I have an earth supplied by the power company - There is an earth wire that
appears out of the bottom of the main power companies fuse where the main
cable comes in.
This is then connected to a black box, sealed with an electric company seal
It then proceeds into my consumer unit - pretty straight forward
From a google search, I gather this is a TN-S system
All fine and dandy
(See the first picture here
I have <nearly> added a generator transfer switch to the equation.
(You can see it on the page above, all that is currently connected is a
cable from the mains in terminals to an isolator
I am planning of removing the meter tails from the CU, and shoving them into
the bottom of this isolator (after the power company has come out to remove
I am a tad confused with the earthing requirements for this...
The instructions with this switch state I need a TT system. where the earth
spike is connected at the generator end.
Also in the transfer switch, the neutral from the generator is connected to
Do I simply bang an earhting rod in the ground next to the generator, and
connect it to the generator's earthing point, then connect this earth to the
CU, so I then have two earth's, one provided by the electricity company, and
one provided by me?
Some advise would be greatly appreciated!
Ha Ha Ha !!!! Sorry. But I've got to laugh. Can I ask who told you this
is going to work ? For one thing, what feeds your consumer unit after your
swap over switch ?
There is no neutral on a generator supply, so how are you isolating the
phase / phase supply from the generator so that it doesn't back feed to the
existing domestic mains supply ? Sticking one phase from the generator into
the ground, would also mean you'd also have to stick the neutral side of the
existing consumer unit into the ground. This won't give you full 240 volts
supply unless the earth impedance is below 1 ohm between your generator
neutral/earth point and your consumer unit neutral terminal.
The only safest way to swap between domestic supply and a generator supply,
is with a proper 100 ampere break before make switch. Like the ones with
the big handle you'd see on an old Frankenstein movie. Does the unit you've
bought have an automatic contactor which is rated at 100 amps and has the
ability to completely break from the domestic supply before it makes contact
with the generator ? If it does, then you don't have to stick anything in
the ground and you can leave it as a safety device. You'd only have to make
the connections from the phase/phase supply of the generator to the existing
consumer unit, but through the swap over switch.
Have a word with the sparks that comes to remove the consumer fuse. He may
have a better idea of what's needed.
Good luck with it.
There will be some tails from the transfer switch!
The transfer switch is a double pole three position switch (break before
In position 1 (mains) the L and N from the CU are connected to the mains via
position 2 is all off
position 3 (Generator) is L and N from the CU is connected to the generator.
The transfer switch does this
When on generator power...
The L of the generator is connected to the L of the CU (Via the transfer
The N if the generator is connected to the N and the E of the CU, again via
the transfer switch
The E does not go through this break before make transfer switch, so is
The question I have, is, the transfer switch manual says there should be an
earth rod next to the generator
I already have an earth supplied by the electric company
will having two earthing points cause a problem?
This is what the transfer switch is all about ;)
So the L and N (red phase and black phase) from the generator will be
connected to the transfer switch directly and the generator earth will be
kept as a safety earth device as it should be then.
The two earth points will be separate entities, but bonded together as a
common earth in case of faults occurring when either supply is used.
But because there is no neutral on the generator, then you'd have to connect
the phase/phase supply from the generator directly to the transfer switch.
Why do you think that one phase from the generator needs to be stuck in the
earth beside the generator unit ?
But there is no neutral on a generator supply. A generator supplies two
phases that are 180 degrees out of phase with each other. On a 240 volts
generator that means you have 120 volts on the black phase and 120 volts on
the red phase, when added together they give the full 240 volts ac supply.
The earth connection is for safety on the generator in case you touch
anything over to short the phases out.
For safety on the generator supply only. Without an earth you stand a
chance of shorting out the phases and not killing the supply by tripping the
generator ELCB (RCD) device.
Which is at, or very, very close to neutral potential. Mainly because the
neutral on the domestic supply is another earth.
Not if you connect the two as one bonded system. When on either generator
or the domestic mains supply, the earth would be supplied by both the
generator earth point and the domestic mains head supply connection.
Then the transfer switch would also be supplied by the black phase and the
red phase from the generator as well. But this can cause problems with
appliances that contain capacitative loads. Because the generator supply
doesn't contain a neutral (Zero Potential), there is also a danger of
appliances which use single phase switching, like light switches, not to be
fully isolated from one of the phases (normally black), so this means you
could still short out a load of 120 volts to earth when running from the
BigWallop (spamguard@_spam_guard.com) wrote:
: So the L and N (red phase and black phase) from the generator will be
: connected to the transfer switch directly and the generator earth will be
: kept as a safety earth device as it should be then.
: The two earth points will be separate entities, but bonded together as a
: common earth in case of faults occurring when either supply is used.
More importantly, if there is no supply from the mains then it is possible
there is no earth (e.g. the supplier's cable has been cut) and therefore
the safety of the whole installation depends on the earth rod.
: But because there is no neutral on the generator, then you'd have to connect
: the phase/phase supply from the generator directly to the transfer switch.
: Why do you think that one phase from the generator needs to be stuck in the
: earth beside the generator unit ?
This is necessary to make it a neutral! Obviously, the same point should
be connected as the neutral as well.
: > > Sticking one phase from the generator into
: > > the ground, would also mean you'd also have to stick the neutral side of
: > the
: > > existing consumer unit into the ground. This won't give you full 240
: > volts
: > > supply unless the earth impedance is below 1 ohm between your generator
: > > neutral/earth point and your consumer unit neutral terminal.
: > When on generator power...
: > The L of the generator is connected to the L of the CU (Via the transfer
: > switch)
: > The N if the generator is connected to the N and the E of the CU, again
: > the transfer switch
: > The E does not go through this break before make transfer switch, so is
: > always connected
: But there is no neutral on a generator supply. A generator supplies two
: phases that are 180 degrees out of phase with each other. On a 240 volts
: generator that means you have 120 volts on the black phase and 120 volts on
: the red phase, when added together they give the full 240 volts ac supply.
: The earth connection is for safety on the generator in case you touch
: anything over to short the phases out.
A generator where there is no connection between the power output and the
earth provides a 240V output. Since it is floating, it is not 120V on
each "phase", it could be 1V on one cable and 239 on the other, it could
be 200V on one side and 40V on the other, it could even be 1000V on one
side and 760V on the other. Without any earth connection, the voltage
in relation to earth is *unknown* (and indeed is *absolutely*
In reality, the generator is not perfectly insulated and therefore the
chassis will have a certain voltage in relation to the output. If you
earth the chassis then the you will cause the outputs to tend to
particular (probably random) values in relation to earth. (Unless the
generator is designed to operate with a "centre tapped earth" - see the
end of this post).
Since houses expect a live supply with around 240V on it and a neutral
with around 0V on it then you need to connect one of the "phase" outputs
of the generator to earth to stop the voltage on it changing at the whim
of the generator insulation (and any other pieces of equipment you might
connect to the supply). Ths of course has the effect of fixing the
other "phase" at 240V.
: > The question I have, is, the transfer switch manual says there should be
: > earth rod next to the generator
: For safety on the generator supply only. Without an earth you stand a
: chance of shorting out the phases and not killing the supply by tripping the
: generator ELCB (RCD) device.
Correct, the earth is necessary for safety of the whole house when
operating from the generator supply. The supply can be treated as any
other TT supply with regard to short circuit and residual current
: > I already have an earth supplied by the electric company
: Which is at, or very, very close to neutral potential. Mainly because the
: neutral on the domestic supply is another earth.
And the generator needs to be installed so the same applies when the
generator is feeding the installation.
: Then the transfer switch would also be supplied by the black phase and the
: red phase from the generator as well. But this can cause problems with
: appliances that contain capacitative loads. Because the generator supply
: doesn't contain a neutral (Zero Potential), there is also a danger of
: appliances which use single phase switching, like light switches, not to be
: fully isolated from one of the phases (normally black), so this means you
: could still short out a load of 120 volts to earth when running from the
: generator supply.
Unless you know exactly what you are doing, I would never run a domestic
installation floating in the manner you have described it, for exactly
the reason you describe. One of the two "phases" coming from the generator
must be connected to earth to give you a neutral (in the general case it
doesn't matter which one but there may be generator specific reasons why
you would chose one or the other).
There is a special case which needs to be understood (especially if the
origin of the generator is uncertain). In many temporary installations
(e.g. building sites) a 240V supply might be provided with the mid point
referenced to the earth. This has the benefit that the maximum voltage
to earth is reduced to 120V RMS with hopefully a reduced risk of injury
in the event of a shock. In this case you truely have 120V on one phase
and 120V on the other. The can be done either by referencing the supply
to earth with a couple of resistors, or by using a generator with the
windings internally connected to earth so as to give 120V on each "phase".
If you have such a generator, then also earthing one of the "phases" as
described above will mean that you short-circuit one of the windings and
you'll only get 120V out of it. (Hopefully the generator has an RCD that
would trip in this circumstance). Such a generator is unsuitable for
domestic supply without modification.
Exactly what Dale said, but a bit more technically than I wanted to go into.
Do this test on the generator.
Place your meter across the red phase and earth, then across the black phase
and earth. Measuring the voltages across each of the phases in this
fashion, will tell you if the generator is suitable for use as a domestic
supply in cases of mains grid failure.
If you get the 120 / 120 configuration, then it is not safe for your home to
have this type of supply. You still run the risk of having a 120 volts
phase supply to earth even if you think an appliance is properly switched
off. 120 volts is still enough to cause burns or even stop your heart,
especially if you're not expecting it to be there.
Correct and I expect that the generators earth point bonds, to use the
words of Bigwallop, the "black phase" to that earth rod.
This is where it becomes complicated. That supply earth may be at a
voltage other than the local earth provided by the earth rod.
Connecting the two together could cause earth currents to flow and in
some circumstances and not necessarily fault conditions these could be
I suggest you get in touch with a properly qualified electrical
engineer with experience of generator installation. It may also be
worth doing a google in this group or uk.tech.electronic-security for
a fairly recent discussion on this. Bigwallop appears to be changing
his tune a little...
Dave. pam is missing e-mail
It is my understanding (it came up recently) that you can bond the supply
earth with your local earth rod. After all, main equipotential bonding also
connects your supply earth to local earth. The local earth rod is essential
for generator supplies, as you can't rely on the supplier earth in the case
of a power cut. This is because the power cut is possibly a cable break,
which is as likely to affect the earthed armour/combined neutral earth
conductor as the phase conductor.
You must ensure that the entire system, or the generator circuit is capable
of running as a TT system. This requires the earth rod, as mention before
and a suitable RCD in the loop somewhere. A 100mA Type S is indicated. This
can go just after the generator and before the transfer switch, in which
case you have TN earthing on mains and TT on generator. Alternatively, the
RCD can go after transfer switch and the supplier's earth disconnected. This
will give you TT at all times. The instructions on the transfer switch
should be followed, however.
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