Electrical – 50 Volts on the Neutral (180 Volts on the Live) bad or safe

generatorgroundingneutralsingle-phase

We have an off-grid UK home on occasion powered by a generator (MOSA GE 6000 SX/GS).

Due to the design of the generator, it has two modes: centre-tapped-earth 110 V, and 230 V. When running in 230 V mode, the two windings in the generator are connected in series; in 110 V mode, the windings are operated in parallel.

The result is that when the house is powered (generator operating in 230 V mode) there is 50 volts between the neutral and earth, and 180 volts between the live and earth.

I am told by an electrician that the neutral should be 0V (tied to earth) while the manufacturers of the generator unsurprisingly say that 50 volts on the neutral is fine.

Which is correct?

System Diagram
generator diagram
Note: Generator is connected to earth by an earth stake connected to the case.

Portion of internal circuit diagram of generator:
enter image description here

Note 1: BL=Neutral (230V), BR=Live (230V).
Note 2: The manufacturer added that the two windings are: connected in series when the generator is in 230 V mode, and connected in parallel when in 110 V mode.

Best Answer

The portable generator is designed primerally to provide a 110V center-tapped earth supply as is normally used on UK construction sites. The 230V output is something of an afterthought which is why you end up with it referenced to earth in a weird way.

In the standards for modern appliances in Europe there is no expectation that the "neutral" pin is at earth potential. So powering modern appliances off the generator should be safe.

OTOH in UK house wiring it is normal to assume that neutral is at earth potential. We don't normally put any overcurrent protection in the neutral and we frequently work on circuits with only the live isolated.

As I see it you have a few options, each with it's pros and cons.

  1. Make sure all circuits are RCD protected with a double pole RCD (note that most RCBOs are only single pole isolating) to mitigate the lack of overcurrent protection in the "neutral" and place warning notices so people don't try to work on circuits that are only single pole isolated.
  2. Set up the consumer unit with double-pole breakers. This is a good option electrically but can get kind of costly as double pole breakers are not widely used.
  3. Modify the generator, remove the existing earth reference and add a new one at the neutral end of the winding. Downside here is you will almost certainly be voiding the warranty and the 110V output will no longer be the center-tapped earth supply expected on UK construction sites.
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