[ac1951[_2_]]

I noticed on a recent thread about cable size to an outside detached
Garage, that there where some issues mentioned regarding whether or
not you where exporting a PME earth.

The points raised that interested me related to the size of earth
conductor and usage of the supply.
In my case I have, already in-place, a 2.5mm twin SWA cable connecting
my garage which converts to a 2.5mm T&E cable on the outside of my
house. This is connected to a 6A RCBO in the CU. (Total distance circa
40M)

In the garage is a 2 way CU with 2x5A mcb's, one feeding the lights
and the other feeding 2xDouble 13A sockets.
I use the sockets for basic power tools (drill, planer, sander etc)
and also electric lawn mower. All the tools are double insulated and
therefore do not have an earth.

Is this installation legal ?

What would you do differently ?

Where can I find a concise explaination of do's/don't regarding the
exporting of a PME.

thanks
Andy

[Pete Verdon]

ac1951 wrote:

In my case I have, already in-place, a 2.5mm twin SWA cable connecting
my garage which converts to a 2.5mm T&E cable on the outside of my
house. This is connected to a 6A RCBO in the CU. (Total distance circa
40M)

In the garage is a 2 way CU with 2x5A mcb's, one feeding the lights
and the other feeding 2xDouble 13A sockets.
I use the sockets for basic power tools (drill, planer, sander etc)
and also electric lawn mower. All the tools are double insulated and
therefore do not have an earth.

Is this installation legal ?

I can't answer that, but to help those who can you will also need to say
what, if anything, is connected to the armour of your SWA. The actual
connection to the armour is in the form of the brass gland(s), but what
is connected onwards from here?

Pete

[John Rumm]

ac1951 wrote:

I noticed on a recent thread about cable size to an outside detached
Garage, that there where some issues mentioned regarding whether or
not you where exporting a PME earth.

The points raised that interested me related to the size of earth
conductor and usage of the supply.
In my case I have, already in-place, a 2.5mm twin SWA cable connecting
my garage which converts to a 2.5mm T&E cable on the outside of my
house. This is connected to a 6A RCBO in the CU. (Total distance circa
40M)

Did you really mean 6A or was that typo for 16A?

What is the imbalance trip current rating for the RCBO?

In the garage is a 2 way CU with 2x5A mcb's, one feeding the lights
and the other feeding 2xDouble 13A sockets.
I use the sockets for basic power tools (drill, planer, sander etc)
and also electric lawn mower. All the tools are double insulated and
therefore do not have an earth.

That means you are not at risk of taking the PME earth into the garden
etc. Which is good. ;-)

Is this installation legal ?

What is the construction of the garage like? Any other services in there
like water or gas?

What would you do differently ?

Probably make the garage a TT install with its own RCD and local earth.

Where can I find a concise explaination of do's/don't regarding the
exporting of a PME.

Well you could say BS7671 is "concise" to the point of being terse at
times. I don't recall seeing any particularly details articles or
coverage in books on the subject - although there are plenty of items
that cover at least a part of the requirements.

This one is ok:

http://www.iee.org/Publish/WireRegs/...s_outdoors.pdf

Failing that, our very own:

http://wiki.diyfaq.org.uk/index.php?...ricity_outside

Touches on a number of salient points (even more so since I added some
today!)


--
Cheers,

John.

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

[robgraham]

On 23 Apr, 21:37, John Rumm wrote:
ac1951 wrote:
I noticed on a recent thread about cable size to an outside detached
Garage, that there where some issues mentioned regarding whether or
not you where exporting a PME earth.

The points raised that interested me related to the size of earth
conductor and usage of the supply.
In my case I have, already in-place, a 2.5mm twin SWA cable connecting
my garage which converts to a 2.5mm T&E cable on the outside of my
house. This is connected to a 6A RCBO in the CU. (Total distance circa
40M)

Did you really mean 6A or was that typo for 16A?

What is the imbalance trip current rating for the RCBO?

In the garage is a 2 way CU with 2x5A mcb's, one feeding the lights
and the other feeding 2xDouble 13A sockets.
I use the sockets for basic power tools (drill, planer, sander etc)
and also electric lawn mower. All the tools are double insulated and
therefore do not have an earth.

That means you are not at risk of taking the PME earth into the garden
etc. Which is good. ;-)

Is this installation legal ?

What is the construction of the garage like? Any other services in there
like water or gas?

What would you do differently ?

Probably make the garage a TT install with its own RCD and local earth.

Where can I find a concise explaination of do's/don't regarding the
exporting of a PME.

Well you could say BS7671 is "concise" to the point of being terse at
times. I don't recall seeing any particularly details articles or
coverage in books on the subject - although there are plenty of items
that cover at least a part of the requirements.

This one is ok:

http://www.iee.org/Publish/WireRegs/...ents/Issue16/2...

Failing that, our very own:

http://wiki.diyfaq.org.uk/index.php?...ricity_outside

Touches on a number of salient points (even more so since I added some
today!)

--
Cheers,

John.

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

A timely posting and I hope my questions do not get taken as a
highjack !

I'm about to deal with a similar situation. The garage is fed with a
6mm T&E from it's own mcb in the house CU - earthing is TN-S and I had
already considered that a TT earth for the garage would be a good idea
(30m plus run from the CU) with the earth to the house disconnedted.

Two questions - as the TT earth is relatively high resistance compared
to the supplier earth, and hence the fault current will not be high,
does the earth cable size have to be any bigger than 2.5mm ? If it is
to be bigger, what is the justification for this?

Secondly, although there is no water pipes, etc, the garage roof is
steel frames; should these be bonded ?

Thanks

Rob

[Andy Wade]

robgraham wrote:

Two questions - as the TT earth is relatively high resistance compared
to the supplier earth, and hence the fault current will not be high,
does the earth cable size have to be any bigger than 2.5mm ? If it is
to be bigger, what is the justification for this?

By "earth cable" do you mean the earthing conductor - i.e. the
connection to the earth electrode? If buried for part of its length
this needs to be 16 mm^2 on grounds of mechanical robustness. If not
buried then 4 mm^2 is OK. In both cases the size can be reduced to 2.5
mm^2 if the conductor is mechanically protected by being run in conduit,
etc.

Secondly, although there is no water pipes, etc, the garage roof is
steel frames; should these be bonded ?

There is no need to bond structural steelwork unless it's in contact
with the ground.

--
Andy

[John Rumm]

robgraham wrote:

I'm about to deal with a similar situation. The garage is fed with a
6mm T&E from it's own mcb in the house CU - earthing is TN-S and I had
already considered that a TT earth for the garage would be a good idea
(30m plus run from the CU) with the earth to the house disconnedted.

Two questions - as the TT earth is relatively high resistance compared
to the supplier earth, and hence the fault current will not be high,
does the earth cable size have to be any bigger than 2.5mm ?

You mean the one in the 6mm^2 T&E at the house?

At a quick glance, no; because the earth fault protection here is
provided by the TN-S earth at the house and not your TT earth. (hence
why you don't need RCD protection at the head end).

Might be worth checking the disconnect time though. You have not said
what proportion of the run is T&E and what is SWA, so we will check the
worst case - all T&E with a round trip resistance phase to CPC of 10.49
mOhms/m. If we take the TN-S external earth fault loop impedance as 0.8
ohms we get 0.8 + 30 x 0.01049 = 1.1147 total. That gives a prospective
fault current of 206A. That should open a 32A on the instant part of its
trip response. For a 40A MCB you will be getting closer to the lower end
of the fault current required[1] to open within five secs, so you will
need to watch the fault withstand time of the CPC - at 5 secs it would
be under sized.

[1] However we are making pessimistic assumptions here, so real life
will probably be better.

If it is
to be bigger, what is the justification for this?

If you are talking about the main earth connection to your earth stake,
then that should be in 6mm^2 as per Fig 4c in the OSG.

Secondly, although there is no water pipes, etc, the garage roof is
steel frames; should these be bonded ?

No need since you are TT and your frame etc is attached to the same
local earth as the CU. There is also no risk that a fault in the supply
could leave your earth floating at a high voltage - as might be the case
with a PME head end.


--
Cheers,

John.

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

[ac1951[_2_]]

On Apr 23, 9:37*pm, John Rumm wrote:

Thanks John see answers to your questions below:-

This is connected to a 6A RCBO in the CU. (Total distance circa 40M)
Did you really mean 6A or was that *typo for 16A?
No definately a 6A but I had thought of changing it to 10A. I seem to
remember when I did some calculation this was OK but I need to double
check unless you know differently.

What is the imbalance trip current rating for the RCBO?
30mA

Is this installation legal ?

What is the construction of the garage like? Any other services in there
like water or gas?

No Water or Gas..concrete block construction timber frame roof with
normal flet and roofing tiles, wooden up-over door, concrete floor.

What would you do differently ?

Probably make the garage a TT install with its own RCD and local earth.



--
Cheers,

John.

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