Ok guys,

Our national grid system appears to be a TT one- where the earth conductor
is connected back to the neutral wire at the service fuse where it enters
the house, *and* there is an earth rod too, connected to same.

I am wanting to install a generator change over unit, which will isolate N
and L from the grid and allow the gen to feed the house. (Honda 6kva)

Question:

Do I need to earth stake the genny from its earth terminal, and / or connect
the generator plug's N and E together to bring the N line of the generator
down to zero potential to create a safe system???

Or what?!

TIA

Tim..

On Mon, 29 Mar 2004 17:28:22 +0000 (UTC), in uk.d-i-y " Tim \(Remove
NOSPAM." strung together this:

Ok guys,

Our national grid system appears to be a TT one- where the earth conductor
is connected back to the neutral wire at the service fuse where it enters
the house,

That's not TT.

*and* there is an earth rod too, connected to same.

That is. I think what someones done is provided a backup TT earth in
addition to the TN-C-S earth supplied by the electric company, just in
case it's lost at their end.

I am wanting to install a generator change over unit, which will isolate N
and L from the grid and allow the gen to feed the house. (Honda 6kva)

Question:

Do I need to earth stake the genny from its earth terminal, and / or connect
the generator plug's N and E together to bring the N line of the generator
down to zero potential to create a safe system???

No. If you fire the genny up and plug something in is it unsafe? It's
the same as powering a few small items in the house throught he
existing wiring. For extra safety you could install an earth spike and
connect it to the generators earth terminal, that's about all you'll
need to do.
--

SJW
A.C.S. Ltd.

" Tim (Remove NOSPAM." wrote in message
...

Our national grid system appears to be a TT one- where the earth
conductor is connected back to the neutral wire at the service fuse
where it enters the house, *and* there is an earth rod too, connected
to same.

That's not TT, that's TN-C-S (PME) with an additional earth electrode.

I am wanting to install a generator change over unit, which will
isolate N and L from the grid and allow the gen to feed the house.
(Honda 6kva)

Do I need to earth stake the genny from its earth terminal, and / or
connect the generator plug's N and E together to bring the N line of
the generator down to zero potential to create a safe system???

Or what?!

Yes to both questions, for a permanant installation. Here's a wiring
diagram that I've posted a couple of times in the last few months, and which
should be fairly self-explanatory. Your exisiting earth electrode, provided
that it's up to scratch and securely connected to the main earth terminal,
should be OK (unless it's the supplier's, in which case you should really
provide your own, connected in parallel).

Since the impedance of the supply from a small genny will be higher than
from the mains, earth fault loop impedance (Zs) values might be too high to
give proper protection. For this reason a 100 or 300 mA RCD should be used
in the generator feed as an aditional precaution.


L o----------------o
\ SW1a
Mains o---------- L to CU
from
meter +-------o
| SW1: DPCO changeover switch
| 100A break before make!
N o----------------o
| \ SW1b
| o---------- N to CU
|
| +---o
| |
| |
Supplier's E o-----------------------------o Main earth terminal
earth | | /
| | |
| | 16mm^2 |
| | earth |
------- |
| | |
| G.P. | | G.P. = Generator
| | | protection,
------- | fuse or CB
| | | plus 100mA RCD
| | |
-- | - | -- |
| G( | | |
| G( | | |
| G( | | | G = generator winding
| G( | | |
| G( | | |
| | | | |
Generator | +---.B | |
frame - | | | | B = bonding connection
-------.-- |
B|\ |
| \-------------+
|
|
|
-----
--- Your earth electrode(s)
- Re 200 ohm



NB 1. You are supposed to consult with your electricity supplier /
distribution network operator before installing switched standby generating
plant.

NB 2. Reg. 551-02-03 requires automatic load shedding arrangements if the
generator is not rated to supply the whole installation.

HTH
--
Andy

"Andy Wade" wrote in message
...
" Tim (Remove NOSPAM." wrote in message
...

Our national grid system appears to be a TT one- where the earth
conductor is connected back to the neutral wire at the service fuse
where it enters the house, *and* there is an earth rod too, connected
to same.

That's not TT, that's TN-C-S (PME) with an additional earth electrode.

I am wanting to install a generator change over unit, which will
isolate N and L from the grid and allow the gen to feed the house.
(Honda 6kva)

Do I need to earth stake the genny from its earth terminal, and / or
connect the generator plug's N and E together to bring the N line of
the generator down to zero potential to create a safe system???

Or what?!

Yes to both questions, for a permanant installation. Here's a wiring
diagram that I've posted a couple of times in the last few months, and
which
should be fairly self-explanatory. Your exisiting earth electrode,
provided
that it's up to scratch and securely connected to the main earth terminal,
should be OK (unless it's the supplier's, in which case you should really
provide your own, connected in parallel).

Since the impedance of the supply from a small genny will be higher than
from the mains, earth fault loop impedance (Zs) values might be too high
to
give proper protection. For this reason a 100 or 300 mA RCD should be
used
in the generator feed as an aditional precaution.


L o----------------o
\ SW1a
Mains o---------- L to CU
from
meter +-------o
| SW1: DPCO changeover switch
| 100A break before make!
N o----------------o
| \ SW1b
| o---------- N to CU
|
| +---o
| |
| |
Supplier's E o-----------------------------o Main earth terminal
earth | | /
| | |
| | 16mm^2 |
| | earth |
------- |
| | |
| G.P. | | G.P. = Generator
| | | protection,
------- | fuse or CB
| | | plus 100mA RCD
| | |
-- | - | -- |
| G( | | |
| G( | | |
| G( | | | G = generator winding
| G( | | |
| G( | | |
| | | | |
Generator | +---.B | |
frame - | | | | B = bonding connection
-------.-- |
B|\ |
| \-------------+
|
|
|
-----
--- Your earth electrode(s)
- Re 200 ohm



NB 1. You are supposed to consult with your electricity supplier /
distribution network operator before installing switched standby
generating
plant.

NB 2. Reg. 551-02-03 requires automatic load shedding arrangements if the
generator is not rated to supply the whole installation.


Many thanks to all!

tim..

On Mon, 29 Mar 2004 17:47:38 GMT, Lurch wrote:

No. If you fire the genny up and plug something in is it unsafe?

Depends...

It's the same as powering a few small items in the house throught
the existing wiring.

Except that without N bonded to E and L E fault will not cause any
fault current to flow (think about it there is no circuit...) but will
raise the N way above E, probably up to the full generator output
voltage. Bear in mind that most switches and MCBs are single pole in L
then things "switched off" aren't. Also as N and E are quite likely to
be interconnected somewhere the installations E could also rise to the
full generator output voltage.

One end of the generators winding needs to be bonded to the frame of
the generator and to local earth and to the main earth terminal of the
installation. It is the bonding of local earth to the main earth
terminal and thus the suppliers earth that can be problematical with
some types of installation.

For extra safety you could install an earth spike and connect it to
the generators earth terminal, that's about all you'll need to do.

Assuming that the gennys earth terminal also bonds to one of the
generators phases.

--
Cheers
Dave. pam is missing e-mail

On Mon, 05 Apr 2004 00:22:16 +0100 (BST), in uk.d-i-y "Dave Liquorice"
strung together this:

On Mon, 29 Mar 2004 17:47:38 GMT, Lurch wrote:

No. If you fire the genny up and plug something in is it unsafe?

Depends...

It's the same as powering a few small items in the house throught
the existing wiring.

Except that without N bonded to E and L E fault will not cause any
fault current to flow (think about it there is no circuit...) but will
raise the N way above E, probably up to the full generator output
voltage. Bear in mind that most switches and MCBs are single pole in L
then things "switched off" aren't.

That would also apply to anything plugged directly into the genny.
It'd be no more 'unsafe' than just plugging something in to an
extension lead on the genny.

Also as N and E are quite likely to
be interconnected somewhere the installations E could also rise to the
full generator output voltage.

Where would they be connected?

One end of the generators winding needs to be bonded to the frame of
the generator and to local earth and to the main earth terminal of the
installation. It is the bonding of local earth to the main earth
terminal and thus the suppliers earth that can be problematical with
some types of installation.

That is why when using a gennerator you should isolate the suppliers
earthing facility with a link, or just unscrew it, and turn opff the
main switch of the CU or use a changeover switch.

For extra safety you could install an earth spike and connect it to
the generators earth terminal, that's about all you'll need to do.

Assuming that the gennys earth terminal also bonds to one of the
generators phases.

I seem to think that most are.
--

SJW
A.C.S. Ltd.

On Mon, 05 Apr 2004 10:57:50 GMT, Lurch wrote:

That would also apply to anything plugged directly into the genny.
It'd be no more 'unsafe' than just plugging something in to an
extension lead on the genny.

Not quite, a single appliance on a extension with a L E fault will
only mean that that appliances N and/or E rise above true E. In an
installation the *whole* installations N and/or E rises. In the case
of E rising all the back boxes and fitting screws of the sockets etc
or exposed earthed metal work on any connected appliance or E bonded
fittings (CH pipe work, taps etc) all becomes a shock hazard...

A raised N shouldn't be so much of hazard but could still present a
"surprise".

Also as N and E are quite likely to be interconnected somewhere the
installations E could also rise to the full generator output
voltage.

Where would they be connected?

I hedged that statement a bit. B-) With proper DP break before make
change over switch there shouldn't be a N E connection, other than the
one intended at the generator.

One end of the generators winding needs to be bonded to the frame
of the generator and to local earth and to the main earth terminal
of the installation. It is the bonding of local earth to the main
earth terminal and thus the suppliers earth that can be
problematical with some types of installation.

That is why when using a gennerator you should isolate the suppliers
earthing facility with a link, or just unscrew it, and turn opff the
main switch of the CU or use a changeover switch.

Personally I'd get the type of supply changed such that the suppliers
earth can remain connected or fit a 3P switch. Having to to a manual
disconnect/connect the earth is just asking for it to be left in the
wrong state at some point.

Assuming that the gennys earth terminal also bonds to one of the
generators phases.

I seem to think that most are.

"think"! I'd make damn sure it was, no assumptions or "thinking".

--
Cheers
Dave. pam is missing e-mail

On Tue, 06 Apr 2004 17:26:20 +0100 (BST), in uk.d-i-y "Dave Liquorice"
strung together this:

So pretty much a 'check what you're doing as different installations
will require different solutions' sort of answer.

Personally I'd get the type of supply changed such that the suppliers
earth can remain connected or fit a 3P switch. Having to to a manual
disconnect/connect the earth is just asking for it to be left in the
wrong state at some point.

Are you suggesting fitting a switch to the earth? I do hope not, it's
a serious contravention of BS7671. What if the switch was to go faulty
on the earth pole alone?

Assuming that the gennys earth terminal also bonds to one of the
generators phases.

I seem to think that most are.

"think"! I'd make damn sure it was, no assumptions or "thinking".

Quite, hence the first line of my reply.
--

SJW
A.C.S. Ltd.

On Tue, 06 Apr 2004 18:48:22 GMT, Lurch wrote:

So pretty much a 'check what you're doing as different installations
will require different solutions' sort of answer.

Yep. B-)

Are you suggesting fitting a switch to the earth? I do hope not,
it's a serious contravention of BS7671. What if the switch was to go
faulty on the earth pole alone?

Same as "forgetting" to adjust the manual link or dis/reconnect it
that you where suggesting, even if the latter obeys the regs I feel it
is rather to prone to Sods Law.

To detect a naff switch, wire a neon across the poles. If the neon on
the closed side glows you have a problem...

--
Cheers
Dave. pam is missing e-mail

On Tue, 06 Apr 2004 22:30:26 +0100 (BST), in uk.d-i-y "Dave Liquorice"
strung together this:

Are you suggesting fitting a switch to the earth? I do hope not,
it's a serious contravention of BS7671. What if the switch was to go
faulty on the earth pole alone?

Same as "forgetting" to adjust the manual link or dis/reconnect it
that you where suggesting, even if the latter obeys the regs I feel it
is rather to prone to Sods Law.

But human error can knacker any good theory, if someone were to
'forget' to switch off the CU then disastrous consequences could occur
when the genny was fired up. A faulty switch is beyond human control.

To detect a naff switch, wire a neon across the poles. If the neon on
the closed side glows you have a problem...

I think we're in bodging territory now. If you want to fit a switch to
the main earth then go ahead, I would strongly advise against it and
would never do it myself though.
--

SJW
A.C.S. Ltd.