A friend of mine is building a boat. On this boat, he has 12V batteries with a 230V/13A transformer.

Recently somebody suggested he should connect the null with the ground. With DC, this is common practice. But how would somebody suggest doing the same with AC?! Polarity switches continuously on AC.

I suspect he either misunderstood the advice or the advisor didn't understand it himself either.

What might the advisor have wanted to tell? There's no reason to assume malicious intent.

  • 2
    \$\begingroup\$ Null? or Hull...? \$\endgroup\$
    – Majenko
    Nov 26, 2014 at 10:51
  • \$\begingroup\$ @Majenko-notGoogle Although the difference is subtle in English, it's not in my native language. Confusion between the two is just about impossible. \$\endgroup\$
    – Mast
    Nov 26, 2014 at 10:57
  • \$\begingroup\$ Ok. It's just that I'm not familiar with "null" in that context. \$\endgroup\$
    – Majenko
    Nov 26, 2014 at 10:59
  • 2
    \$\begingroup\$ @Majenko-notGoogle An electric socket has three 'sides': Phase, null and ground. \$\endgroup\$
    – Mast
    Nov 26, 2014 at 11:02
  • 1
    \$\begingroup\$ @Kitana In general, on a boat, the "large amount of metal" is the big lump underneath designed to keep the boat upright (the "keel"). If that's not in the water, then you're either on land, or the boat's upside down and a ground connection is the least of your worries. \$\endgroup\$
    – Majenko
    Nov 26, 2014 at 23:51

3 Answers 3


I think the "adviser" may have been talking about connecting the socket's earth to the inverter's neutral / cold / null point:


simulate this circuit – Schematic created using CircuitLab

With this arrangement, if there is a short or other fault causing current to flow through the earth connection, it will either cause enough current to flow to trip / blow the MCB or Fuse, or cause a mismatch in the live / neutral currents which would trip an RCD (GFCI). You could also add an ELCB into the mix as well, and that would function normally.

  • \$\begingroup\$ Wouldn't the GFCI be pulled anyway? The ground of the socket is wired to the ground of the meterbox (located between the transformer and the boat's sockets). Although I'm not familiar with the exact internal circuitry of said box, I always assume there's a current meter (probably some relay) inside pulling a switch when the current goes over 30mA. \$\endgroup\$
    – Mast
    Nov 26, 2014 at 12:09
  • \$\begingroup\$ I don't know. All this is supposition and guesswork. We don't know how the boat is wired, what the inverter provides, what fuses / trips there are in the circuit, if any. This is how I would wire the earth pin of the socket up, but I would be reticent about connecting it to anything else that isn't part of that AC circuit due to how a typical inverter works. Details about the internal workings of the inverter would be useful too since there are different ways of generating the AC waveform. \$\endgroup\$
    – Majenko
    Nov 26, 2014 at 12:16
  • \$\begingroup\$ This connection will trip the RCD/GFCI in normal operation by diverting >20ma from the neutral line to the ground connection. \$\endgroup\$
    – user16324
    Nov 26, 2014 at 20:50
  • \$\begingroup\$ @BrianDrummond Where to? \$\endgroup\$
    – Majenko
    Nov 26, 2014 at 23:24
  • \$\begingroup\$ aaaah wait, the question is ambiguous; he asks about a 230/12 transformer (not a 12/230V inverter). I (and Peter B) have been interpreting that as a 230v to 12V transformer to charge from a shore power hookup when available : the fact that he mentions 13A which is a standard UK outlet supports that. If he actually means a 12V to 230V inverter then you are entirely correct, neutral and ground should be connected at source - i.e. at the inverter. And this supply must never be connected accidentally to shore power. Some clarification required, I think... \$\endgroup\$
    – user16324
    Nov 26, 2014 at 23:45

But how would somebody suggest doing the same with AC?! Polarity switches continuously on AC.

This is not advise or complete answer for your main question, just explanation about GND.

Voltage is difference between two terminals.

In the other words - there is no voltage in one terminal. Voltage must be between two terminals.

GND can be used as point of reference and sometimes as conductor in some cases (cars, boats) to reduce cost of wiring. It is also commonly used method. You can read more about grounding here: Where is the ground/negative for overhead power lines?

If you connect one terminal to GND and say "this is zero" - other terminal relatively to this will have variable voltage - sometimes positive, sometimes negative.

enter image description here

If you have a boat and you will connect 12V battery - to GND and say "this is zero" - other terminal will have +12V relatively to GND.

If you connect battery + to GND - other terminal will have -12V relatively to your ground.


The shore power neutral must not be connected to vessel ground. When the inverter is operating, the on-board neutral must be connected to vessel ground.

The shore power "safety ground" should be connected to the vessel ground, preferably through a Galvanic Isolator.

The above rules are from ABYC (USA) and CSA (Canada), but, according to a British book I have, apply there as well.

  • \$\begingroup\$ Marine (and mobile/RV) inverters have built-in bond switching to bond neutral and ground when powered on, but break the bond when powered off so it doesn't bond the shorepower connection. \$\endgroup\$
    – Johnny
    Nov 26, 2014 at 22:06

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