Earthing

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 make)

In position 1 (mains) the L and N from the CU are connected to the mains via this switch

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 switch) 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 always connected

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 ;)

Thanks!

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 generator supply.

You're welcome.

You should also have a read through these pages:

And this might help you a bit as well:

And this:

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 substantial.

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...

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.

Christian.

: 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 : via : > 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* unmeasureable).

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 : an : > 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 protection.

: > 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.

Have fun,

Dale.

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.