I'm shortly going to be renting out a property which I'm buying, and
before I do I'll be getting the electrics checked over.

Before that - pre-empting the results of the PIR - I think I should
probably upgrade the CU to a modern RCD-protected job, even if only a
butt-covering exercise (and I'll also first need to install main
equipotential bonding twixt gas and water pipes).

I'm aware I may be opening a can of worms by upgrading the CU and then
finding the RCD won't set owing to a hitherto undetected issue with the
existing wiring, but I'm accepting that. I'm not intending to touch the
existing wiring as such, unless necessary - it looks OK - ie I'm not
into adding any new circuits etc.

I would be very grateful for advice as to what specification of CU to go
for, especially as I'm not familar with the 17th edn regs.

Existing syatem seems to have 1 lighting circuit and 1 ring main (not
sure why there are apparently 3 circuit breakers though?):

What would be most appropriate as a like-for-like replacement - a 2-way
(3-way if needed?) CU, with full RCD protection? Or split-load with the
lights not on RCD? Something else?

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

Thanks
David

Owain wrote:
On Nov 13, 8:02 pm, Lobster wrote:
Existing syatem seems to have 1 lighting circuit and 1 ring main (not
sure why there are apparently 3 circuit breakers though?):

15A for an immersion heater, probably.

Under 17th Edition
- all circuits need RCD protection (with limited exceptions for
specific cable types/mounting methods)
- whole-house RCD is not permitted

Presumably the property is on the small size, but it would be wise to
allow for future needs (and the special offer packs of CU+MCBs are
usually about 10-way) which usually work out cheaper than buying a
smaller CU with main switch and using RCBOs for every circuit.

MK 16 module, 10 way, populated Dual RCD Consumer Unit including MCBs
is £80 on scrooficks. Gives plenty of room for expansion for an
electric cooker, shower, additional lighting and power circuits etc.

Bear in mind if you are renting it out and all the lights fail, that
needs an emergency response, if the ceiling lights fail but the wall
lights still work that gives you more breathing space to get it
fixed.

I just don't fall for this "the lights have gone out so I will fall down the
stars" routine.

We do not turn into lemmings when the lights go out.


--
Adam

Lobster wrote:
I'm shortly going to be renting out a property which I'm buying, and
before I do I'll be getting the electrics checked over.

Before that - pre-empting the results of the PIR - I think I should
probably upgrade the CU to a modern RCD-protected job, even if only a
butt-covering exercise (and I'll also first need to install main
equipotential bonding twixt gas and water pipes).

I'm aware I may be opening a can of worms by upgrading the CU and then
finding the RCD won't set owing to a hitherto undetected issue with
the existing wiring, but I'm accepting that. I'm not intending to
touch the existing wiring as such, unless necessary - it looks OK -
ie I'm not into adding any new circuits etc.

I would be very grateful for advice as to what specification of CU to
go for, especially as I'm not familar with the 17th edn regs.

Existing syatem seems to have 1 lighting circuit and 1 ring main (not
sure why there are apparently 3 circuit breakers though?):

What would be most appropriate as a like-for-like replacement - a
2-way (3-way if needed?) CU, with full RCD protection? Or split-load
with the lights not on RCD? Something else?

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

Thanks
David

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

It looks like some bonding is already done.

--
Adam

ARWadsworth wrote:

Lobster wrote:

What would be most appropriate as a like-for-like replacement - a
2-way (3-way if needed?) CU, with full RCD protection? Or split-load
with the lights not on RCD? Something else?

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

It looks like some bonding is already done.

Would you do a Periodic before starting a CU change on something like
this?
My parents want theirs changing, but being as most of the wiring is 50
years old, I'm not going to be changing anything until I get it all
tested. They really need a full rewire, but they dont want the
re-decoration that would involve, so it'll be a compromise of fitting a
CU with proper bonding.
Ta
Alan,
--
To reply by e-mail, change the ' + ' to 'plus'.

A.Lee wrote:
ARWadsworth wrote:

Lobster wrote:

What would be most appropriate as a like-for-like replacement - a
2-way (3-way if needed?) CU, with full RCD protection? Or
split-load with the lights not on RCD? Something else?

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

It looks like some bonding is already done.

Would you do a Periodic before starting a CU change on something like
this?

If I was at work then yes as a) I could then warn the customer that there
will be extra costs as the RCD is going to trip and b) I can use the results
on the certificate I need to issue.

If it was for a property that I owned then no. I'd do the PIR after the CU
swap and fix any faults as required. A PIR for 3 circuits should not take
long.

My parents want theirs changing, but being as most of the wiring is 50
years old, I'm not going to be changing anything until I get it all
tested. They really need a full rewire, but they dont want the
re-decoration that would involve, so it'll be a compromise of fitting
a CU with proper bonding.

You need to do the test and fix any faults first. You know that your mother
will complain if the sockets trip:-)

Are you intending to fit a 17th edition compliant CU?
And
http://esc.org.uk/pdfs/business-and-...BPG1v2_web.pdf
probably applys to your parents.

Cheers
--
Adam

On 14/11/2010 09:11, ARWadsworth wrote:
wrote:
I'm shortly going to be renting out a property which I'm buying, and
before I do I'll be getting the electrics checked over.

Before that - pre-empting the results of the PIR - I think I should
probably upgrade the CU to a modern RCD-protected job, even if only a
butt-covering exercise (and I'll also first need to install main
equipotential bonding twixt gas and water pipes).

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

Thanks for the advice.

Thinking on this further a bit - the reason for making the change is
purely to make it safer for tenants and to cover my butt (see above!) -
I have no plans at all to extend the wiring. I'd like to provide RCD
protection if possible, for safety reasons.

Looking at the current CU, it looks as if original wired fuses have been
replaced by MCBs - is it possible to get hold of plug-in RCBOs in the
same way? If so, it seems that would meet my requirements in full?

If not... then how important does the panel reckon it is that I do
upgrade the CU? I'll certainly be getting the house wiring checked
anyway, and one improvement I could easily do is replace the double
socket by the back door (which could be used for electrical appliances
in the back yard) with an RCD-protected item.

It looks like some bonding is already done.

Indeed, but AFAICR not to the water main (on the opposite side of the
house!)

Thanks
David

Lobster wrote:

Looking at the current CU, it looks as if original wired fuses have been
replaced by MCBs

Yes.

is it possible to get hold of plug-in RCBOs in the
same way? If so, it seems that would meet my requirements in full?

No, because they need neutral connections too.

Lobster wrote:
On 14/11/2010 09:11, ARWadsworth wrote:
wrote:
I'm shortly going to be renting out a property which I'm buying, and
before I do I'll be getting the electrics checked over.

Before that - pre-empting the results of the PIR - I think I should
probably upgrade the CU to a modern RCD-protected job, even if only
a butt-covering exercise (and I'll also first need to install main
equipotential bonding twixt gas and water pipes).

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

Thanks for the advice.

Thinking on this further a bit - the reason for making the change is
purely to make it safer for tenants and to cover my butt (see above!)
- I have no plans at all to extend the wiring. I'd like to provide
RCD protection if possible, for safety reasons.

Looking at the current CU, it looks as if original wired fuses have
been replaced by MCBs - is it possible to get hold of plug-in RCBOs
in the same way? If so, it seems that would meet my requirements in
full?

I am afraid not. And if I could invent one I would be very rich.

If not... then how important does the panel reckon it is that I do
upgrade the CU? I'll certainly be getting the house wiring checked
anyway, and one improvement I could easily do is replace the double
socket by the back door (which could be used for electrical appliances
in the back yard) with an RCD-protected item.

Under the 16th edition RCDs were required on sockets that could reasonably
be expected to supply portable equipment outdoors.

Now that is a wide definition. Could you reasonably expect someone trying to
cut the grass with a faulty lawnmower that trips the RCD socket to then try
it in another non RCD protected socket in the house?

The 17th edition is a little tighter in it's rules - it requires all sockets
to have RCD protection apart from a very few selective sockets that almost
certainly do not apply to your installation.

Assuming that the place is a long term investment and you can change the CU
yourself for less than £100 (and still have the electrician check it all out
and issue a PIR for the same price as checking out the old fuse box and
issuing a PIR) I would say swap it. Your arse is coverered and apart from
the PIR it is DIY.


It looks like some bonding is already done.

Indeed, but AFAICR not to the water main (on the opposite side of the
house!)

Bugger! Never mind.

--
Adam

On 15/11/10 18:47, Lobster wrote:
On 14/11/2010 09:11, ARWadsworth wrote:
wrote:
I'm shortly going to be renting out a property which I'm buying, and
before I do I'll be getting the electrics checked over.

Before that - pre-empting the results of the PIR - I think I should
probably upgrade the CU to a modern RCD-protected job, even if only a
butt-covering exercise (and I'll also first need to install main
equipotential bonding twixt gas and water pipes).

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

Thanks for the advice.

Thinking on this further a bit - the reason for making the change is
purely to make it safer for tenants and to cover my butt (see above!) -
I have no plans at all to extend the wiring. I'd like to provide RCD
protection if possible, for safety reasons.

Looking at the current CU, it looks as if original wired fuses have been
replaced by MCBs - is it possible to get hold of plug-in RCBOs in the
same way? If so, it seems that would meet my requirements in full?

If not... then how important does the panel reckon it is that I do
upgrade the CU? I'll certainly be getting the house wiring checked
anyway, and one improvement I could easily do is replace the double
socket by the back door (which could be used for electrical appliances
in the back yard) with an RCD-protected item.

It looks like some bonding is already done.

Indeed, but AFAICR not to the water main (on the opposite side of the
house!)

Thanks
David
You could put an RCD before the consumber unbit
which would then trip everything if theres a fault-
so maybe also put a few battery maintained emergency lights at the top
of the stairs.

If you rewire you legally have to put in mains powered fire alarms, i
believe.
[g]

george [dicegeorge] wrote:
On 15/11/10 18:47, Lobster wrote:
On 14/11/2010 09:11, ARWadsworth wrote:
wrote:
I'm shortly going to be renting out a property which I'm buying,
and before I do I'll be getting the electrics checked over.

Before that - pre-empting the results of the PIR - I think I should
probably upgrade the CU to a modern RCD-protected job, even if
only a butt-covering exercise (and I'll also first need to install
main equipotential bonding twixt gas and water pipes).

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

Thanks for the advice.

Thinking on this further a bit - the reason for making the change is
purely to make it safer for tenants and to cover my butt (see
above!) - I have no plans at all to extend the wiring. I'd like to
provide RCD protection if possible, for safety reasons.

Looking at the current CU, it looks as if original wired fuses have
been replaced by MCBs - is it possible to get hold of plug-in RCBOs
in the same way? If so, it seems that would meet my requirements in
full? If not... then how important does the panel reckon it is that I do
upgrade the CU? I'll certainly be getting the house wiring checked
anyway, and one improvement I could easily do is replace the double
socket by the back door (which could be used for electrical
appliances in the back yard) with an RCD-protected item.

It looks like some bonding is already done.

Indeed, but AFAICR not to the water main (on the opposite side of the
house!)

Thanks
David
You could put an RCD before the consumber unbit
which would then trip everything if theres a fault-
so maybe also put a few battery maintained emergency lights at the top
of the stairs.

If you rewire you legally have to put in mains powered fire alarms, i
believe.
[g]

No, you don't have to install fire alarms or smoke alarms with a rewire.

--
Adam

On 14/11/2010 09:11, ARWadsworth wrote:

wrote:

I'm aware I may be opening a can of worms by upgrading the CU and then
finding the RCD won't set owing to a hitherto undetected issue with
the existing wiring, but I'm accepting that. I'm not intending to
touch the existing wiring as such, unless necessary - it looks OK -
ie I'm not into adding any new circuits etc.

I would be very grateful for advice as to what specification of CU to
go for, especially as I'm not familar with the 17th edn regs.

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

Am revisiting this thread as am now near the time when I need to take
the plunge, plus I've done a bit of reading up on the 17th edn jungle.

Having looked at the old CU again, there are clearly only actually 2
active circuits - one lights, one ring main. While I have no plans to
rectify that far-from-ideal situation, I can imagine it might prove
necessary to divvy up the current circuits or add new ones at some point
in the future.

So - am wondering: can't I achieve the same (and equally compliant)
level of protection by using a 17th edn dual-RCD CU with standard MCBs,
instead of 1 RCBO per circuit? - ie, lights on one RCD and ringmain on
the other? Or am I missing something? AFAICS it would be a slightly
cheaper way of doing it anyway; and it certainly would be if I did ever
want to add new circuits in the future.

Thanks
David

Lobster wrote:
On 14/11/2010 09:11, ARWadsworth wrote:

wrote:

I'm aware I may be opening a can of worms by upgrading the CU and
then finding the RCD won't set owing to a hitherto undetected issue
with the existing wiring, but I'm accepting that. I'm not intending
to touch the existing wiring as such, unless necessary - it looks
OK - ie I'm not into adding any new circuits etc.

I would be very grateful for advice as to what specification of CU
to go for, especially as I'm not familar with the 17th edn regs.

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

Am revisiting this thread as am now near the time when I need to take
the plunge, plus I've done a bit of reading up on the 17th edn jungle.

Having looked at the old CU again, there are clearly only actually 2
active circuits - one lights, one ring main. While I have no plans to
rectify that far-from-ideal situation, I can imagine it might prove
necessary to divvy up the current circuits or add new ones at some
point in the future.

So - am wondering: can't I achieve the same (and equally compliant)
level of protection by using a 17th edn dual-RCD CU with standard
MCBs, instead of 1 RCBO per circuit? - ie, lights on one RCD and
ringmain on the other? Or am I missing something? AFAICS it would
be a slightly cheaper way of doing it anyway; and it certainly would
be if I did ever want to add new circuits in the future.

Thanks
David


No reason why not.

The price difference between a 5 way CU with 2 RCBOs and a 17th edition CU
is minor, but the cost to add circuits on the latter is much cheaper.

--
Adam

On 05/12/2010 18:16, ARWadsworth wrote:
wrote:
On 14/11/2010 09:11, ARWadsworth wrote:

wrote:

I'm aware I may be opening a can of worms by upgrading the CU and
then finding the RCD won't set owing to a hitherto undetected issue
with the existing wiring, but I'm accepting that. I'm not intending
to touch the existing wiring as such, unless necessary - it looks
OK - ie I'm not into adding any new circuits etc.

I would be very grateful for advice as to what specification of CU
to go for, especially as I'm not familar with the 17th edn regs.

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

Am revisiting this thread as am now near the time when I need to take
the plunge, plus I've done a bit of reading up on the 17th edn jungle.

Having looked at the old CU again, there are clearly only actually 2
active circuits - one lights, one ring main. While I have no plans to
rectify that far-from-ideal situation, I can imagine it might prove
necessary to divvy up the current circuits or add new ones at some
point in the future.

So - am wondering: can't I achieve the same (and equally compliant)
level of protection by using a 17th edn dual-RCD CU with standard
MCBs, instead of 1 RCBO per circuit? - ie, lights on one RCD and
ringmain on the other? Or am I missing something? AFAICS it would
be a slightly cheaper way of doing it anyway; and it certainly would
be if I did ever want to add new circuits in the future.

No reason why not.

The price difference between a 5 way CU with 2 RCBOs and a 17th edition CU
is minor, but the cost to add circuits on the latter is much cheaper.

Great, thanks for the reassurance Adam: glad I've apparently correctly
got my head round it!

David

On 05/12/2010 18:16, ARWadsworth wrote:
wrote:
On 14/11/2010 09:11, ARWadsworth wrote:

wrote:

I'm aware I may be opening a can of worms by upgrading the CU and
then finding the RCD won't set owing to a hitherto undetected issue
with the existing wiring, but I'm accepting that. I'm not intending
to touch the existing wiring as such, unless necessary - it looks
OK - ie I'm not into adding any new circuits etc.

I would be very grateful for advice as to what specification of CU
to go for, especially as I'm not familar with the 17th edn regs.

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

Am revisiting this thread as am now near the time when I need to take
the plunge, plus I've done a bit of reading up on the 17th edn jungle.

Having looked at the old CU again, there are clearly only actually 2
active circuits - one lights, one ring main. While I have no plans to
rectify that far-from-ideal situation, I can imagine it might prove
necessary to divvy up the current circuits or add new ones at some
point in the future.

So - am wondering: can't I achieve the same (and equally compliant)
level of protection by using a 17th edn dual-RCD CU with standard
MCBs, instead of 1 RCBO per circuit? - ie, lights on one RCD and
ringmain on the other? Or am I missing something? AFAICS it would
be a slightly cheaper way of doing it anyway; and it certainly would
be if I did ever want to add new circuits in the future.


No reason why not.

The price difference between a 5 way CU with 2 RCBOs and a 17th edition CU
is minor, but the cost to add circuits on the latter is much cheaper.

Well that was lucky! Because having bought the 17th edn CU I've just
discovered that actually there are three circuits, in that what was
evidently formerly a radial for an immersion heater is now used by the
combi boiler instead.

So my next question - given that I will have one ring main and one
lighting circuit, each on separate RCDs - which side should I wire in
the radial for the boiler, with the ring main or lights? Worried about
loss of frost protection in the winter when an RCD trips...

Thanks
David

Lobster wrote:
On 05/12/2010 18:16, ARWadsworth wrote:
wrote:
On 14/11/2010 09:11, ARWadsworth wrote:

wrote:

I'm aware I may be opening a can of worms by upgrading the CU and
then finding the RCD won't set owing to a hitherto undetected
issue with the existing wiring, but I'm accepting that. I'm not
intending to touch the existing wiring as such, unless necessary
- it looks OK - ie I'm not into adding any new circuits etc.

I would be very grateful for advice as to what specification of CU
to go for, especially as I'm not familar with the 17th edn regs.

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

Am revisiting this thread as am now near the time when I need to
take the plunge, plus I've done a bit of reading up on the 17th edn
jungle. Having looked at the old CU again, there are clearly only
actually 2
active circuits - one lights, one ring main. While I have no plans
to rectify that far-from-ideal situation, I can imagine it might
prove necessary to divvy up the current circuits or add new ones at
some point in the future.

So - am wondering: can't I achieve the same (and equally compliant)
level of protection by using a 17th edn dual-RCD CU with standard
MCBs, instead of 1 RCBO per circuit? - ie, lights on one RCD and
ringmain on the other? Or am I missing something? AFAICS it would
be a slightly cheaper way of doing it anyway; and it certainly would
be if I did ever want to add new circuits in the future.


No reason why not.

The price difference between a 5 way CU with 2 RCBOs and a 17th
edition CU is minor, but the cost to add circuits on the latter is
much cheaper.

Well that was lucky! Because having bought the 17th edn CU I've just
discovered that actually there are three circuits, in that what was
evidently formerly a radial for an immersion heater is now used by the
combi boiler instead.

So my next question - given that I will have one ring main and one
lighting circuit, each on separate RCDs - which side should I wire in
the radial for the boiler, with the ring main or lights? Worried
about loss of frost protection in the winter when an RCD trips...

You have never said what the property is used for. Is it occupied? If so
then stick the boiler on the same RCD as the lights. Most people tend to
notice when it is cold and the lights are not working.


--
Adam

On 15/12/2010 22:04, ARWadsworth wrote:
wrote:
On 05/12/2010 18:16, ARWadsworth wrote:
wrote:
On 14/11/2010 09:11, ARWadsworth wrote:

wrote:

I'm aware I may be opening a can of worms by upgrading the CU and
then finding the RCD won't set owing to a hitherto undetected
issue with the existing wiring, but I'm accepting that. I'm not
intending to touch the existing wiring as such, unless necessary
- it looks OK - ie I'm not into adding any new circuits etc.

I would be very grateful for advice as to what specification of CU
to go for, especially as I'm not familar with the 17th edn regs.

Here's the current configuration, which no doubt will prompt other
questions!:
http://img508.imageshack.us/img508/1033/77871334.jpg

I would use a 5 way CU and 3 RCBOs.
The 15A MCB would probably have been for an immersion heater.

Am revisiting this thread as am now near the time when I need to
take the plunge, plus I've done a bit of reading up on the 17th edn
jungle. Having looked at the old CU again, there are clearly only
actually 2
active circuits - one lights, one ring main. While I have no plans
to rectify that far-from-ideal situation, I can imagine it might
prove necessary to divvy up the current circuits or add new ones at
some point in the future.

So - am wondering: can't I achieve the same (and equally compliant)
level of protection by using a 17th edn dual-RCD CU with standard
MCBs, instead of 1 RCBO per circuit? - ie, lights on one RCD and
ringmain on the other? Or am I missing something? AFAICS it would
be a slightly cheaper way of doing it anyway; and it certainly would
be if I did ever want to add new circuits in the future.


No reason why not.

The price difference between a 5 way CU with 2 RCBOs and a 17th
edition CU is minor, but the cost to add circuits on the latter is
much cheaper.

Well that was lucky! Because having bought the 17th edn CU I've just
discovered that actually there are three circuits, in that what was
evidently formerly a radial for an immersion heater is now used by the
combi boiler instead.

So my next question - given that I will have one ring main and one
lighting circuit, each on separate RCDs - which side should I wire in
the radial for the boiler, with the ring main or lights? Worried
about loss of frost protection in the winter when an RCD trips...

You have never said what the property is used for. Is it occupied? If so
then stick the boiler on the same RCD as the lights. Most people tend to
notice when it is cold and the lights are not working.

Just a normal residential home... so in terms of frost protection I
suppose the main concern would be when nobody's home, eg the classic
going away for the Xmas holidays?

So thinking about what might cause a trip while the home is empty...
security light bulb (or inside light left on) blowing? faulty appliance
left plugged in? Dunno. I suppose it's best on the lighting circuit
(though from personal experience in my home, every time a bulb blows, my
RCD pops)

David

Lobster wrote:

Am revisiting this thread as am now near the time when I need to
take the plunge, plus I've done a bit of reading up on the 17th
edn jungle. Having looked at the old CU again, there are clearly
only actually 2
active circuits - one lights, one ring main. While I have no
plans to rectify that far-from-ideal situation, I can imagine it
might prove necessary to divvy up the current circuits or add new
ones at some point in the future.

So - am wondering: can't I achieve the same (and equally
compliant) level of protection by using a 17th edn dual-RCD CU
with standard MCBs, instead of 1 RCBO per circuit? - ie, lights
on one RCD and ringmain on the other? Or am I missing something?
AFAICS it would be a slightly cheaper way of doing it anyway; and
it certainly would be if I did ever want to add new circuits in
the future.


No reason why not.

The price difference between a 5 way CU with 2 RCBOs and a 17th
edition CU is minor, but the cost to add circuits on the latter is
much cheaper.

Well that was lucky! Because having bought the 17th edn CU I've
just discovered that actually there are three circuits, in that
what was evidently formerly a radial for an immersion heater is now
used by the combi boiler instead.

So my next question - given that I will have one ring main and one
lighting circuit, each on separate RCDs - which side should I wire
in the radial for the boiler, with the ring main or lights? Worried
about loss of frost protection in the winter when an RCD trips...

You have never said what the property is used for. Is it occupied?
If so then stick the boiler on the same RCD as the lights. Most
people tend to notice when it is cold and the lights are not working.

Just a normal residential home... so in terms of frost protection I
suppose the main concern would be when nobody's home, eg the classic
going away for the Xmas holidays?

So thinking about what might cause a trip while the home is empty...
security light bulb (or inside light left on) blowing? faulty
appliance left plugged in? Dunno. I suppose it's best on the
lighting circuit (though from personal experience in my home, every
time a bulb blows, my RCD pops)

Faulty fridge or freezer, faulty CH pump:-)

I suppose if we have a full week below zero degrees that you can pop in and
check if the place is left empty for that week. Of course you will have to
check the gas and electric meters are not pre payment meters with no credit
left on them

There is not really much to over engineer on your setup. And don't forget
there are thousands of people out there with the CH on RCD protected
circuits that have no problems.

--
Adam