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

On 25/06/2018 01:08, 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?

I thought it was normal to use a 100mA time delay RCD at the 'main' CU?

It does mean the wiring to each rented room would need to have
mechanical protection or be surface wired, etc.

On 25/06/2018 01:08, 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.

So not an acceptable solution...

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.

If its clipped to the surface, then it does not require RCD protection.
(assuming the earthing system is not TT)

Neither of those options strike me as fully satisfactory. How's good to do this?

If you need RCD protection for the submain, then use a type S device at
the main CU.




--
Cheers,

John.

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

On 25/06/18 01:08, 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


The link circuit is classed as a distribution circuit. It need not be
RCD protected if it meets various criteria. Being visible is one.
Another is to mechanically protect it, eg SWA, heavy steel conduit.

However, I have this setup for a sub-CU that deals with "outside things"
(3 circuits). I opted for a single RCBO up the head end.

But I think in your case, as renters are more likely to cause trips and
you don't want other renters being affected, a non RCD distribution
circuit to the room and 2 RCBOs would give the best setup (lights don't
go out of they trip the socket circuit).

On 25/06/18 07:50, Fredxx wrote:
On 25/06/2018 01:08, 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?

I thought it was normal to use a 100mA time delay RCD at the 'main' CU?

It does mean the wiring to each rented room would need to have
mechanical protection or be surface wired, etc.



That's for TT earthing. 100mA Type S RCD does *not* provide "personal
protection" which is the factor here.

On 25/06/2018 13:32, Tim Watts wrote:
On 25/06/18 07:50, Fredxx wrote:
On 25/06/2018 01:08, 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?

I thought it was normal to use a 100mA time delay RCD at the 'main' CU?

It does mean the wiring to each rented room would need to have
mechanical protection or be surface wired, etc.



That's for TT earthing. 100mA Type S RCD does *not* provide "personal
protection" which is the factor here.

I thought the 100mA was primarily for fire protection rather than just a
requirement for TT earthing?

In this instance I had also assumed that each room would have its own CU
with two RCDs or perhaps one with some form of emergency lighting.

Its also not unknown for lighting to be on a permanent, separate
circuit, such it is not powered through a coin slot meter.

On 25/06/2018 14:30, Fredxx wrote:
On 25/06/2018 13:32, Tim Watts wrote:
On 25/06/18 07:50, Fredxx wrote:
On 25/06/2018 01:08, 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?

I thought it was normal to use a 100mA time delay RCD at the 'main' CU?

It does mean the wiring to each rented room would need to have
mechanical protection or be surface wired, etc.



That's for TT earthing. 100mA Type S RCD does *not* provide "personal
protection" which is the factor here.

Not if there are downstream 30mA trip devices...

I thought the 100mA was primarily for fire protection rather than just a
requirement for TT earthing?

Both - either for installation protection, or for circumstances where
the earth loop impedance is too high to reliably trip a MCB under fault
conditions.

In this instance I had also assumed that each room would have its own CU
with two RCDs or perhaps one with some form of emergency lighting.

If they are independent, (e.g. RCBOs) then you can skip the lighting
unless you are doing something particularly risky in there...

Its also not unknown for lighting to be on a permanent, separate
circuit, such it is not powered through a coin slot meter.


--
Cheers,

John.

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

On 25/06/2018 12:22, John Rumm wrote:
On 25/06/2018 01:08, 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.

So not an acceptable solution...

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.

If its clipped to the surface, then it does not require RCD protection.
(assuming the earthing system is not TT)

Neither of those options strike me as fully satisfactory. How's good
to do this?

If you need RCD protection for the submain, then use a type S device at
the main CU.




Just to add to that last bit. It depends on why you need the RCD protection.
If you need RCD protection other than for a TT supply (ignoring stables
etc) because the regs demand it due eg buried T&E then it must be 30mA
non time delayed.

--
Adam

On 25/06/2018 01:08, 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?


Surface clipped T&E does not need RCD protection. You can always trunk
it if you prefer.

Ideally the rooms should have 2 RCBOs in their CU.

--
Adam

On 25/06/2018 14:30, Fredxx wrote:
On 25/06/2018 13:32, Tim Watts wrote:
On 25/06/18 07:50, Fredxx wrote:
On 25/06/2018 01:08, 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?

I thought it was normal to use a 100mA time delay RCD at the 'main' CU?

It does mean the wiring to each rented room would need to have
mechanical protection or be surface wired, etc.



That's for TT earthing. 100mA Type S RCD does *not* provide "personal
protection" which is the factor here.

I thought the 100mA was primarily for fire protection rather than just a
requirement for TT earthing?

That would be 300mA RCD eg stables


In this instance I had also assumed that each room would have its own CU
with two RCDs or perhaps one with some form of emergency lighting.

That would be nice, but of no use if there is a 30mA RCD in series with it.

Its also not unknown for lighting to be on a permanent, separate
circuit, such it is not powered through a coin slot meter.


Yes, if done properly. Certainly for the communal areas. I am not sure
what the regs are for em lighting inside a HMO room. It certainly had
non on the last one I worked in.

--
Adam

On Monday, 25 June 2018 12:22:33 UTC+1, John Rumm wrote:
On 25/06/2018 01:08, tabbypurr 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.

So not an acceptable solution...

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.

If its clipped to the surface, then it does not require RCD protection.
(assuming the earthing system is not TT)

Neither of those options strike me as fully satisfactory. How's good to do this?

If you need RCD protection for the submain, then use a type S device at
the main CU.

I didn't think to mention the earthing: it's TT. So everything must be RCDed.

Leaving the lights out of the metering would make life a lot easier, presumably the secondary CUs could be dispensed with entirely. But it does create some problems down the line, so I'm not sure that'll be chosen.


NT

On Monday, 25 June 2018 16:53:35 UTC+1, ARW wrote:
On 25/06/2018 14:30, Fredxx wrote:
On 25/06/2018 13:32, Tim Watts wrote:
On 25/06/18 07:50, Fredxx wrote:
On 25/06/2018 01:08, tabbypurr 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?

I thought it was normal to use a 100mA time delay RCD at the 'main' CU?

It does mean the wiring to each rented room would need to have
mechanical protection or be surface wired, etc.



That's for TT earthing. 100mA Type S RCD does *not* provide "personal
protection" which is the factor here.

I thought the 100mA was primarily for fire protection rather than just a
requirement for TT earthing?

That would be 300mA RCD eg stables


In this instance I had also assumed that each room would have its own CU
with two RCDs or perhaps one with some form of emergency lighting.

That would be nice, but of no use if there is a 30mA RCD in series with it.

Its also not unknown for lighting to be on a permanent, separate
circuit, such it is not powered through a coin slot meter.


Yes, if done properly. Certainly for the communal areas. I am not sure
what the regs are for em lighting inside a HMO room. It certainly had
non on the last one I worked in.

The communal areas are unmetered.

Is it acceptable to use just one CU, like so:
Shared 300mA RCD feeds an MCB for each room.
Each MCB output goes to a meter and back into the main CU, where it feeds 2x RCBOs for lighting & sockets.
There are then no secondary CUs.


NT

On 26/06/2018 10:47, wrote:
On Monday, 25 June 2018 16:53:35 UTC+1, ARW wrote:

In this instance I had also assumed that each room would have its own CU
with two RCDs or perhaps one with some form of emergency lighting.

That would be nice, but of no use if there is a 30mA RCD in series with it.

Its also not unknown for lighting to be on a permanent, separate
circuit, such it is not powered through a coin slot meter.


Yes, if done properly. Certainly for the communal areas. I am not sure
what the regs are for em lighting inside a HMO room. It certainly had
non on the last one I worked in.

The communal areas are unmetered.

Is it acceptable to use just one CU, like so:
Shared 300mA RCD feeds an MCB for each room.

How many rooms?

Each MCB output goes to a meter and back into the main CU, where it feeds 2x RCBOs for lighting & sockets.
There are then no secondary CUs.

You would need a separate set of busbars for each room then, in addition
to a set for the main feeds to the meters. That may prove tricky
depending on how many rooms there are. So you would need a main switch
position with your type S RCD, plus a MCB for each room's meter feed.
You then need a main switch for each room plus 2 x RCBOs.

So if it were say three rooms, that would be 5 ways on the supply side,
plus 3 x 4 ways on the secondary side. 17 ways in total and 4 discrete
sets of neutral bars. You may find some CUs designed for a 4 way split,
Adam will probably be better able to advise.

Since its TT and you need all circuits RCD protected, you will have to
ensure any submains are routed in such a way that you don't trigger the
requirement that they be protected by a 30mA RCD, else you lose
discrimination.





--
Cheers,

John.

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

On 26/06/18 10:40, wrote:
On Monday, 25 June 2018 12:22:33 UTC+1, John Rumm wrote:
On 25/06/2018 01:08, tabbypurr 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.

So not an acceptable solution...

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.

If its clipped to the surface, then it does not require RCD protection.
(assuming the earthing system is not TT)

Neither of those options strike me as fully satisfactory. How's good to do this?

If you need RCD protection for the submain, then use a type S device at
the main CU.

I didn't think to mention the earthing: it's TT. So everything must be RCDed.


100mA Type S at the head end

30mA RCBOs in the room.

But you will still need to treat the cable as non-RCD protected for the
purposes of how it is routed as buried non mechanically protected cables
need to have RCD protection at 30mA/40mS.

So to avoid confusion:

100mA Type S: Provides fault protection on the cable as L-E fault
currents with a TT rod will generally not trip an MCB or fuse in the
required time.

30mA/40mS RCD for personal (shock) protection.


Leaving the lights out of the metering would make life a lot easier, presumably the secondary CUs could be dispensed with entirely. But it does create some problems down the line, so I'm not sure that'll be chosen.

If you are worried about the cable route, use SWA - it's going to be
small for these circuits, it's cheap enough and you can run it how you like.

On 26/06/18 10:47, wrote:
On Monday, 25 June 2018 16:53:35 UTC+1, ARW wrote:
On 25/06/2018 14:30, Fredxx wrote:
On 25/06/2018 13:32, Tim Watts wrote:
On 25/06/18 07:50, Fredxx wrote:
On 25/06/2018 01:08, tabbypurr 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?

I thought it was normal to use a 100mA time delay RCD at the 'main' CU?

It does mean the wiring to each rented room would need to have
mechanical protection or be surface wired, etc.



That's for TT earthing. 100mA Type S RCD does *not* provide "personal
protection" which is the factor here.

I thought the 100mA was primarily for fire protection rather than just a
requirement for TT earthing?

That would be 300mA RCD eg stables


In this instance I had also assumed that each room would have its own CU
with two RCDs or perhaps one with some form of emergency lighting.

That would be nice, but of no use if there is a 30mA RCD in series with it.

Its also not unknown for lighting to be on a permanent, separate
circuit, such it is not powered through a coin slot meter.


Yes, if done properly. Certainly for the communal areas. I am not sure
what the regs are for em lighting inside a HMO room. It certainly had
non on the last one I worked in.

The communal areas are unmetered.

Is it acceptable to use just one CU, like so:
Shared 300mA RCD feeds an MCB for each room.
Each MCB output goes to a meter and back into the main CU, where it feeds 2x RCBOs for lighting & sockets.
There are then no secondary CUs.

Why 300mA RCD? 100mA is more usual for TT

I think you'd be wise to make the socket RCBO available in the room if
these are rented in a small enclosure. This is what Premier Inn do for
each room (in most of the ones I've been in) - then the guest can at
least reset it if they trip it.

On 26/06/2018 11:42, John Rumm wrote:
On 26/06/2018 10:47, wrote:
On Monday, 25 June 2018 16:53:35 UTC+1, ARWÂ* wrote:

In this instance I had also assumed that each room would have its
own CU
with two RCDs or perhaps one with some form of emergency lighting.

That would be nice, but of no use if there is a 30mA RCD in series
with it.

Its also not unknown for lighting to be on a permanent, separate
circuit, such it is not powered through a coin slot meter.


Yes, if done properly. Certainly for the communal areas. I am not sure
what the regs are for em lighting inside a HMO room. It certainly had
non on the last one I worked in.

The communal areas are unmetered.

Is it acceptable to use just one CU, like so:
Shared 300mA RCD feeds an MCB for each room.

How many rooms?

Each MCB output goes to a meter and back into the main CU, where it
feeds 2x RCBOs for lighting & sockets.
There are then no secondary CUs.

You would need a separate set of busbars for each room then, in addition
to a set for the main feeds to the meters. That may prove tricky
depending on how many rooms there are. So you would need a main switch
position with your type S RCD, plus a MCB for each room's meter feed.
You then need a main switch for each room plus 2 x RCBOs.


The 30mA RCBOs need to be DP switched.


--
Adam

On Tuesday, 26 June 2018 11:42:01 UTC+1, John Rumm wrote:
On 26/06/2018 10:47, tabbypurr wrote:
On Monday, 25 June 2018 16:53:35 UTC+1, ARW wrote:

In this instance I had also assumed that each room would have its own CU
with two RCDs or perhaps one with some form of emergency lighting.

That would be nice, but of no use if there is a 30mA RCD in series with it.

Its also not unknown for lighting to be on a permanent, separate
circuit, such it is not powered through a coin slot meter.


Yes, if done properly. Certainly for the communal areas. I am not sure
what the regs are for em lighting inside a HMO room. It certainly had
non on the last one I worked in.

The communal areas are unmetered.

Is it acceptable to use just one CU, like so:
Shared 300mA RCD feeds an MCB for each room.

How many rooms?

4

Each MCB output goes to a meter and back into the main CU, where it feeds 2x RCBOs for lighting & sockets.
There are then no secondary CUs.

You would need a separate set of busbars for each room then,
in addition
to a set for the main feeds to the meters. That may prove tricky
depending on how many rooms there are. So you would need a main switch
position with your type S RCD, plus a MCB for each room's meter feed.
You then need a main switch for each room plus 2 x RCBOs.

why a main switch for each room?

So if it were say three rooms, that would be 5 ways on the supply side,
plus 3 x 4 ways on the secondary side. 17 ways in total and 4 discrete
sets of neutral bars.

RCBOs means each circuit neutral fed from its own RCBO

You may find some CUs designed for a 4 way split,
Adam will probably be better able to advise.

Since its TT and you need all circuits RCD protected, you will have to
ensure any submains are routed in such a way that you don't trigger the
requirement that they be protected by a 30mA RCD, else you lose
discrimination.

they're surface wired, clipped direct


NT

On 26/06/2018 22:19, ARW wrote:
On 26/06/2018 11:42, John Rumm wrote:


You would need a separate set of busbars for each room then, in
addition to a set for the main feeds to the meters. That may prove
tricky depending on how many rooms there are. So you would need a main
switch position with your type S RCD, plus a MCB for each room's meter
feed. You then need a main switch for each room plus 2 x RCBOs.


The 30mA RCBOs need to be DP switched.

You need DP isolation for TT, but the switch at the head of each sub
section would achieve that would it not? Or am I missing something?


--
Cheers,

John.

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

On 27/06/2018 00:55, wrote:
On Tuesday, 26 June 2018 11:42:01 UTC+1, John Rumm wrote:
On 26/06/2018 10:47, tabbypurr wrote:
On Monday, 25 June 2018 16:53:35 UTC+1, ARW wrote:

In this instance I had also assumed that each room would have its own CU
with two RCDs or perhaps one with some form of emergency lighting.

That would be nice, but of no use if there is a 30mA RCD in series with it.

Its also not unknown for lighting to be on a permanent, separate
circuit, such it is not powered through a coin slot meter.


Yes, if done properly. Certainly for the communal areas. I am not sure
what the regs are for em lighting inside a HMO room. It certainly had
non on the last one I worked in.

The communal areas are unmetered.

Is it acceptable to use just one CU, like so:
Shared 300mA RCD feeds an MCB for each room.

How many rooms?

4

That's going to be pushing it a bit for one CU...

Each MCB output goes to a meter and back into the main CU, where it feeds 2x RCBOs for lighting & sockets.
There are then no secondary CUs.

You would need a separate set of busbars for each room then,
in addition
to a set for the main feeds to the meters. That may prove tricky
depending on how many rooms there are. So you would need a main switch
position with your type S RCD, plus a MCB for each room's meter feed.
You then need a main switch for each room plus 2 x RCBOs.

why a main switch for each room?

Its one way of getting round the limitations on the number of available
neutral bus bars - you could return the circuit neutrals to the neutral
pole of the switch (since there will only be two circuit). It also gives
you a convenient method of isolation for the room.

So if it were say three rooms, that would be 5 ways on the supply side,
plus 3 x 4 ways on the secondary side. 17 ways in total and 4 discrete
sets of neutral bars.

RCBOs means each circuit neutral fed from its own RCBO

and in turn those need to be fed from somewhere...

[1] Adam makes a good point that I did overlook though. Using single
pole RCBOs like this potentially causes a loss of discrimination if you
were to ever get a neutral to earth short. It would trip the local RCBO,
but that would not clear the actual fault which would still be visible
to the head end RCD. Hence it would trip the whole installation.

The only way round that would be using double pole RCBOs, or using
another type S RCD in the place of each of the main switches.

You may find some CUs designed for a 4 way split,
Adam will probably be better able to advise.

Since its TT and you need all circuits RCD protected, you will have to
ensure any submains are routed in such a way that you don't trigger the
requirement that they be protected by a 30mA RCD, else you lose
discrimination.

they're surface wired, clipped direct

So at the moment, no need for a 30mA RCD on the submains.



--
Cheers,

John.

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

On 27/06/18 13:58, John Rumm wrote:
[1] Adam makes a good point that I did overlook though. Using single
pole RCBOs like this potentially causes a loss of discrimination if you
were to ever get a neutral to earth short. It would trip the local RCBO,
but that would not clear the actual fault which would still be visible
to the head end RCD. Hence it would trip the whole installation.

The only way round that would be using double pole RCBOs, or using
another type S RCD in the place of each of the main switches.

And as a side note, Hager sell DP RCBOs (industrial range) but they are
compatible with their consumer boards.

On 27/06/2018 13:58, John Rumm wrote:
On 27/06/2018 00:55, wrote:
On Tuesday, 26 June 2018 11:42:01 UTC+1, John RummÂ* wrote:
On 26/06/2018 10:47, tabbypurr wrote:

You would need a separate set of busbars for each room then,
in addition
to a set for the main feeds to the meters. That may prove tricky
depending on how many rooms there are. So you would need a main switch
position with your type S RCD, plus a MCB for each room's meter feed.
You then need a main switch for each room plus 2 x RCBOs.

why a main switch for each room?

Its one way of getting round the limitations on the number of available
neutral bus bars - you could return the circuit neutrals to the neutral
pole of the switch (since there will only be two circuit). It also gives
you a convenient method of isolation for the room.

So if it were say three rooms, that would be 5 ways on the supply side,
plus 3 x 4 ways on the secondary side. 17 ways in total and 4 discrete
sets of neutral bars.

RCBOs means each circuit neutral fed from its own RCBO

and in turn those need to be fed from somewhere...

[1] Adam makes a good point that I did overlook though. Using single
pole RCBOs like this potentially causes a loss of discrimination if you
were to ever get a neutral to earth short. It would trip the local RCBO,
but that would not clear the actual fault which would still be visible
to the head end RCD. Hence it would trip the whole installation.

The only way round that would be using double pole RCBOs, or using
another type S RCD in the place of each of the main switches.




So if the main switch for each room is done away with because we are
using DP 30mA RCBOs then that is 2 ways for the main 100mA switch and
assuming that the DP RCBOs are double width then a 21 way module CU will
work.

Schneider do single module DP RCBOs if you want a smaller CU but only on
a C curve.

There is now need for extra bus bars as the RCBOs neutrals would be
directly fed from the neutral bus bar on the outgoing side of the main
switch.


--
Adam

On 27/06/2018 18:39, ARW wrote:
On 27/06/2018 13:58, John Rumm wrote:
On 27/06/2018 00:55, wrote:
On Tuesday, 26 June 2018 11:42:01 UTC+1, John RummÂ* wrote:
On 26/06/2018 10:47, tabbypurr wrote:

You would need a separate set of busbars for each room then,
in addition
to a set for the main feeds to the meters. That may prove tricky
depending on how many rooms there are. So you would need a main switch
position with your type S RCD, plus a MCB for each room's meter feed.
You then need a main switch for each room plus 2 x RCBOs.

why a main switch for each room?

Its one way of getting round the limitations on the number of
available neutral bus bars - you could return the circuit neutrals to
the neutral pole of the switch (since there will only be two circuit).
It also gives you a convenient method of isolation for the room.

So if it were say three rooms, that would be 5 ways on the supply side,
plus 3 x 4 ways on the secondary side. 17 ways in total and 4 discrete
sets of neutral bars.

RCBOs means each circuit neutral fed from its own RCBO

and in turn those need to be fed from somewhere...

[1] Adam makes a good point that I did overlook though. Using single
pole RCBOs like this potentially causes a loss of discrimination if
you were to ever get a neutral to earth short. It would trip the local
RCBO, but that would not clear the actual fault which would still be
visible to the head end RCD. Hence it would trip the whole installation.

The only way round that would be using double pole RCBOs, or using
another type S RCD in the place of each of the main switches.




So if the main switch for each room is done away with because we are
using DP 30mA RCBOs then that is 2 ways for the main 100mA switch and
assuming that the DP RCBOs are double width then a 21 way module CU will
work.

Schneider do single module DP RCBOs if you want a smaller CU but only on
a C curve.

There is now need for extra bus bars as the RCBOs neutrals would be


Sorry - NO need for extra bus bars



--
Adam

Tim Watts wrote:

as a side note, Hager sell DP RCBOs

List price is north of £135+VAT though ...

On 27/06/2018 19:21, Andy Burns wrote:
Tim Watts wrote:

as a side note, Hager sell DP RCBOs

List price is north of £135+VAT though ...


Cheap then?

https://uk.rs-online.com/web/p/products/7909443

--
Adam

On 27/06/2018 19:54, ARW wrote:
On 27/06/2018 19:21, Andy Burns wrote:
Tim Watts wrote:

as a side note, Hager sell DP RCBOs

List price is north of £135+VAT though ...


Cheap then?

https://uk.rs-online.com/web/p/products/7909443



Just spotted these.

https://www.tlc-direct.co.uk/Products/WYNHXS1B06.html

https://www.electrium.co.uk/media/20...%20(We b).pdf

Not sure if the second link will work.

So Wylex are finally back in the game.

--
Adam

On Wednesday, 27 June 2018 18:39:08 UTC+1, ARW wrote:
On 27/06/2018 13:58, John Rumm wrote:
On 27/06/2018 00:55, tabbypurr wrote:
On Tuesday, 26 June 2018 11:42:01 UTC+1, John RummÂ* wrote:
On 26/06/2018 10:47, tabbypurr wrote:

You would need a separate set of busbars for each room then,
in addition
to a set for the main feeds to the meters. That may prove tricky
depending on how many rooms there are. So you would need a main switch
position with your type S RCD, plus a MCB for each room's meter feed.
You then need a main switch for each room plus 2 x RCBOs.

why a main switch for each room?

Its one way of getting round the limitations on the number of available
neutral bus bars - you could return the circuit neutrals to the neutral
pole of the switch (since there will only be two circuit). It also gives
you a convenient method of isolation for the room.

So if it were say three rooms, that would be 5 ways on the supply side,
plus 3 x 4 ways on the secondary side. 17 ways in total and 4 discrete
sets of neutral bars.

RCBOs means each circuit neutral fed from its own RCBO

and in turn those need to be fed from somewhere...

[1] Adam makes a good point that I did overlook though. Using single
pole RCBOs like this potentially causes a loss of discrimination if you
were to ever get a neutral to earth short. It would trip the local RCBO,
but that would not clear the actual fault which would still be visible
to the head end RCD. Hence it would trip the whole installation.

The only way round that would be using double pole RCBOs, or using
another type S RCD in the place of each of the main switches.




So if the main switch for each room is done away with because we are
using DP 30mA RCBOs then that is 2 ways for the main 100mA switch and
assuming that the DP RCBOs are double width then a 21 way module CU will
work.

each room would need 4 ways for itself, so the rooms alone would take up 16 ways, then there's the rest of the circuits. It would take more than a 21 way.

Schneider do single module DP RCBOs if you want a smaller CU but only on
a C curve.

There is now need for extra bus bars as the RCBOs neutrals would be
directly fed from the neutral bus bar on the outgoing side of the main
switch.

A bunch of Wylex RCBOs are available to use, the cost of all new DP RCBOs is not going down well.

There are some things I'm not seeing here. Why would socket & lighting need separate RCD action when the norm is to fit a split CU where lots of circuits all share each RCD?


NT

On 27/06/18 19:21, Andy Burns wrote:
Tim Watts wrote:

as a side note, Hager sell DP RCBOs

List price is north of £135+VAT though ...

I have one and they're nothing like that in reality...

On 27/06/2018 18:39, ARW wrote:

There is now need for extra bus bars as the RCBOs neutrals would be
directly fed from the neutral bus bar on the outgoing side of the main
switch.

That would mean that you are taking your live feed to the RCBO from the
return submain from the external meter, and the neutral feed from the
origin of the supply submain to the meter.

Probably not a problem electrically (the neutral connections on the
meter are likely just looped through anyway), but not quite as elegant
as using the neutral return from the meter.


--
Cheers,

John.

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

On 28/06/2018 11:07, John Rumm wrote:
On 27/06/2018 18:39, ARW wrote:

There is now need for extra bus bars as the RCBOs neutrals would be
directly fed from the neutral bus bar on the outgoing side of the main
switch.

That would mean that you are taking your live feed to the RCBO from the
return submain from the external meter, and the neutral feed from the
origin of the supply submain to the meter.

Probably not a problem electrically (the neutral connections on the
meter are likely just looped through anyway), but not quite as elegant
as using the neutral return from the meter.




The neutral is only there to allow the meter to measure the live current.

A meter will work without a neutral return.
Safest way to bypass a meter? - remove the neutral.


--
Adam

On 28/06/2018 19:02, ARW wrote:
On 28/06/2018 11:07, John Rumm wrote:
On 27/06/2018 18:39, ARW wrote:

There is now need for extra bus bars as the RCBOs neutrals would be
directly fed from the neutral bus bar on the outgoing side of the
main switch.

That would mean that you are taking your live feed to the RCBO from
the return submain from the external meter, and the neutral feed from
the origin of the supply submain to the meter.

Probably not a problem electrically (the neutral connections on the
meter are likely just looped through anyway), but not quite as elegant
as using the neutral return from the meter.




The neutral is only there to allow the meter to measure the live current.

Indeed, I realise it will work ok, it was more a case of my being wary
of doing something unexpected for a future maintainer.

A meter will work without a neutral return.
Safest way to bypass a meter? - remove the neutral.


--
Cheers,

John.

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

On Monday, 25 June 2018 01:08:08 UTC+1, tabby 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?
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?
I take it there's no regulatory problem with using one secondary CU for 4 circuits rather than 4 separate CUs.
Thanks everyone, getting close to plan 2 I think.


NT

On 29/06/2018 00:19, wrote:
On Monday, 25 June 2018 01:08:08 UTC+1, tabby 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?
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?

They must be 30mA RCD.


I take it there's no regulatory problem with using one secondary CU for 4 circuits rather than 4 separate CUs.
Thanks everyone, getting close to plan 2 I think.

No problem other than inconvenience with a fault.

You did make it sound like each meter was just supplying one room with
just a light and socket circuit. So what are the 4 circuits?


--
Adam

On 29/06/2018 00:19, wrote:
On Monday, 25 June 2018 01:08:08 UTC+1, tabby 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.

The submain feeds then return to a *pair* of basic 17th edition bundle
deal style CUs (i.e. split load type with two normal RCDs and a
selection of MCBs). Feed two rooms from each of the secondary CUs. You
will probably get a good enough selection of MCBs with the CUs to meet
your needs.

I take it there's no regulatory problem with using one secondary CU for 4 circuits rather than 4 separate CUs.

That bit is ok.




--
Cheers,

John.

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

On Friday, 29 June 2018 18:32:50 UTC+1, ARW wrote:
On 29/06/2018 00:19, tabbypurr wrote:
On Monday, 25 June 2018 01:08:08 UTC+1, tabby 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?
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?

They must be 30mA RCD.

If the cables feeding the meters require 30mA RCDs I'm seeing a problem with that. Either
a) there is no RCD further downline, and any current imabalnce causes both sockets & lights to go out in a room, or
b) there is another RCD, but no discrimination, so luck of the draw which trips & still often both circuits go off on a leakage fault.


I take it there's no regulatory problem with using one secondary CU for 4 circuits rather than 4 separate CUs.
Thanks everyone, getting close to plan 2 I think.

No problem other than inconvenience with a fault.

The 4 feeds would be electrically unconnected to each other, wouldn't make any difference to fault behaviour. So the secondary CU would have 4 inputs, 1 from each landlord's meter, not the usual 1.

You did make it sound like each meter was just supplying one room with
just a light and socket circuit. So what are the 4 circuits?

that's right, one return from the meter for each room. Each of which then feeds socket & light circuits.


NT

On Friday, 29 June 2018 21:51:21 UTC+1, John Rumm wrote:
On 29/06/2018 00:19, tabbypurr wrote:
On Monday, 25 June 2018 01:08:08 UTC+1, tabby 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

the question is really whether it's required or not.

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.

I don't think I follow. The whole point of time delayed RCDs is to achieve discrimination. But if the 30mA RCDs are SP, of course there would be no discrimination on a N-E fault. I gather that's not permissible.


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).

those options look excessively expensive


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.

we can put those in the main CU

The submain feeds then return to a *pair* of basic 17th edition bundle
deal style CUs (i.e. split load type with two normal RCDs and a
selection of MCBs). Feed two rooms from each of the secondary CUs. You
will probably get a good enough selection of MCBs with the CUs to meet
your needs.

IIUC you mean separate the 2 halves of the split CU so each RCD supplies one room. That brings me back to the question of whether lighting/socket discrimination within each room is a requirement. Life is easier if it isn't.

I take it there's no regulatory problem with using one secondary CU for 4 circuits rather than 4 separate CUs.

That bit is ok.

thanks everyone, busy making progress on other work atm.


NT

On 29/06/2018 21:51, John Rumm wrote:
On 29/06/2018 00:19, 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.


--
Adam

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.

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.

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

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?


NT

On 30/06/2018 08:15, ARW wrote:
On 29/06/2018 21:51, John Rumm wrote:
On 29/06/2018 00:19, 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.

Won't that mean the feeds to each room will not be 30mA RCD protected
and will need to be surfaced wired, or in SWA.

Do you need the lighting in a HMO room to be on a separate RCD to mains
outlets, or is the light in the landing on a different RCD circuit
sufficient?

On 30/06/2018 01:47, wrote:
On Friday, 29 June 2018 18:32:50 UTC+1, ARW wrote:
On 29/06/2018 00:19, tabbypurr wrote:
On Monday, 25 June 2018 01:08:08 UTC+1, tabby 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?
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?

They must be 30mA RCD.

If the cables feeding the meters require 30mA RCDs I'm seeing a problem with that. Either

A 30mA trip RCD (i.e. shock protection) would only be required for the
feeds to and from the meters if the cables are both hidden, and do not
had sufficient protection (either mechanically or by virtue of burial
depth). If they are surface wired, then you can do without 30mA protection.

They will need infrastructure / fire protection since its a TT install -
but that can be via a type S RCD




--
Cheers,

John.

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

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.

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.

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.

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

(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.

--
Cheers,

John.

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