket circuits.

I ask this question as i have just read above that a rcd protected mcb
should not be used for outdoor lighting.
  #9   Report Post  
Christian McArdle
 
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I ask this question as i have just read above that a rcd protected mcb
should not be used for outdoor lighting.


It is fine to do so, but you may not wish to share the RCD with another
circuit, as leakage may get quite high in wet weather. Personally, I
wouldn't want any exterior circuits to share an RCD (i.e. on a split load)
with any circuits internal to the building.

I've used RCBOs for this, but you could just use an MCB off the non-RCD side
and fit an additional RCD just for that circuit in a separate box (or an
entire consumer unit for exterior electrics, if there is a lot of it).

Christian.


  #10   Report Post  
Pete C
 
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On Wed, 27 Oct 2004 00:40:45 +0100, Kalico wrote:

Well, thanks to all for the advice.
As an aside, I have some rented houses that have a 100mA RCD on the
whole of the CU. Must have been a common way to re-wire them when
they were done approximately 10 years ago. Should I now re-wire them
split load. I hear the arguements about falling down stairs but think
it more likely a tenant will put a nail through a wire or similar.


Hi,

If better protection is required, a faster 30mA RBCO on the ring main
and possibly cooker circuit may help, as they're more vulnerable to
stray nails and dodgy appliances.

Maybe others can comment on whether a 100mA RCD will normally trip
when a light bulb blows.

cheers,
Pete.


  #11   Report Post  
The Natural Philosopher
 
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Pete C wrote:

On Wed, 27 Oct 2004 00:40:45 +0100, Kalico wrote:


Well, thanks to all for the advice.
As an aside, I have some rented houses that have a 100mA RCD on the
whole of the CU. Must have been a common way to re-wire them when
they were done approximately 10 years ago. Should I now re-wire them
split load. I hear the arguements about falling down stairs but think
it more likely a tenant will put a nail through a wire or similar.



Hi,

If better protection is required, a faster 30mA RBCO on the ring main
and possibly cooker circuit may help, as they're more vulnerable to
stray nails and dodgy appliances.

Maybe others can comment on whether a 100mA RCD will normally trip
when a light bulb blows.


No, but a 30mA one may.

I have yet to fully sort my setup, but a 30mA RCD was unuseable on the
house - all new wiring, no faults. Just lots of leakage from various
things. Mainly eletronics of which I have a LOT.

100mA has been utterly reliable - only tripped when washing machine
motor coils shorted to ground. And in the odd thunderstorm.

Still looking for slender RCBO's to fit teh MCB positions where needed.

cheers,
Pete.

  #12   Report Post  
Buxnot
 
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Andrew Gabriel wrote:
In article ,
Kalico writes:
I remember this being discussed some time ago, but not sure if we
reached a conclusion.


I don't have the regs or On-Site guide on me, but I think
a single RCD protecting a whole installation has to be at least
100mA, which means it's unsuitable to provide protection against
electrocution for appliances used outdoors and in some cases in
a room containing a bath or shower.


Our installation is a strange one. It is a 1950s house that was
completely rewired about 12 years ago.

The wylex consumer unit contains a 100mA RCD which doubles as the main
switch. Downstream from that are 2 lighting circuits (not upstairs and
down but left-half-of-house and right-half-of-house - god knows why it's
done this way).

Also downstream from the 100mA RCD is a 30mA RCD. Downstream from that
are all the electric circuits including water heater (always off as we
only use gas) cooker, socket rings (again left-half-of-house and
right-half-of-house).

We're on a PME system.

I can only assume that the circuits have been wired left-half/right-half
to allow working on circuits on the same floor while still getting power
from an extension cable in another room. All highly dubious to me, I
just turn off the whole 30mA RCD. At least we can still get lights with
that turned off....


  #13   Report Post  
Kalico
 
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On Wed, 27 Oct 2004 22:41:56 +0100, Pete C
wrote:

On Wed, 27 Oct 2004 00:40:45 +0100, Kalico wrote:

Well, thanks to all for the advice.
As an aside, I have some rented houses that have a 100mA RCD on the
whole of the CU. Must have been a common way to re-wire them when
they were done approximately 10 years ago. Should I now re-wire them
split load. I hear the arguements about falling down stairs but think
it more likely a tenant will put a nail through a wire or similar.


Hi,

If better protection is required, a faster 30mA RBCO on the ring main
and possibly cooker circuit may help, as they're more vulnerable to
stray nails and dodgy appliances.

Maybe others can comment on whether a 100mA RCD will normally trip
when a light bulb blows.

cheers,
Pete.

Not in my experience. In fact, the 100mA trips have never tripped
unless they 'should', ie when there has been the type of fault that an
RCD is designed for.

Rob


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  #14   Report Post  
Martin Angove
 
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In message ,
"Buxnot" wrote:


Our installation is a strange one. It is a 1950s house that was
completely rewired about 12 years ago.

The wylex consumer unit contains a 100mA RCD which doubles as the main
switch.


[...]

Also downstream from the 100mA RCD is a 30mA RCD. Downstream from that
are all the electric circuits including water heater (always off as we
only use gas) cooker, socket rings (again left-half-of-house and
right-half-of-house).

We're on a PME system.


The usual cause for installing a 100mA RCD to protect everything,
followed by a 30mA for sockets is where the earth is provided locally -
i.e. by an earth rod or similar. In these "series" setups the 100mA
should be time-delayed in order to allow the 30mA to have a chance to
trip first.

If, as you say, you have a PME system then the 100mA RCD is not usually
required. Be cautious though - in my area the local distribution company
upgraded everyone n years ago to be PME *capable* but no more - most
installations are still TT. This could be confusing for anyone who
doesn't know about it because the distribution company replaced the
"cutout" (the bit their street cable goes into which includes the fuse
before the meter), and most of these cutouts have labels on them saying
"warning, this is a TN-C-S or PME system" (or something similar).

The fact is that unless there is an earth wire going from the main earth
terminal into the neutral block of this cutout, then the setup is still
TT. Getting your earth into this terminal varies from area to area.
Around here Western Power will install a short tail and a brown earth
terminal for nothing, whereas in Derby (forgot the name of the
company) the charge is 70 quid.

I can only assume that the circuits have been wired left-half/right-half
to allow working on circuits on the same floor while still getting power
from an extension cable in another room. All highly dubious to me, I
just turn off the whole 30mA RCD. At least we can still get lights with
that turned off....


There's nothing at all dubious about having a left/right split versus an
up/down split. All the regulations require is that installations are
split into a sensible number of sensibly protected circuits. Just
because up/down is *normal* doesn't mean it's the only way, so long as
the labelling is clear.

Lighting is usually done up/down because it is much easier to do it that
way - especially in an unboarded attic it is the work of a couple of
hours to run cables over every room to lighting points. Sockets are a
little more difficult as they usually come up from below (upstairs).

Some houses have everything together. The usual way to do it in terraces
around here is 30A ring main for all sockets (up and down), 30A cooker,
15A immersion (if installed) and 5A lights (up and down). When replacing
a consumer unit it is often quite easy to split the lights as there are
usually two cables from the same 5A fuse, and one often goes straight up
into the attic.

When I rewired our house (1920s council semi) it got a lot more
complicated than that; I have three ring circuits; "east", "west" and
"kitchen", one 30A radial for the utility room and external power and an
upstairs and downstairs lighting circuit (and a couple of other minor
things).

The lights are not RCD protected, the three rings are on a common RCD
and the 30A radial is an RCBO. Part of the reason for the east/west
split was the way we renovated the house. By doing it this way we still
had power upstairs and down in the part of the house we weren't working
on. There was no reason from a loading point of view to split east/west
or indeed at all, given that the kitchen has its own circuit.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Feet Smell? Nose Run? Hey, you're upside down!
  #15   Report Post  
Christian McArdle
 
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There's nothing at all dubious about having a left/right split versus an
up/down split.


Indeed, for lighting circuits, the safest solution is to randomly assign
light fittings to one of two circuits. This way, if the MCB trips, it is
more likely that a nearby light is on and working, so you are less likely to
fall. In the event that the fault can't be cleared, the house is then more
usable, without a complete floor out of commission.

Christian.





  #16   Report Post  
Martin Angove
 
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In message ,
"Christian McArdle" wrote:

There's nothing at all dubious about having a left/right split versus an
up/down split.


Indeed, for lighting circuits, the safest solution is to randomly assign
light fittings to one of two circuits. This way, if the MCB trips, it is
more likely that a nearby light is on and working, so you are less likely to
fall. In the event that the fault can't be cleared, the house is then more
usable, without a complete floor out of commission.


Arranging the wiring for this can be fun though, and labelling is
vital; most people would for example expect all 1mm2 cables in the loft
to be the same lighting circuit.

There are also issues where you, for example, have two or more fittings
in the same room and wish to switch them from the same location - there
is then power from two separate circuits behind the n-gang light
switch. Mind you, this can also be the case for a hall/landing 2-way
arrangement in a "conventionally" wired (i.e. up/down) house.

It's a good idea in theory though!

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
Two free issues: http://www.livtech.co.uk/ Living With Technology
.... Modem, said the gardener when he'd finished the lawn..
  #17   Report Post  
Andy Wade
 
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Martin Angove wrote:

When replacing a consumer unit it is often quite easy to split the
lights as there are usually two cables from the same 5A fuse, and one
often goes straight up into the attic.


It's probably worth adding that if you split a lighting circuit like
that you must be very careful to ensure that the two circuits created
are electrically separate. In particular you need to know how
hall/landing two-way circuits are wired so that you don't end up with a
live (phase) feed coming from circuit A and the corresponding neutral
return going to circuit B. A good test before connecting the circuits
to the new CU is to strap the L & N of each circuit together and measure
the insulation resistance between the two with your 'megger', with all
lamps in place and all lights switched on.

If the circuit being split is wired in then modern twin/triple-and-earth
way the problem is unlikely to occur, although it could if somewhat
non-standard methods have been used. Where the wiring is singles in
conduit the issue is highly likely to occur and due diligence is required.

--
Andy
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