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.