Floating network In mobile systems with its own power source one can specifically choose not to use a TT network but to use an IT network. In an IT network the phase and neutral are not coupled to another potential like earth. The voltages created by the independent power source are floating; this is very safe and simple.
If a conductor or housing, powered by the independent power source, is touched no electricity will flow. No current can flow to earth because the earthing conductor is absent and the electric circuit to earth is not complete. This is a similar situation as the safety transformer in a bathroom. If only 1 conductor is touched no current can flow. Inverters and generators are in principle nothing more than the source of two potential differences with a difference of 230 Volt. Touching will not lead to a current flow because this voltage is unable to create power. Independent power systems are more and more often connected to mains. As soon as a mains connection is made a RCD needs to be installed, as to satisfy the requirements of the TT or TN network where the mobile network is now connected to. Theoretically, a network can be a TT network when connected to mains and at the same time be a floating IT network while the generator or inverter is in use. The external earth potential is only operational if the energy from this supply is used as well. If the used energy is not taken from the mains supply but from the inverter or generator supply, no earth leakage current can flow to the physical earth, trough the water back to shore. If a person comes into contact with a conductor, the circuit: phase – person – earth – star point – phase has not been completed and no current will flow through that person. The external earth is not part of this electrical circuit. Fuses that protect for high currents in an overload or short circuit situation naturally remain operational. However, the RCD will never detect an earth leakage current because there is a bypass around the RCD in the network. Pressing the test button on the RCD is also useless. A false impression that the RCD is operational can be given, while in reality the RCD is not operational. When the test button on a RCD is pressed, an internal bypass is activated, simulating an earth leak, so the RCD can be electrically and mechanical tested. This by no means tests the whole installation, only the RCD itself. This will lead to confusion and/or dangerous situations. It is for these reasons recommended to follow the principles of the TT network, also for situations when the installation is not connected to mains power. From IT network to TT network The switch from IT to TT network has to accommodate for a connection being made between a phase, coming from the floating network, and earth. Like earlier explained; the difference between these is only a potential difference. When earth is connected to one of the phases, the network will automatically become a TT network. It is important to realize that this earth is by no means similar to the earth in a house installation. This is a different kind of electric circuit, i.e. power plant versus inverter or generator. The earth is sometimes physical equal, but it belongs to 2 separate electrical circuits that are independent of each other. Of course, it is allowed and possible to connect the 2 earths together. I other words; the generator or inverter can supply power and be connected to the hull of the boat while that hull s also connected to earth (star point) of the shore supply. Current can never flow from one energy source to the other.
Almost all modern-day inverters and generators have one of their phases connected to ground. In that way they conform to a TT network. Never the less, this will always need to be checked prior to installation. All Victron Energy inverters and have the neutral connected to the casing and to the earth connection. Measurements should be: Earth to Neutral = 0V Neutral to Phase = 230V
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