If your Grounding system (lets say a ground wire tied to the engine
block) is not connected to the blue output lead of the isolating transformer, then that blue lead is not a neutral since by definition, a neutral is an intentionally grounded conductor. What you have instead is two ungrounded (hot) conductors, one of which may have a grounding connection downstream through the frame of a piece of equipment
When you say the Earth output of the transformer has 150V, I assume that you mean you are measuring 150V between the transformer ground terminal which would be tied to the frame of the transformer, and one of the ungrounded conductors.
When you establish a neutral on the boat side of the transformer by grounding one of the transformer legs, you can expect to read 230V from hot to neutral, or hot to ground (because the meter completes a path from hot, through the low impedance ground and back through the neutral to the supply.
When your ground is floating, and you read from an ungrounded hot wire to ground, a circuit somehow has to be completed from the hot supply wire from the transformer through the meter to the ground system and return to the supply hot on the other side of the transformer coil.
Otherwise you will read zero volts. With your floating ground reading 150V you are measuring part of a series circuit between one ungrounded hot from the supply back to the other. Current
is flowing from the hot through the meter leads and meter, through the ground terminal/frame of the transformer, through the ground system trying to find a path back to the other side of the supply (transformer). It is finding a path but its not a low impedance path such as it would be if that side of the transformer was intentionally grounded. Rather, it is finding a connection between the grounding system and the wiring
from the transformer through the various devices. 80V is being dropped along that path (230V-150V). That path potentially includes (get it, potentially...that was a play on words
) any ac grounding conductor or path, any DC negative or ground path, any bonding conductors or path and the path from any connected device in the system back to the supply transformer.
American NEC and ABYC standards (I'm not an expert on ABYC standards) require a separately derived system such as our isolating transformer to have one of the transformer taps grounded in order to establish a new grounding system and allow sufficient fault current to flow back to the source through a low impedance path so it can trip a breaker. This creates a Neutral or more properly a grounded conductor. European and other standards may be relying on a residual current device (measuring the difference between incoming and outgoing current ) to ensure the safety
of people and livestock. Perhaps that is why the standards differ?
The problem with this stuff is that sometimes the attempted answers only serve to confuse more...