Hello,

Does anyone know of a change over switch to switch between grid
electricity and a local generator? TLC sell two, one is:
http://www.tlc-direct.co.uk/Products/KMT338212I.html

rated at 32A; the other is rated at 63A.

The plan is to have two consumer units. The main one would be grid
connected only and supply most circuits. The second, smaller, CU would
be switched between grid and locally generated power. This second CU
would have three 6A MCBs: one for lights upstairs, one for lights
downstairs, and one for the CH boiler.

Now set-up like this the maximum load is 3*6=18A. Since this is well
below the 32A rating of the switch, can I use this switch? My concern
is that the CU RCD is rated at 80A as are the tails to the CU. If
someone added more MCBs in the future, for the freezer for example,
this could take the load above 32A and the switch would go bang before
everything else.

Should I protect the switch with a 30A fuse to prevent this? Or would
it be the responsibility of whomever added to the system in the future
to make the necessary changes and either add a fuse or upgrade the
switch?

I feel a 80A switch would be the best solution, if you know of one?

I appreciate this kind of work needs a qualified electrician and I
will do so but it's nice to understand what is involved so that I can
understand when I get quotes and different people tell me what they
will do and for how much. It also means I will know what they are on
about and whether what they say needs doing or whether I am being
talked into extra unnecessary work!

Thanks.

Stephen

Take a look at this switch:

http://www.briggsandstrattongenerato...ertransfer.htm

A more expensive option, but designed just for the job by those who know
about generators.

I use this switch (with a 6.5kVA generator) wired into my main Consumer Unit.
When the power fails, I switch all circuits (via their MCBs) off, power up
the genny, switch over, and then select which circuits to turn on.

Obviously lights up and down, plus the ground floor ring main (serves
freezer and boiler ).

url:http://myreader.co.uk/msg/1391142633.aspx

Stephen wrote:
Hello,

Does anyone know of a change over switch to switch between grid
electricity and a local generator? TLC sell two, one is:
http://www.tlc-direct.co.uk/Products/KMT338212I.html

rated at 32A; the other is rated at 63A.

The plan is to have two consumer units. The main one would be grid
connected only and supply most circuits. The second, smaller, CU would
be switched between grid and locally generated power. This second CU
would have three 6A MCBs: one for lights upstairs, one for lights
downstairs, and one for the CH boiler.

Now set-up like this the maximum load is 3*6=18A. Since this is well
below the 32A rating of the switch, can I use this switch? My concern
is that the CU RCD is rated at 80A as are the tails to the CU. If
someone added more MCBs in the future, for the freezer for example,
this could take the load above 32A and the switch would go bang before
everything else.


The 32A switch is fine as long as: -

1. The mcb in the grid cu is no larger than 32A.
2. The mcb in the generator is no larger than 32A.

This means that a maximum of 32A can be drawn through the switch
irrespective of the current carrying capacity of the cu or its tails.

However, since you would power the generator CU from a 32A outgoing way
on the grid CU (via the changeover switch, natch), you wouldn't need
16mm2 or 25mm2 tails on the generator cu; you could just use 4mm2 T&E.


HTH
Rumble

On Fri, 13 Mar 2009 23:00:39 +0000, Dave Osborne
wrote:

The 32A switch is fine as long as: -

1. The mcb in the grid cu is no larger than 32A.
2. The mcb in the generator is no larger than 32A.

This means that a maximum of 32A can be drawn through the switch
irrespective of the current carrying capacity of the cu or its tails.

However, since you would power the generator CU from a 32A outgoing way
on the grid CU (via the changeover switch, natch), you wouldn't need
16mm2 or 25mm2 tails on the generator cu; you could just use 4mm2 T&E.

Thank you. I wasn't thinking of doing it like that: I was going to
take the tails into a Henley block and then run separate feeds to the
grid CU and small CU. The problem then was that the feed from the
Henley block would be fused at 80A, much higher than the rating of the
switch.

Your solution is much tidier: I will not need the Henley block, nor a
fuse before the switch, so less boxes to put on the wall. And like you
say, I can use, smaller, more manageable cable and run from a spare
32A MCB in the grid CU.

Thanks very much,

On Fri, 13 Mar 2009 23:00:39 +0000, Dave Osborne
wrote:

you wouldn't need
16mm2 or 25mm2 tails on the generator cu; you could just use 4mm2 T&E.

Sorry to add to my own reply but I have just thought: isn't 4mm^2 T&E
frowned on in this group because it has a small CPC? Would 6mm^2 T&E
be better? I know CPC size is important; I remember reading the
"taking electric outside" in the FAQ but OTOH we're probably talking a
couple of metres at most running from one CU to the other via the
switch.

One disadvantage of running the second CU from the first is that both
have RCDs. Wouldn't this mean that if something caused the small CU to
trip it would also trip the first CU? I suppose I could specify the
second CU to have just a switch rather than an RCD but that would
still result in the main CU RCD tripping on faults in the second CU,
and by not having an RCD in the second box it means there's no RCD
protection when fed from off-grid electricity. I suppose the ideal
solution would be to upgrade my main CU to a split load one and feed
the second CU from the un-RCDed part.

At the moment it is one RCD for everything, which I know this group
doesn't like. How do split load CUs fit in with the 17th edition?
Isn't everything supposed to have an RCD now?

Thanks,
Stephen.

Stephen wrote:
On Fri, 13 Mar 2009 23:00:39 +0000, Dave Osborne
wrote:

you wouldn't need
16mm2 or 25mm2 tails on the generator cu; you could just use 4mm2 T&E.

Sorry to add to my own reply but I have just thought: isn't 4mm^2 T&E
frowned on in this group because it has a small CPC? Would 6mm^2 T&E
be better?

Over a short distance it makes no odds. Use 6mm2 if you prefer.

I know CPC size is important; I remember reading the
"taking electric outside" in the FAQ but OTOH we're probably talking a
couple of metres at most running from one CU to the other via the
switch.

One disadvantage of running the second CU from the first is that both
have RCDs. Wouldn't this mean that if something caused the small CU to
trip it would also trip the first CU?

No. Under some circumstances, they would both trip; under others, only
one would trip (and it could be either!).

I suppose I could specify the
second CU to have just a switch rather than an RCD but that would
still result in the main CU RCD tripping on faults in the second CU,

True, but that's the way it works now, so you would be no worse off.

and by not having an RCD in the second box it means there's no RCD
protection when fed from off-grid electricity.

True, but since you're probably only going to do lighting, the boiler
and the freezer from your genny, you may consider that having the genny
RCD protected is not a high priority. Also, you could protect the genny
by an RCD before the changeover switch if you wanted.

I suppose the ideal
solution would be to upgrade my main CU to a split load one and feed
the second CU from the un-RCDed part.


Absolutely, if you chose not to bring your CU installation up to 17th
standards.

At the moment it is one RCD for everything, which I know this group
doesn't like.

Only because of its potential nuisance value.

How do split load CUs fit in with the 17th edition?
Isn't everything supposed to have an RCD now?


Well, notionally, yes, all final circuits do under the 17th.
However, the feed to the second CU is a sub-main, so no RCD protection
is required (unless you have a TT supply of course).

John Rumm or Owain will hopefully be along in a bit to comment
further(I'm not really up to speed on the 17th).

Thanks,
Stephen.

On Sun, 15 Mar 2009 18:32:31 +0000, Dave Osborne
wrote:

Over a short distance it makes no odds. Use 6mm2 if you prefer.

I was going to look up the equation and double check, but that's what
I thought because it's such a small length.

One disadvantage of running the second CU from the first is that both
have RCDs. Wouldn't this mean that if something caused the small CU to
trip it would also trip the first CU?

No. Under some circumstances, they would both trip; under others, only
one would trip (and it could be either!).

Which could be a nuisance, though hopefully RCD trips should be very
rare.

I suppose I could specify the
second CU to have just a switch rather than an RCD but that would
still result in the main CU RCD tripping on faults in the second CU,

True, but that's the way it works now, so you would be no worse off.

But perhaps now is the time to fix that since I'll be having the other
work done at the same time. OTOH how much of a problem is it having
everything on the one RCD because RCDs shouldn't trip that often.

Also, you could protect the genny
by an RCD before the changeover switch if you wanted.

That's true but two boxes (CU and RCD) probably will cost twice having
one RCD'ed CU so it might be simpler to do it all in one box.

I suppose the ideal
solution would be to upgrade my main CU to a split load one and feed
the second CU from the un-RCDed part.

Absolutely, if you chose not to bring your CU installation up to 17th
standards.

Can you get such an upgrade now thought? Surely everything has to be
done to 17th ed. now?

How do split load CUs fit in with the 17th edition?
Isn't everything supposed to have an RCD now?

I've had a quick look through Screwfix and it looks as though some
17ed boxes have two non-protected circuits before the RCD. I think you
are supposed to use RCBOs in those though.

I notice Screwfix sell Crabtree type-B MCBs but their RCBOs are type
C. Why is that? Why aren't they both the same type?

I am also puzzled because there seem to be split load boxes with two
RCDs. How does one trip without taking out the other?

John Rumm or Owain will hopefully be along in a bit to comment
further(I'm not really up to speed on the 17th).

Are they electricians? I never knew that. I'll pay special attention
to their posts.

Thanks,
Stephen.

On Sun, 15 Mar 2009 21:26:59 +0000, Stephen wrote:

I've had a quick look through Screwfix and it looks as though some
17ed boxes have two non-protected circuits before the RCD. I think you
are supposed to use RCBOs in those though.

I was going to mention screwfix as a flyer came through the door showinga
Wylex split load CU complete with 2 RCDs, Main switch, and MCBs plus some
white electrical bits single pole double sockets and pendant fitteings for
£100...

I am also puzzled because there seem to be split load boxes with two
RCDs. How does one trip without taking out the other?

'cause they aren't wired in series? I would assume that the incoming mains
goes through the main switch then splits to each RCD. This gives you the
required 30mA shock protection on all circits whilst maintaining some
circuits if one RCD trips.

--
Cheers
Dave.

Stephen wrote:

I suppose the ideal
solution would be to upgrade my main CU to a split load one and feed
the second CU from the un-RCDed part.
Absolutely, if you chose not to bring your CU installation up to 17th
standards.

Can you get such an upgrade now thought? Surely everything has to be
done to 17th ed. now?

Have a look at this document:

http://www.elecsa.org.uk/downloads/p...0Conductor.pdf

http://preview.tinyurl.com/c7pmw8

There is no general requirement that you have to bring an electrical
installation up to the current regulations, however, there are
circumstances in which you have some obligation to improve your
installation.

In particular, if you renew your consumer unit, you have to do so in
compliance with the latest version of the regualtions and at the same
time ensure that your main equipotential bonding is up to scratch.


How do split load CUs fit in with the 17th edition?
They're pretty much de-rigueur.

Isn't everything supposed to have an RCD now?

Well, yes, but only because of a new requirement for circuits to be
buried 50mm deep or have mechanical protection or be RCD protected means
that for replacing CU's, RCD's/RCBO's galore is the way forward.


I've had a quick look through Screwfix and it looks as though some
17ed boxes have two non-protected circuits before the RCD. I think you
are supposed to use RCBOs in those though.

Maybe. As I said before, only final circuits need to be protected.
Sub-mains don't.

I notice Screwfix sell Crabtree type-B MCBs but their RCBOs are type
C. Why is that? Why aren't they both the same type?

Up until the 17th edition, RCBO's were almost exclusively used in
industrial environments, where Type C MCB's are very common.

I suggest you consider http://www.tlc-direct.co.uk for your electrical
supplies.


I am also puzzled because there seem to be split load boxes with two
RCDs. How does one trip without taking out the other?

The RCD's are not daisy-chained. There is effectively a Henley block
inside the consumer unit.


John Rumm or Owain will hopefully be along in a bit to comment
further(I'm not really up to speed on the 17th).

Are they electricians? I never knew that. I'll pay special attention
to their posts.

John is a clever bugger who is up to speed on a number of subjects.

Owain is (afaik) an electrician who I for one never seem to disagree with.

If Christian McArdle were still around, I would recommend him as well,
but he dropped out some years ago. Anyone know what happened to him?

On Sun, 15 Mar 2009 22:43:42 +0000 (GMT), "Dave Liquorice"
wrote:

I am also puzzled because there seem to be split load boxes with two
RCDs. How does one trip without taking out the other?

'cause they aren't wired in series? I would assume that the incoming mai=
ns
goes through the main switch then splits to each RCD. This gives you the=

required 30mA shock protection on all circits whilst maintaining some
circuits if one RCD trips.

Thanks. I haven't got a split load box so hadn't realised the busbars
were in parallel; that explains it.

On Sun, 15 Mar 2009 23:30:33 +0000, Dave Osborne
wrote:

There is no general requirement that you have to bring an electrical
installation up to the current regulations, however, there are
circumstances in which you have some obligation to improve your
installation.

In particular, if you renew your consumer unit, you have to do so in
compliance with the latest version of the regualtions and at the same
time ensure that your main equipotential bonding is up to scratch.

Thanks for the informative post. So it would appear that though other
aspects of the installation need not be upgraded, you must always
upgrade CUs to the latest regs?

Can only the electricity company upgrade the earth connection from the
CU to the meter?

How do split load CUs fit in with the 17th edition?
They're pretty much de-rigueur.

I was thinking of the old split-load with one half without a RCD but
if I have to go 17th ed, these are now out of the question (so why are
they still being sold?)

I suggest you consider http://www.tlc-direct.co.uk for your electrical
supplies.

I am already a happy TLC customer

Thanks,
Stephen.