Having bought a small generator I'm now reading the manual supplied
with it - the manual is not very good.
In the manual is a diagram showing the earthing point (frame) of the
generator being connected to a ground spike, but there's no helpful
words to advise that this should be done.
Am I supposed to connect the frame of the generator to an earthing
spike driven into the ground when in use? What size spike?
This is only a tiddly little generator - 240v/950W rating.
On Fri, 7 Nov 2003 21:07:32 -0000, sparky wrote:
Order of postings corrected to make sense.
I'm keeping quiet but if the OP searches groups.google.com back in
this group and uk.tech.electronic-security, he'll find a couple of
lengthy recent threads... Still they boil down to get expert advice
from an qualified spark with experience of generator installations.
Dave. pam is missing e-mail
That's why I'm not replying to any more of them. Some people don't
understand the differences between the domestic supply and a generator
supply system, so they get stuck in this Live and Neutral thing. When you
try to tell them the difference, they look at you as though you've got two
heads. So now I've given up. :-))
On Sat, 08 Nov 2003 05:15:42 GMT, BigWallop wrote:
I fully understand where you are coming from and I believe we actually
agree, there seems to be a "separated by a common language" problem.
Local Earth - Properly installed connection to the local physical
ground via an earth spike etc.
Supply Earth - The "earth" connection provided by the public
Generator assumed to have no center tap to the alternator windings or
if it does the center tap is *NOT* connected to *ANYTHING*. A
generator with a center tap connected to local earth cannot make a
safe connection into a normal installation as both generator phases
are fixed at half the supply potential away from local earth.
For simplicity I have omited mentioning anything about how any
connections are made. Obviously the public supply L and N need to be
fully isolated from the CU before the generator phases are connected
and vice versa.
I agree that simply to connect the two "phases" from a generator to
the L and N terminals of a CU is unsafe. Both generator phases are
floating with respect to local earth. It is quite likely that the N
line will be at a significant voltage above local earth, <ouch> if you
come in contact with it. Single pole switches will not make the
connected equipment "safe" as they only isolate the L connection.
To over come this problem you pick one of the generator phases,
doesn't matter which, and bond that with correctly sized cables to
local earth, at the generator. This local earth bonded generator phase
you connect to the CUs N terminal. The unbonded generator phase to the
CUs L terminal. You now have a situation where the N wiring is at, or
very close to, local earth potential and all the L wiring is fixed to
be at the nominal output voltage of the generator. Single pole
switches now work "correctly".
Now the tricky bit. All the circuit protective wiring (aka the "earth"
wires in T&E etc) in the building are connected back to the main earth
termal next to the CU. The main earth terminal is connected to the
supply earth and equipotential bonding is also run from the main earth
terminal to the incoming service pipes etc
Trouble is that the supply earth could be at a different potential to
the local earth. Depending on the public supply type I think the
dangers of connecting the local earth to supply earth change. For TT
(local earth spike) and TN-C-S (PME) installations I don't *think*
there is much to worry about, as the supply earth is derived at or
very close to the premises.
I do have worries about making a local earth/supply earth connection
on a TN-S (supply earth provided from supply cable armour or metalic
sheath) system. This is the part that one needs to take advice on from
a qualified spark with experience of generator instalations.
Finally should the generator chassis be connected to the local earth?
Dave. pam is missing e-mail
All interesting stuff, and thanks for taking the time to point it out.
It would be so, so easy for someone to buy a genny and wire it into
the house mains without knowing what the hell they are doing. Thank
goodness for a newsgroup like this one I say!
Now if the proposed building regs part P covered genny sales.....
BigWallop (spamguard@_spam_guard.com) wrote:
: > On Sat, 08 Nov 2003 05:15:42 GMT, BigWallop wrote:
: > > Some people don't understand the differences between the domestic
: > > supply and a generator supply system, so they get stuck in this Live
: > > and Neutral thing.
: > I fully understand where you are coming from and I believe we actually
: > agree, there seems to be a "separated by a common language" problem.
: > B-)
: And if the generator has any safety earth leakage device, then placing any
: of the phase to earth will activate the device and kill the supply from the
The neutral connection must be made before any earth leakage detection
device. Dismemberment of the generator may be necessary to achieve this.
: > Local Earth - Properly installed connection to the local physical
: > ground via an earth spike etc.
: > Supply Earth - The "earth" connection provided by the public
: > electricity company.
: That's the situation, yes.
: > Generator assumed to have no center tap to the alternator windings or
: > if it does the center tap is *NOT* connected to *ANYTHING*. A
: > generator with a center tap connected to local earth cannot make a
: > safe connection into a normal installation as both generator phases
: > are fixed at half the supply potential away from local earth.
: > For simplicity I have omited mentioning anything about how any
: > connections are made. Obviously the public supply L and N need to be
: > fully isolated from the CU before the generator phases are connected
: > and vice versa.
: Right. Exactly.
: > I agree that simply to connect the two "phases" from a generator to
: > the L and N terminals of a CU is unsafe. Both generator phases are
: > floating with respect to local earth. It is quite likely that the N
: > line will be at a significant voltage above local earth, <ouch> if you
: > come in contact with it. Single pole switches will not make the
: > connected equipment "safe" as they only isolate the L connection.
No, it depends entirely on how good your insulation is. Assuming
good insulation all round, it means that you become the neutral
reference when you touch one of the conductors and you don't feel
a thing. This is fine unless there is anything else drawing a
current to earth at the same time...
: > To over come this problem you pick one of the generator phases,
: > doesn't matter which, and bond that with correctly sized cables to
: > local earth, at the generator. This local earth bonded generator phase
: > you connect to the CUs N terminal. The unbonded generator phase to the
: > CUs L terminal. You now have a situation where the N wiring is at, or
: > very close to, local earth potential and all the L wiring is fixed to
: > be at the nominal output voltage of the generator. Single pole
: > switches now work "correctly".
: Then the phase you've chosen to stick in the earth will need less than one
: ohm impedance to be of any in supplying a neutral at the consumer unit and
: this would need to be calculated correctly in all weather and ground effect
: situations. It may also trip any safety devices on the generator itself.
: So that needs further investigation.
Why is such a low earth impedance required? Fault currents will rarely
flow through the rod itelf and in the rare cases they do they should be
picked up by an RCD. The rod only provides a reference for the whole
system. Think of the arrangement as having the very low earth fault
loop impedance of a TN-C-S system for faults to bonded equipment
(generator permitting!) but the relatively high earth fault loop
impedance of a TT system for true faults to ground. This does mean that
an RCD on the live and neutral from the generator is essential to catch
true earth faults though. I would also be wary of earth fault loop
impedance figures when looking at the supply from a small generator -
I suspect there will be quite a lot of resistance in the generator and
an RCD would be required even if you have the best earth rod in the
: > Now the tricky bit. All the circuit protective wiring (aka the "earth"
: > wires in T&E etc) in the building are connected back to the main earth
: > termal next to the CU. The main earth terminal is connected to the
: > supply earth and equipotential bonding is also run from the main earth
: > terminal to the incoming service pipes etc
: > Trouble is that the supply earth could be at a different potential to
: > the local earth. Depending on the public supply type I think the
: > dangers of connecting the local earth to supply earth change. For TT
: > (local earth spike) and TN-C-S (PME) installations I don't *think*
: > there is much to worry about, as the supply earth is derived at or
: > very close to the premises.
: But if the supply earth is receiving back fed half phase supply from the
: generator, which has a phase stuck in the ground at less than one ohm
: impedance, then the earth, no matter where it is coming from, will have
: picked up this charge and will supply it back to the connections that
: already exist.
??? But if it is a true earth then you have the luxury of attempting
to charge the whole planet and nobody will care. The problem occurs
when you have more than one earth and they are not quite the same.
The biggest problem is the effect you may inadvertantly have on your
For a TT supply, there is no problem whatsoever. TT is convenient in
that there is only one possible earth reference, the earth rod. This
would provide the earth reference and the installation protective
conductors. One side of the generator, all metalwork bonding, etc would
be connected to this. The external supply only gives you live and
neutral and there is no risk of earth loops.
For a TN-C-S supply, you will have two earths, the supply earth (derived
from the supply neutral) and your own earth rod. Your own earth rod is
required to provide an earth in the event that the supply cable is cut.
I would be tempted to connect the PME earth and the earth rod together
but there are two caveats: 1) There is a risk that you might experience
more rapid than expected corrosion of your earth rod due to any voltage
present on the supply neutral derived earth during normal operating
conditions (this shouldn't be an issue since otherwise any cross bonded
buried pipework would also suffer) and 2) under fault conditions during
generator operation the voltage on your earth rod will rise taking the
voltage on the supply neutral conductor with it. This shouldn't be a
problem assuming that everything locally is RCD protected, cross bonded
and the earth through the rod is reasonable but it may cause some
current to flow into the supply neutral and I don't know if the local
electricity supplier would be expecting this. I don't know the official
answer in this circumstance - it will either be specified in the relevant
IEE regs or you will need to consult your electricity supplier.
: > I do have worries about making a local earth/supply earth connection
: > on a TN-S (supply earth provided from supply cable armour or metalic
: > sheath) system. This is the part that one needs to take advice on from
: > a qualified spark with experience of generator instalations.
The same applies to a TN-S system as for a TN-C-S. There is a risk
that under fault conditions you will cause an increase in the potential
of your bonding which can be seen at the end of your supply cable (on
the sheath rather than the neutral in this instance). Again, assuming
RCD protection, proper cross bonding and a reasonable earth rod there
shouldn't be an issue but I would check with the electricity supplier.
: The two earth potentials must be totally separate. No domestic supply from
: the grid can be connected to the system if you are going to change over to a
: generator supply. Any common connection between them will also become part
: of the supply from both. They must be totally separated in the phase
: connections and earth potential connections. Totally.
I don't think that is hugely practicable and in most cases there
shouldn't be a problem. In the case of TN-S and TN-C-S supplies I would
do a bit of research or consult the supplier to understand what their
concerns are with regard to their earth but keeping the earths separate
will mean that there are switching devices in the earth, a very bad idea
in my opinion. Cross bonding will generally mean that the two earths
are effectively connected anyway. If there is any significant voltage
difference between the two available earths, then I strongly recommend
that you do not use mains applicances outside the house!
Keeping the neutrals and phases separate is obviously a given unless you
have some relatively unusual arrangements with your supplier and a whole
load of protective equipment to ensure that you don't feed into a dead
: > Finally, should the generator chassis be connected to the local earth?
: > Yes.
: But then you'll supply the earth connections with both local and supply
: earth potential. Not a great idea in my experience.
The generator chassis should be connected to earth and the local one
makes the most sense since that is the one guaranteed to be present all
the time. As I suggested above, connecting the local and supplier earths
together is probably the answer anyway.
This is an IT system - /isole terre/ - floating source and earthed
metalwork. It's used in some industrial situations where supply
disconnnection in the event of an earth fault would be more dangerous then
leaving the power on. It's fine in the sense that a single earth fault
won't cause a problem - in fact the fault will go unnoticed until a second
one occurs. For that reason IT systems have to employ some form of earth
monitoring to draw attention to the fault.
Running your house as an IT system is not permitted, since the indirect
contact protection is designed on the assumption that it's a TN system -
either TN-S or TN-C-S (PME) - and no earth monitoring is employed.
It isn't - although the statement "will need less than one ohm impedance" is
pretty meaningless; did he mean impedance to earth, or something else?
Even the earthing resistance at public supply network susbtations only has
to be less than 20 ohms for PME networks (although it is typically far
Without the RCD it's no different to a non-RCD protected circuit on the TN
public supply. This won't catch what you call "true earth faults" either,
but such faults shouldn't be possible since all exposed-conductive-parts
should be connected to the main earth terminal via their respective CPC's.
Anything supplied outside the main equipotential zone should be RCD-fed of
I agree. The higher impedance of the generator supply means that the value
of Ze (the part of the earth loop impedance exterenal to the house
installation) is higher than the value assumed for design, so you can't rely
on disconnection time requirements being met, etc. A 100mA time delayed RCD
is the best solution here. (If the house installation is TT-earthed then
this RCD will (should) already exist and there's no need for another one on
Quite. Whatever he was getting at, it's so badly expressed that I haven't a
clue what he meant, except that he seems obsessed with generator outputs
Yes, I agree with all that.
And that. The requirement for an independent means of earthing is in BS
7671 (Reg. 551-04-03).
I don't think that's a problem at all. There are other circumstances in
which local earth elecrodes bonded to the PME terminal are recommended.
Again, no problem here that I'm aware of. If your generator to house TN
system is properly installed then earth fault currents are contained within
your own copper conductors. Anything flowing to earth should either be
negligible, or, as you say, quickly interrupted by the RCD. But in any case
you do need to consult the supplier - this requirement is in BS 7671, Reg.
551-01-01: "Requirements of the electricity supplier shall be ascertained
before a generating set is installed in an installation which is connected
to the public supply".
As I've said before, BS 7671 has three pages of requirements relating to
generators (which includes UPS's by the way) in section 551. Anyone
contemplating a permanent genny installation should purchase and study, and
consult their supplier. If in doubt consult a professional electrical
Here's my attempt at an ASCII wiring diagram:
Mains o----------> L to CU
meter + -------o
| SW1: DPCO changeover switch
| 100A break before make!
| \ SW1b
| o----------> N to CU
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
--- Your earth electrode(s)
- Re < 200 ohm
N.B. Reg. 551-02-03 requires automatic load shedding arrangements if the
generator is not rated to supply the whole installation.
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