Just a small point here. It has a 40A RCD and two mcbs. On the top of
the casing there are two terminal strips, one neutral and one earth. A
blue neutral wire is supplied that goes from the neutral terminal strip
to the "bottom" of the RCD switch. A small link is provided that
links the RCD with the two mcbs at the bottom of the RCD and mcbs.

The instructions are not that good. Looking at it, it seems the live
enters the RCD at the bottom as the neutral does too. Is that so? I
would think the live and neutral enter the RCD switch at the top and
out the bottom to the mcbs totally isolating them. Is the neutral wire
from neutral terminal to the bottom of the RCD switch to make the RCD
operate? That means the neutral across the RCD is bridged.

As the live enters the mcbs at the bottom the neutral must do as well,
with the out wores at the top.

wrote:

Just a small point here. It has a 40A RCD and two mcbs. On the top of
the casing there are two terminal strips, one neutral and one earth. A
blue neutral wire is supplied that goes from the neutral terminal strip
to the "bottom" of the RCD switch. A small link is provided that
links the RCD with the two mcbs at the bottom of the RCD and mcbs.

I'm not familiar with that particular CU, but is sounds like a perfectly
standard layout.

The instructions are not that good. Looking at it, it seems the live
enters the RCD at the bottom as the neutral does too. Is that so?

Well, no. From the description given above the incoming supply must go
to the top RCD terminals because the bottom ones would appear to be the
outgoing feeds to the 'live' (strictly, phase) busbar and to the neutral
bar.

The two outgoing final circuits are taken from the top terminal of each
MCB and the neutral bar. The earth bar is common to everything: the
incoming earth (or local earth rod, if it's a separately earthed TT
installation), the outgoing circuit earths and any main bonding conductors.

HTH - but if not, please provide a photo somewhere.

--
Andy

Andy Wade wrote:
wrote:

Thanks for the reply.

Just a small point here. It has a 40A RCD and two mcbs. On the top of
the casing there are two terminal strips, one neutral and one earth. A
blue neutral wire is supplied that goes from the neutral terminal strip
to the "bottom" of the RCD switch. A small link is provided that
links the RCD with the two mcbs at the bottom of the RCD and mcbs.

The small link is for the live and is called a bus bar. It is three
lugs that slot into the live terminals of all three switches of RCD and
two mcbs.

I'm not familiar with that particular CU, but is sounds like a perfectly
standard layout.

The instructions are not that good. Looking at it, it seems the live
enters the RCD at the bottom as the neutral does too. Is that so?

Well, no. From the description given above the incoming supply must go
to the top RCD terminals because the bottom ones would appear to be the
outgoing feeds to the 'live' (strictly, phase) busbar and to the neutral
bar.

Yes, that appears logical. Live in top of RCD and lives out to mcbs via
a small bus bar. It is the neutral wire from the neutral terminal that
goes directly to the output terminal of the RCD, which has thrown me.
The neutral is not switched through the RCD, although a terminal is
there to switch it on the RCD. I assess that this is to make the RCD
work not being familiar of operation.

The two outgoing final circuits are taken from the top terminal of each
MCB and the neutral bar.

If an mcb has a switching mechanism for both live and neutral (two
terminals on the input and output) can these be used and not take the
neutrals from the common neutral terminal strip? It must be safer to
isolate both live and neutral when the mcbs are off.

The earth bar is common to everything: the
incoming earth (or local earth rod, if it's a separately earthed TT
installation), the outgoing circuit earths and any main bonding conductors.

HTH - but if not, please provide a photo somewhere.

Sorry, I can't get a picture of it just yet. They are on the shelves at
Wickes. ;-) Once again thanks.

--
Andy

wrote:

The small link is for the live and is called a bus bar. It is three
lugs that slot into the live terminals of all three switches of RCD and
two mcbs.

Yes, and the Neutral bus bar will be elsewhere in the CU and fed via a
link cable or rod from the other terminal on the *bottom* of the RCD.

Well, no. From the description given above the incoming supply must go
to the top RCD terminals because the bottom ones would appear to be the
outgoing feeds to the 'live' (strictly, phase) busbar and to the neutral
bar.


Yes, that appears logical. Live in top of RCD and lives out to mcbs via
a small bus bar. It is the neutral wire from the neutral terminal that
goes directly to the output terminal of the RCD, which has thrown me.

The Neutral of the supply is connected to the *top* of the RCD - next to
the Phase from the supply. The outputs from the RCD are at the bottom,
one to the phase bus bar (metal thing with lugs), the other (typically
[1]) via a cable to a neutral bus bar - multiway cable terminal with
screw downs.

[1] some small CUs with only a few ways actually mount the neutral bus
bar directly into the output terminal at the base of the RCD or main
switch.

The neutral is not switched through the RCD, although a terminal is
there to switch it on the RCD. I assess that this is to make the RCD
work not being familiar of operation.

No - the neutral *is* switched via the RCD. Both phase and netural must
pass out and return via the RCD in order for it to operate correctly.

The two outgoing final circuits are taken from the top terminal of each
MCB and the neutral bar.


If an mcb has a switching mechanism for both live and neutral (two

They don't. MCBs in this type of CU are usually single pole and switch
the phase only.


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

John Rumm wrote:
wrote:

The small link is for the live and is called a bus bar. It is three
lugs that slot into the live terminals of all three switches of RCD and
two mcbs.

Yes, and the Neutral bus bar will be elsewhere in the CU and fed via a
link cable or rod from the other terminal on the *bottom* of the RCD.

Well, no. From the description given above the incoming supply must go
to the top RCD terminals because the bottom ones would appear to be the
outgoing feeds to the 'live' (strictly, phase) busbar and to the neutral
bar.

Yes, that appears logical. Live in top of RCD and lives out to mcbs via
a small bus bar. It is the neutral wire from the neutral terminal that
goes directly to the output terminal of the RCD, which has thrown me.

The Neutral of the supply is connected to the *top* of the RCD - next to
the Phase from the supply. The outputs from the RCD are at the bottom,
one to the phase bus bar (metal thing with lugs), the other (typically
[1]) via a cable to a neutral bus bar - multiway cable terminal with
screw downs.

[1] some small CUs with only a few ways actually mount the neutral bus
bar directly into the output terminal at the base of the RCD or main
switch.

The neutral is not switched through the RCD, although a terminal is
there to switch it on the RCD. I assess that this is to make the RCD
work not being familiar of operation.

No - the neutral *is* switched via the RCD. Both phase and netural must
pass out and return via the RCD in order for it to operate correctly.

The two outgoing final circuits are taken from the top terminal of each
MCB and the neutral bar.

Thanks. I get it now. The CU has two terminal strips, earth and
neutral. The live and neutral come in and connect to the "top" of the
RCD switch. The earth goes straight to the earth terminal. The output
of the RCD switch has the live going to the two mcbs via a amll bus bar
link, the neutral to the neutral terminal strip. When the RCD is off
there is no supply to the live bus bar or neutral terminal strip,
cutting out the lives and neutrals to all circuits. This makes sense.
The mcbs have the live from the output of the mcb and take take the
neutral from the common neutral terminal trip.

wrote:

Thanks. I get it now. The CU has two terminal strips, earth and
neutral. The live and neutral come in and connect to the "top" of the
RCD switch. The earth goes straight to the earth terminal. The output
of the RCD switch has the live going to the two mcbs via a amll bus bar
link, the neutral to the neutral terminal strip. When the RCD is off
there is no supply to the live bus bar or neutral terminal strip,
cutting out the lives and neutrals to all circuits. This makes sense.
The mcbs have the live from the output of the mcb and take take the
neutral from the common neutral terminal trip.

Spot on.



--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 26 Dec 2005 13:39:44 -0800, wrote:




Thanks. I get it now. The CU has two terminal strips, earth and
neutral. The live and neutral come in and connect to the "top" of the
RCD switch. The earth goes straight to the earth terminal. The output
of the RCD switch has the live going to the two mcbs via a amll bus bar
link, the neutral to the neutral terminal strip. When the RCD is off
there is no supply to the live bus bar or neutral terminal strip,
cutting out the lives and neutrals to all circuits. This makes sense.
The mcbs have the live from the output of the mcb and take take the
neutral from the common neutral terminal trip.

For completeness.....

In a larger CU with the possibility of configuring a so-called "split"
arrangement, there are normally two neutral bars connected initially
with a link.

The supply phase and neutral normally go into an incomer switch
(non-RCD if it isn't a TT system). The phase on the load side goes
to a bus bar which hooks together MCBs that are not being connected
via an RCD. The neutral on the load side of the incomer is connected
to the neutral bus bar.

If there is to be no RCD at all (unlikely these days), MCBs are fitted
all the way along and the link between the neutral bus bars stays in
place.

If there is an RCD, then the phase bus bar is cut appropriately and
the RCD fitted. The RCD takes its supply from the load terminals of
the incoming switch. The load phase connection of the RCD goes to a
second bus bar to feed downstream RCDs and then there is a neutral
connection from the load neutral of the RCD to the second neutral bus
bar. *Very Important*. The link between the two neutral bus bars
must be removed.

*Also important* When wiring circuits, the neutral connections must
go to the correct neutral bus bar dependent on whether the circuit is
upstream or downstream of the RCD.



--

..andy

In article .com,
wrote:
Thanks. I get it now. The CU has two terminal strips, earth and
neutral. The live and neutral come in and connect to the "top" of the
RCD switch. The earth goes straight to the earth terminal. The output
of the RCD switch has the live going to the two mcbs via a amll bus bar
link, the neutral to the neutral terminal strip. When the RCD is off
there is no supply to the live bus bar or neutral terminal strip,
cutting out the lives and neutrals to all circuits. This makes sense.
The mcbs have the live from the output of the mcb and take take the
neutral from the common neutral terminal trip.

Just the same as any CU. The master switch - or RCD - is double pole. MCBs
- or even fuses - are only on the line. Split load CUs are more confusing
as you need two neutral busses - one for the RCD protected side and one
for the non RCD one.

--
*TEAMWORK...means never having to take all the blame yourself *

Dave Plowman London SW
To e-mail, change noise into sound.