Should there be a connection between neutral and earth onboard my boat?

In the ‘Any questions?’ section of Yachting Monthly’s October 2004 edition, reader Richard Peperell raised a significant question regarding shorepower earthing which warranted a reply beyond the space available in the magazine. Here is the original question, along with Nigel Calder’s extended explanation.


Dear YM – I have a 52ft steel ketch with shorepower, connected through an isolating transformer and RCD (residual current device). In the March issue of YM’s ‘Any Questions’, Nigel Calder states there should not be an on-board connection between neutral and earth, but my research with the Metal Boat Society and Mastervolt suggests that the neutral and earth should be connected. Please could I hear Nigel’s views – perhaps this is an exception for metal boats?

Nigel also comments that reverse polarity does not create a hazard when an isolating transformer is fitted. Could he explain this, and also give me some tips on how to make or buy a reverse polarity tester?

Richard Peperell, via email

Nigel Calder replies:
Earthing (grounding in US parlance) is difficult to address in a short note, especially given significant differences in the approach between Europeans and Americans. I hope the following will address the specific questions:

1. With an isolation transformer there is no direct connection between the shorepower circuit and the boat’s circuits – power is transferred magnetically. This has several beneficial effects, one of which is that the polarity of the on board circuits is established on board, and is not affected by the polarity of the shoreside connection. Given such a ‘polarized’ boat, a reverse polarity device is not needed. On other boats, all that is needed to test for reverse polarity is a high-impedance (at least 25,000 ohms in the USA – I’ll explain this in a moment) LED circuit between the neutral and earth circuits. If the polarity gets reversed on board, the neutral becomes ‘hot’ and the light now glows.

2. With an isolation transformer, the way polarity is established on board is by tying the neutral and the earth together at the transformer. In fact, the basic rule for all on board AC power is that the earth and neutral should be tied together at the generating source. In a typical installation (without an isolation transformer) the generating source is on shore (the neutral and earth are tied together at the dockside source), not on the boat, but with an on board AC generator, an isolation transformer, or a DC-to-AC inverter, these devices become the ‘generating source’ and as such the earth and neutral should be tied together at this point (or some electrical equivalent – normally the case of the device is used, but it could be a physically different point so long as it has the same electrical equivalence). I think Mastervolt, the Metal Boat Society and I are all saying the same thing, and maybe I wasn’t clear enough in my earlier comments about the distinction between typical shorepower installations and those with isolation transformers. If you have an isolation transformer, the neutral and earth should be tied together at the transformer.

3. A DC-to-AC inverter with a ‘pass-through’ capability (i.e. when the boat is plugged into shorepower, the inverter switches to shorepower) is an interesting situation. When in the ‘pass through’ mode (i.e. the boat is operating on shorepower) the neutral to earth connection must be at the generating source (shorepower, or the on board side of an isolation transformer if one is fitted) but when the inverter is supplying AC power it becomes the generating source, and as such the neutral to earth connection needs to be made at the inverter. Marine inverters have switches that make this connection when in invert mode and as such are different from many shoreside inverters. You should always buy a marine inverter for a boat.

4. The Europeans and Americans have different philosophies when it comes to RCD devices (which measure leakage of current from the ‘hot’ and neutral wires to earth, and trip when the current exceeds a certain level). In Europe, the trip limit is generally set at 30 mA (30 milliamps) whereas in the USA it is set at 5 mA as a result of tests conducted many years ago that established 5 mA as the level at which leakage current in the water began to effect muscle control in humans (this was determined by suspending student volunteers in a swimming pool and ramping up the current!). In the damp marine atmosphere 5 mA is low enough to be susceptible to nuisance trips, so the Americans protect individual outlets on the boat (this puts only one circuit, and the leakage current from that circuit, on each RCD, as opposed to putting the whole boat, with its accumulated leakage currents, on a single device) whereas the Europeans put the 30 mA RCD on the incoming shorepower line and use the single device to protect all the circuits on the boat. European standards then assume that this will protect against leaks to earth and as such when a whole boat RCD is fitted permit the boat to be wired without the AC earth circuits connected to DC negative (something that is otherwise important from a safety perspective). On boats that do not have an isolation transformer, not making this connection helps to prevent galvanic corrosion (especially important with metal boats). However, in the USA a considerable amount of data has been collected from domestic RCD use to show that failure rates are high (principally due to lightning-induced voltage surges) so even with a whole boat RCD the American standards advocate the AC earth to DC negative connection, and then recommend either a galvanic isolator, or an isolation transformer, to break the galvanic circuit back to shore. (I know some readers will be scratching their heads at this point! Galvanic isolation is a another complex topic I do not have space to address here.)

5. The minimum 25,000 ohm resistance on American reverse polarity devices is established by determining the resistance it will take on a 120 volt circuit to keep current flow below the 5 mA RCD threshold (otherwise the reverse polarity warning light, which makes a connection between the neutral and earth, would trip an RCD). At 120 volts, this is 24,000 ohms (120/0.005 = 24,000). Applying the same logic to a 30 mA RCD device on a 240 volt circuit, you arrive at 8,000 ohms (240/0.030 = 8,000) – i.e. the minimum resistance on a reverse polarity indicating circuit should be above 8,000 ohms.

6. Reverse polarity from other boats will not necessarily make the water live while concomitantly a boat without reverse polarity can make the water live. The basic rule is to not get in the water around boats plugged into shorepower! You can buy simple testers that plug into any receptacle to warn of various fault conditions – it is worth carrying one on board. You can make a crude test for voltage in the water by touching one probe of an AC voltmeter to any earthed circuit (e.g. the earth socket of a dockside outlet, so long as you know what you are doing and get the correct socket) and dangling the other probe in the water.

This stuff is difficult! I hope the above serves to elucidate rather than further confuse!!