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We recently installed a new generator (Mosa GE6000SX) and during the troubleshooting process for something else, the electrician noticed that when the generator was running but disconnected with no load (just the meter), he observed 40-50 volts between the neutral and the earth (it is a 240 Volt AC single phase generator).

The electrician seemed to think it was a fault with the generator that should be rectified by the manufacturer. However when I spoke with the supplier's technical support today, they informed me that a 40-50 volt reading between the neutral and the earth was "normal".

According to the manufacturer, it supports 110 V and 230 V modes. In 230 V mode, the windings inside are connected in series, while in 110 V mode, they are connected in parallel. As the generator offers split-phase 110 V, the earth is centre-tapped on the 110V windings. i.e. in 110 volt mode, it produces 110v with a centre tapped earth

Should I be concerned about this?

The generator ultimately powers an off-grid home when the renewable power system is low on battery and recharging (inverter bypass).

My understanding is that in a UK home, neutral should be tied to earth at the generation source, as safety features (such as the fuses in the plugs) don't operate on the neutral, so according to the electrician, this is a potentially unsafe configuration.

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    Does the inverter bypass function in your system switch the neutral, or only the hots? – ThreePhaseEel Jan 3 '17 at 23:57
  • Yes - the inverter bypasses the Live and Neutral (240V L-N single phase) – xirt Jan 4 '17 at 0:08
  • I have seen up to 60V on an unloaded inverter type generator when a 60watt light bulb was added as a load as suggested by the tech support person the voltage dropped to less than 3v with bulbs on both legs it was .5 to 1.5v you may want to try this test. I thought the very expensive generator had a problem but it was eliminated by a very small load. That worked for me because who would run the system unloaded except for cool down. – Ed Beal Jan 4 '17 at 0:16
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    One more piece to the puzzle we need: You didn't say where the reading was taken from and whether the generator was connected to the inverter. From your post, it sounds like you had the generator electrically isolated and were measuring the generator output. Is this correct? – Mister Tea Jan 4 '17 at 15:21
  • That is correct. We also get the same reading at the distribution board in the house when the inverter is running on bypass straight through to the generator. The reading was taken from the 230V single phase outlet (commando) on the generator while running, not connected to anything else. – xirt Jan 6 '17 at 6:48
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It turns out that the OP's model of generator is peculiar -- its 120V is set up in the UK jobsite fashion (i.e. two 60V hot legs and a neutral), which when combined with the way the 120/240V switching works on the 120/240V models of this generator, yields a neutral at an odd spot.

The only fix is to use a 240/240 isolation transformer to establish a new neutral point, or to rewire the generator (which'd void its NRTL listing).

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If it is completely isolated from the house (neutral and ground disconnected) there should be a voltage of up to half of the output for a single phase generator.
( a split phase, or three phase generator would have a much lower voltage with respect to earth, but still not zero )

The reason for this is the way AC works.

AC is very similar to a radio signal. In fact a radio signal is just AC at a much higher frequency where at least one half is connected to an antenna rather than directly to a load.

Question: How do you think those non-contact voltage testers work?
Answer: They have a tiny antenna and radio that picks up 50/60 Hertz in them.


If the neutral is internally connected to the frame of the generator, it will have a higher coupling to the earth than the hot side. So it will be a lower voltage with respect to the earth than the hot side.
( You can think of it as the earth is absorbing some of the signal )

Here is a way to prove it (if it is internally connected to the frame even through a resistor), set up the multimeter between neutral of the generator and the earth, and have someone else touch the frame of the generator. You should see the voltage drop at least a few millivolts because it now has an even better coupling to the earth. It should drop even more if they put their bare skin on moist dirt, a ground rod, or in water.
( assuming the generator is completely isolated from everything else )

If you have a single phase (and not split single phase) generator, it is very likely there is no difference between hot and neutral internally except that the neutral is usually grounded.

It may be that it is designed to use your house's neutral to ground bonding connection to prevent a ground loop, and to prevent the frame of the generator becoming the high side if there was a major wiring issue with your house.


Here is a way to confuse just about every electrician.

Stick one side of the multimeter on the ground rod, and the other in the ground a few feet away. You will never have a voltage of zero volts AC in that case, if the ground rod is connected to the neutral/ground of the electrical grid.
( in an off-grid installation it still might not be zero volts )
This is because the grid is never completely balanced. One phase might have more supply or load, which would pull the neutral off of zero volts.

You can even just hold one probe against your hand while the other is on the ground rod, and it will show some AC voltage if you aren't using a high-z multimeter.

Electricians know about electrical codes, they usually know next to nothing about electromagnetic radiation (radio signals).

  • Unfortunately we also get the same reading when the generator is not isolated. – xirt Jan 6 '17 at 6:49
  • @xirt If it is that voltage between ground and neutral, then it is in fact still isolated. The neutral is supposed to be grounded by your houses bonding the neutral to ground. You should then test it with no load as there will be some voltage loss over the wires that go out to it. The only point you have to worry about is the voltage from the neutral/ground bonding point, and the earth (dirt). If that is good, no other voltage reference is relevant. – Brad Gilbert Jan 6 '17 at 17:16

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