Okay, I am confused!

As I understand it (Please correct me if I am wrong!) if you have a TN-S
installation (Earth supplied via the armor on the mains supply) then if you
install a generator, you cant rely on this earth. - seems sensible, as the
fault that has taken you off supply could be because your cable has been
cut.

So you have to install an earth rod if you have a generator connected to the
installation with a transfer switch (Rather than if you just run things from
the generator from an extension lead).

Now where I am unclear is, I understand you have to switch the earth
connection when you changeover to the generator, you can't have both the
TN-S earth and the rod connected to your earthing system at the same time.
Why is this?

Surely the bonded water main would act like an earth rod in a TN-S supply
anyway?

Or is it just you must disconnect the suppliers TN-S earth when running the
generator - and if so, why?

Or am I barking up the wrong tree here?

Ta!

"Sparks" wrote in message
...
Okay, I am confused!

As I understand it (Please correct me if I am wrong!) if you have a TN-S
installation (Earth supplied via the armor on the mains supply) then if
you install a generator, you cant rely on this earth. - seems sensible, as
the fault that has taken you off supply could be because your cable has
been cut.

I can't fully explain why, but if you are using a generator through a change
over switch then you must convert to a TT system with a local earth rod, and
have the generator floating N strapped to E.

Tim..

"Tim.." wrote in message
...

"Sparks" wrote in message
...
Okay, I am confused!

As I understand it (Please correct me if I am wrong!) if you have a TN-S
installation (Earth supplied via the armor on the mains supply) then if
you install a generator, you cant rely on this earth. - seems sensible,
as the fault that has taken you off supply could be because your cable
has been cut.

I can't fully explain why, but if you are using a generator through a
change over switch then you must convert to a TT system with a local earth
rod, and have the generator floating N strapped to E.

Tim..

That's fine, but does this mean the local earth rod MUST be disconnected
during "normal" supply and the electricity companies earth MUST be
disconnected during generator supply - or can they both be left connected at
all times?

:-)

On Feb 3, 5:40 pm, "Sparks" wrote:
"Tim.." wrote in message

...





"Sparks" wrote in message
. ..
Okay, I am confused!

As I understand it (Please correct me if I am wrong!) if you have a TN-S
installation (Earth supplied via the armor on the mains supply) then if
you install a generator, you cant rely on this earth. - seems sensible,
as the fault that has taken you off supply could be because your cable
has been cut.

I can't fully explain why, but if you are using a generator through a
change over switch then you must convert to a TT system with a local earth
rod, and have the generator floating N strapped to E.

Tim..

That's fine, but does this mean the local earth rod MUST be disconnected
during "normal" supply
No, not at all. The local earth rod is connected at all times.

and the electricity companies earth MUST be
disconnected during generator supply
Yes. Otherwise you could end up feeding voltage through to the
linesman trying to repair the cable.

- or can they both be left connected at
all times?

My understanding is that "converting to a TT system" means that you
disconnect the nice safe earth the supply company provide, and use an
earch rod /instead/. That way you just have to switch over the
neutral and live when you switch to generator mode.


With luck, one of the professionals will be along shortly to tell us
all the actual requirements

Martin Bonner wrote:
On Feb 3, 5:40 pm, "Sparks" wrote:

That's fine, but does this mean the local earth rod MUST be disconnected
during "normal" supply
No, not at all. The local earth rod is connected at all times.

Agreed.

and the electricity companies earth MUST be
disconnected during generator supply
Yes.

Well, no, actually. Absolutely no. There is *no* requirement to
disconnect from the supplier's earth. In fact the regs generally forbid
inserting switching devices in protective (earth) conductors, for fairly
obvious reasons. Only hard links needing a tool to disconnect them are
allowed for testing purposes.

My understanding is that "converting to a TT system" means that you
disconnect the nice safe earth the supply company provide, and use an
earch rod /instead/. That way you just have to switch over the
neutral and live when you switch to generator mode.

Nothing needs to be converted to TT; you run the generator supply as a
TN-S system with a 3-core cable (or 2+armour) between genny and house
switchgear (5-core if it's 3-ph). One side of the generator winding (or
the star-point for 3-ph) is designated to be the neutral and is earthed
at the generator end, together with the all the generator's exposed
metalwork. (In other words the earth and neutral conductors of the
cable are connected together (and to earth) at the generator end.)

The need for an RCD comes about because the Ze value (effectively the
supply's line-earth impedance seen at the origin of the house
installation) may be a fair bit higher from the genny than from the much
stiffer public mains, so the standard assumptions that ensure that fuses
blow or MCBs trip sufficiently quickly don't necessarily hold true when
on the genny. A common RCD here ensures that quick disconnection will
always occur (possibly at the expense of discrimination, but that's
usually acceptable on a standby system that's only used occasionally).

--
Andy

"Andy Wade" wrote in message
...
Martin Bonner wrote:
On Feb 3, 5:40 pm, "Sparks" wrote:

That's fine, but does this mean the local earth rod MUST be disconnected
during "normal" supply
No, not at all. The local earth rod is connected at all times.

Agreed.

and the electricity companies earth MUST be
disconnected during generator supply
Yes.

Well, no, actually. Absolutely no. There is *no* requirement to
disconnect from the supplier's earth. In fact the regs generally forbid
inserting switching devices in protective (earth) conductors, for fairly
obvious reasons. Only hard links needing a tool to disconnect them are
allowed for testing purposes.

My understanding is that "converting to a TT system" means that you
disconnect the nice safe earth the supply company provide, and use an
earch rod /instead/. That way you just have to switch over the
neutral and live when you switch to generator mode.

Nothing needs to be converted to TT; you run the generator supply as a
TN-S system with a 3-core cable (or 2+armour) between genny and house
switchgear (5-core if it's 3-ph). One side of the generator winding (or
the star-point for 3-ph) is designated to be the neutral and is earthed at
the generator end, together with the all the generator's exposed
metalwork. (In other words the earth and neutral conductors of the cable
are connected together (and to earth) at the generator end.)

The need for an RCD comes about because the Ze value (effectively the
supply's line-earth impedance seen at the origin of the house
installation) may be a fair bit higher from the genny than from the much
stiffer public mains, so the standard assumptions that ensure that fuses
blow or MCBs trip sufficiently quickly don't necessarily hold true when on
the genny. A common RCD here ensures that quick disconnection will always
occur (possibly at the expense of discrimination, but that's usually
acceptable on a standby system that's only used occasionally).

--
Andy

Thanks for that Andy!

A few more question for you, if you don't mind!...

With an 8KVA generator, what size earth would you require between....
The generator & the earth rod?
The Generator and the consumer unit?

Ta!

Sparks wrote:

A few more question for you, if you don't mind!...

With an 8KVA generator, what size earth would you require between....
The generator & the earth rod?

Treat as for an earthing conductor. If you're using ordinary G/Y
insulated earth wire and it's buried for part of its length it needs to
be 16 mm^2. For bare copper in the ground you'd need 25 mm^2. If it's
not buried it could be 4 mm^2, or even 2.5 if protected in conduit.

Just use 16 insulated and you can't go wrong. Remember that the
connection to the rod needs to be accessible unless welded, soldered or
brazed etc., and labelled "safety electrical connection do not disconnect".

The Generator and the consumer unit?

Electrically than can be quite small, because RCD protection is in
place. What size and type of cable connects the generator to the
transfer switch? The armour of 2-core SWA will be quite OK, assuming
it's 6 mm^2 or larger. For T&E the included CPC will be OK. If you're
using singles or single-sheathed wires then I'd be inclined to make it
one size smaller than the line & neutral. The minimum size for any
separate protective conductor that is not in conduit or trunking is 4 mm^2.

--
Andy

"Martin Bonner" wrote in message
...
On Feb 3, 5:40 pm, "Sparks" wrote:
"Tim.." wrote in message

...





"Sparks" wrote in message
. ..
Okay, I am confused!

As I understand it (Please correct me if I am wrong!) if you have a
TN-S
installation (Earth supplied via the armor on the mains supply) then
if
you install a generator, you cant rely on this earth. - seems
sensible,
as the fault that has taken you off supply could be because your cable
has been cut.

I can't fully explain why, but if you are using a generator through a
change over switch then you must convert to a TT system with a local
earth
rod, and have the generator floating N strapped to E.

Tim..

That's fine, but does this mean the local earth rod MUST be disconnected
during "normal" supply
No, not at all. The local earth rod is connected at all times.

and the electricity companies earth MUST be
disconnected during generator supply
Yes. Otherwise you could end up feeding voltage through to the
linesman trying to repair the cable.

- or can they both be left connected at
all times?

My understanding is that "converting to a TT system" means that you
disconnect the nice safe earth the supply company provide, and use an
earch rod /instead/. That way you just have to switch over the
neutral and live when you switch to generator mode.


That is exactly what is done! MY neighbour has just had it done, and I have
quizzed the Spark this morning. He had a pole mounted earth provided by the
electricity company (which was actually found to be disconnected, but thats
another story!) but now has a local earth spike at the property linked by
10mm cable (only has to be 4mm according to the regs now- madness!)

Tim..

On Mon, 4 Feb 2008 14:01:19 -0000, Tim.. wrote:

but now has a local earth spike at the property linked by 10mm cable
(only has to be 4mm according to the regs now- madness!)

And presumably a 100mA time delayed "whole house" RCD? Under TT the
earthing doesn't have to be quite so chunky as it is assumed to have too
high an impedance to allow really hefty fault currents flow to blow fuses
or trip MCBs in the required time. Hence the 100mA time delayed wholes
house RCD.

--
Cheers
Dave.

"Andy Wade" wrote in message
...
Sparks wrote:

A few more question for you, if you don't mind!...

With an 8KVA generator, what size earth would you require between....
The generator & the earth rod?

Treat as for an earthing conductor. If you're using ordinary G/Y
insulated earth wire and it's buried for part of its length it needs to be
16 mm^2. For bare copper in the ground you'd need 25 mm^2. If it's not
buried it could be 4 mm^2, or even 2.5 if protected in conduit.

I have 10mm between the generator box and the spike, but it is not
protected, but the run is only about 30 cms, along a fence.
Is this OK, or should I put it in some conduit?

Just use 16 insulated and you can't go wrong. Remember that the
connection to the rod needs to be accessible unless welded, soldered or
brazed etc., and labelled "safety electrical connection do not
disconnect".

Do you need to protect the connection form the rain, in somthing like this?
http://www.tlc-direct.co.uk/Products/MTETC.html
If I got one of these, it would mean the conduit can juct push into it.

The Generator and the consumer unit?

Electrically than can be quite small, because RCD protection is in place.

I have a 100mA RCD in the transfer box (Inside) is this OK?
(There is a thermal type fuse on the generator that pops out if there is a
fault)

What size and type of cable connects the generator to the transfer switch?
The armour of 2-core SWA will be quite OK, assuming it's 6 mm^2 or larger.
For T&E the included CPC will be OK. If you're using singles or
single-sheathed wires then I'd be inclined to make it one size smaller
than the line & neutral. The minimum size for any separate protective
conductor that is not in conduit or trunking is 4 mm^2.

Okay, currently it is 6mm T&E between the CU and a IP66 box on the house
(Run of about 10m)
It then changes to 4mm SWA from this plastic box to one of these
http://www.tlc-direct.co.uk/Products...32slash3P.html
This run is about 4m
This connects to one of these
http://www.tlc-direct.co.uk/Products...32slash3S.html
Mounted on the box the generator lives in.
From the back of this, it terminates in another 32A plug, whch is connected
to a flying length of SWA I have installed on the generator panel (It only
comes with 2 16A sockets as standard for some reason)

I did want 6mm SWA, but the dozey tit who laid the new patio got 4mm, I only
realised after he had finished.
I do have a spare conduit layed in, so I can upgrade this is needed - do I
need to? (The generator is acually only 7.5KVA)
Basically the older red one of these!
http://www.mowers-online.co.uk/itm01414.htm

--
Andy

Thanks Andy!

Martin Bonner wrote:
With luck, one of the professionals will be along shortly to tell us
all the actual requirements

and lo, one of the professionals was along :-)

On Feb 4, 12:58 am, Andy Wade wrote:
On Feb 3, 5:40 pm, "Sparks" wrote:
That's fine, but does this mean the local earth rod MUST be disconnected
during "normal" supply
No, not at all. The local earth rod is connected at all times.

Agreed.

and the electricity companies earth MUST be
disconnected during generator supply
Yes.

Well, no, actually. Absolutely no. There is *no* requirement to
disconnect from the supplier's earth.
OK. That's why I wanted a professional.
In fact the regs generally forbid
inserting switching devices in protective (earth) conductors, for fairly
obvious reasons.
Oh sure. I wasn't saying anything about a switch! I just thought
that they company earth was permanently disconnected. Presumably, it
would be /allowed/ to disconnect the company earth, but a silly thing
to do (why disconnect a perfectly good earth).

Sparks wrote:

I have 10mm between the generator box and the spike, but it is not
protected, but the run is only about 30 cms, along a fence.
Is this OK, or should I put it in some conduit?

It should really be 16, or in conduit.

Do you need to protect the connection form the rain, in somthing like this?
http://www.tlc-direct.co.uk/Products/MTETC.html
If I got one of these, it would mean the conduit can juct push into it.

That's one option. The conduit adaptor solvent welds on to the end of
the conduit. Those little boxes don't leave much working space inside
though - I prefer to use a plastic earth pit, such as
http://www.screwfix.com/prods/16092/...tion-Earth-Pit
which you can finish neatly at ground level.

I have a 100mA RCD in the transfer box (Inside) is this OK?
(There is a thermal type fuse on the generator that pops out if there is
a fault)

Well, having the RCD at the far end of the feeder means that you're
relying on the generator's cut-out for protection in the event of a
cable fault. With that 4 mm^2 you only have a 1.5 mm^2 earth! You'd
have to ask the genny manufacturer about its fault characteristics, and
you might find it hard to get an answer. Much simpler to have the RCD
at the genny end - alternatively upgrade to 10 mm^2 SWA, or run a
separate 6 or 10 mm^2 earth alongside the T&E.

Basically the older red one of these!
http://www.mowers-online.co.uk/itm01414.htm

Have you established the generator's internal wiring details? Some
gennies like that are earthed to a centre-tap (115-0-115 V out), which
obviously is not what you need. Make sure you disconnect any centre-tap
connection before running it up with the neutral end earthed...

--
Andy

Martin Bonner wrote:

OK. That's why I wanted a professional.

You didn't get one though...

In fact the regs generally forbid inserting switching devices in
protective (earth) conductors, for fairly obvious reasons.

Having checked, there is an exception allowed in the new edition for
this and earth switching is allowed where essential, subject to various
conditions [see 537.1.5 for details].

Oh sure. I wasn't saying anything about a switch! I just thought
that they company earth was permanently disconnected. Presumably, it
would be /allowed/ to disconnect the company earth, but a silly thing
to do (why disconnect a perfectly good earth).

There's no compulsion to use a supplier's earth facility, either TN-S or
TN-C-S/PME. The TT option is always open, but of course conversion
would usually involve fitting at least one additional RCD. I don't see
any need or point in doing it though, if there's a good TN earth
available, unless the supplier or distributor imposes it as a condition
of having a generator.

--
Andy

"Andy Wade" wrote in message
...
Sparks wrote:

I have 10mm between the generator box and the spike, but it is not
protected, but the run is only about 30 cms, along a fence.
Is this OK, or should I put it in some conduit?

It should really be 16, or in conduit.

Do you need to protect the connection form the rain, in somthing like
this?
http://www.tlc-direct.co.uk/Products/MTETC.html
If I got one of these, it would mean the conduit can juct push into it.

That's one option. The conduit adaptor solvent welds on to the end of the
conduit. Those little boxes don't leave much working space inside
though - I prefer to use a plastic earth pit, such as
http://www.screwfix.com/prods/16092/...tion-Earth-Pit
which you can finish neatly at ground level.

I have a 100mA RCD in the transfer box (Inside) is this OK?
(There is a thermal type fuse on the generator that pops out if there is
a fault)

Well, having the RCD at the far end of the feeder means that you're
relying on the generator's cut-out for protection in the event of a cable
fault. With that 4 mm^2 you only have a 1.5 mm^2 earth!

It's 6mm T&E and three core 4mm SWA

You'd have to ask the genny manufacturer about its fault characteristics,
and you might find it hard to get an answer. Much simpler to have the RCD
at the genny end

Sounds like the best idea, Will install a 100mA RCD in the generator box.

- alternatively upgrade to 10 mm^2 SWA, or run a separate 6 or 10 mm^2
earth alongside the T&E.

Basically the older red one of these!
http://www.mowers-online.co.uk/itm01414.htm

Have you established the generator's internal wiring details? Some
gennies like that are earthed to a centre-tap (115-0-115 V out), which
obviously is not what you need. Make sure you disconnect any centre-tap
connection before running it up with the neutral end earthed...

From the earth pin on the generator, it seems to be putting out 115 between
the L and E and the same with N and E

If I strap one of these to Earth, I get...
230v between L and N
230v between L and E
0v between N and E

It didn't go bang or blow it's fuse!

Do I still need to disconnect somthing?


--
Andy

Thanks again!

In an earlier contribution to this discussion,
Sparks wrote:


From the earth pin on the generator, it seems to be putting out 115
between the L and E and the same with N and E

If I strap one of these to Earth, I get...
230v between L and N
230v between L and E
0v between N and E

It didn't go bang or blow it's fuse!

Do I still need to disconnect somthing?


No, you'll be fine! Your figures suggest that the earth was floating, and
sat in the middle when left to its own devices, but was quite happy to be
strapped to one side.

That's what I've done with my Honda generator. I was pretty sure the earth
was floating but initally connected N to E using a 60w light bulb rather
than a hard link on the basis that if it *wasn't* floating, the bulb would
glow and I would be able to measure a finite voltage across it. Result: no
glow and no voltage, so I replaced the bulb with a hard link.
--
Cheers,
Roger
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Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!

On Wed, 6 Feb 2008 22:04:57 -0000, Roger Mills wrote:

That's what I've done with my Honda generator. I was pretty sure the
earth was floating but initally connected N to E using a 60w light bulb
rather than a hard link on the basis that if it *wasn't* floating, the
bulb would glow and I would be able to measure a finite voltage across
it. Result: no glow and no voltage, so I replaced the bulb with a hard
link.

One also assumes that you have stuck a rudy great label on the set or made
the link *very* visible so that some one coming along expecting the two
phases to be flaoting (as is normal for a portable generator) is aware of
the link. Trouble is they might not be aware of the safety implications
such a link has...

Best to leave the Gen set unmodified and put the link in the connector you
use to connect to the "normal" wiring scheme. With a warning notice and
instruction not to operate the set with that connector unless the earth
spike is also connected to the sets frame.

--
Cheers
Dave.

In an earlier contribution to this discussion,
Dave Liquorice wrote:

On Wed, 6 Feb 2008 22:04:57 -0000, Roger Mills wrote:

That's what I've done with my Honda generator. I was pretty sure the
earth was floating but initally connected N to E using a 60w light
bulb rather than a hard link on the basis that if it *wasn't*
floating, the bulb would glow and I would be able to measure a
finite voltage across it. Result: no glow and no voltage, so I
replaced the bulb with a hard link.

One also assumes that you have stuck a rudy great label on the set or
made the link *very* visible so that some one coming along expecting
the two phases to be flaoting (as is normal for a portable generator)
is aware of the link. Trouble is they might not be aware of the
safety implications such a link has...

Best to leave the Gen set unmodified and put the link in the
connector you use to connect to the "normal" wiring scheme. With a
warning notice and instruction not to operate the set with that
connector unless the earth spike is also connected to the sets frame.

The link is in a well-labelled 13A plug plugged into one of the two outlets,
with the actual load being plugged into the other one.

I don't anticipate that anyone other than me will use it, anyway!
--
Cheers,
Roger
______
Email address maintained for newsgroup use only, and not regularly
monitored.. Messages sent to it may not be read for several weeks.
PLEASE REPLY TO NEWSGROUP!