AC DC common ground

[Dave Lochner]

The simplest, non-technical way to consider AC wiring on a boat is to think of the boat as a big extension cord with a power strip on the end. That is more or less what it is.

The wiring mistakes, especially the AC Neutral and Ground connection come because the boat is treated as a house, which it is not.

At home, would you connect the AC neutral and AC ground on the power strip? Probably not, so don't do it on a boat.

The AC Ground and DC negative should be connected at one point, usually on the engine block.

[boat driver]

Originally Posted by CharlieJ

@roland_stockham #9


DC neutral?? If you truly meant DC ground or B-, than the latest version of ISO 13297 that I hold (2012) contradicts your statement.

ISO 13297-2012


roland_stockham #9


The "big copper stake" has nothing to do with fault clearance! Current is returned via the neutral in a USA 120V system. The safety ground (green wire) is a low impedance redundant conductive path back to the source, usually the secondary of the marina's transformer, to allow enough fault current to pass to trip a protective device somewhere in the circuit.

This is a seriously dangerous subject. Please, do not feel obligated to type something if you do not consider yourself well versed in the subject matter, as misinformation can do some actual harm if it is applied by a novice.

this is not to be taken lightly.
first, couple comments are discussing neutral and other "descriptions" . obviously these authors do not understand electricity- so emphasize, no comments unless you are certified.

the ground in the us' system is not a neutral. it is specifically a ground.

for the rest of the world- electricity does not concern itself with global boundaries.
It is simply physics.
research the principles and specifically the differences. Don't do it correct-
short term is sacrifice your underwater metals.
long term maybe sacrifice your friends.

[Pete7]

Originally Posted by roland stockham

Also depends where the boat was built. May have changed but in Europe it was forbidden to connect the A/C ground to the DC neutral. This seems sensible, D/C neutral is a 'floating ground' because ti is a closed loop on the boat it does not need a reference ground and does not care what voltage it is relative to the world outside the boat. The A/C side however must always have H2 or neutral joining the ground wire at the power source. If this is an onboard generator or inverter the ground and H2 or neutral should connect there. If A/C comes aboard from shore power then the H2 or neutral goes back ashore with the ground. This creates a ground that is reference to the outside world. The marina should have a big copper stake in the ground to maintain this reference.

The original wiring of my 1988 yacht has the A/C Earth (US Ground) connected to the D/C negative (US Neutral). When I recently installed a new inverter, I wired the case Earth connection to the D/C negative supply which connects to the anode etc via bus bars.

This is fairly standard in the UK, though it does provide some lengthy and lively discussions on UK forums.

Biggest problem in Europe is some marinas are wired the wrong way round so you hear of people making their own fly leads to reverse the positive and negative A/C supply.

Thank god for solar, we rarely plug in now unless we are out sailing in the depths of winter.

[boatpoker]

Originally Posted by Pete7

The original wiring of my 1988 yacht has the A/C Earth (US Ground) connected to the D/C negative (US Neutral).

It must be a language or terminology thing ...
"D/C negative" is not "US neutral".

[duetto]

Many thanks to everyone for the replies. I will be adding a connection for a common AC DC ground.

[sailingchiro]

Originally Posted by RigBig

The shore power grounding bus (green wire) should be connected to the DC ground (Black or Yellow Wire). DO NOT connect the AC neutral (White Wire) to the DC ground (Black or Yellow Wire)/AC ground (Green Wire) together.

That was my problem. My inverter charger had green and whites connected together. My boat was blowing entire marina GFCI. I was not a popular dude that 98 degree day. Disconnected all my green wires to see which whites were shorted to greens. All three were in inverter charger. I removed the jumper in the invertor charger and no more problems.

[boatpoker]

Originally Posted by sailingchiro

That was my problem. My inverter charger had green and whites connected together. My boat was blowing entire marina GFCI. I was not a popular dude that 98 degree day. Disconnected all my green wires to see which whites were shorted to greens. All three were in inverter charger. I removed the jumper in the invertor charger and no more problems.

You do know that the ground must be bonded to the neutral at the power source right ? ... and that the inverter is considered a "power source", right ?

A proper inverter does this switching/unswitching automatically. Did you leave this switching alone or just randomly disconnect ground/neutrals ?

[Rotten Ricky]

As a former industrial electrician schooled in European technology but having worked mainly in North American, I can attest that we should be very careful about assumptions about the terminology used. Casual slang compounds the problem. What we assume can kill us. In this case a little knowledge can be a VERY dangerous thing!

One other important matter is that wiring itself is very rarely an issue unless overloaded or the insulation is damaged. Assuming the circuit is wired correctly in the first place, it is the terminations and connections that are the weak point and almost always the problem. Whether it is industrial, domestic or marine circumstances, poor connections are the primary reason for malfunction and fire. Ask your insurer!

Poor contacts - particularly from corrosion - allow arcing that creates heat that results in fires. Use of a thin smear of dialectic grease on terminations will help prevent this, even with non-tinned conductors. This is especially important in high-saline atmospheres near the equator.

I implore people to occasionally check connections so that this cannot happen. Just because it has worked for a month or a year or a decade, does not mean it is safe. In particular, AC systems, because the current is alternating 50 or 60 times a second, have a vibrating effect and tend to loosen over time. If you have a receptacle/plug, switch etc., that is hot to the touch when under load, check or replace it. Every year I check the tightness of the wiring in my home. Every year I find something not tight.

Check your connections!

Cheers, RR.

[Three888s]

Will try to explain in as few as words as possible. In USA, Ac Colors- supply-Black wire, white wire- common, green wire-ground. Main panel you may have common and ground connected. Any sub panel off of main panel common and ground must not be connected. Main panel is where the panel is coming from marina. Correct ground at main panel to be properly grounded should have 1 or 2 8 ft min grounding rods driven into the earth or a water line or such that travels through the earth. Never a gas line. Most sub panels on a boat, common and ground should never be connected. Most sub panels depending on age have a green screw that should be removed. This screw connects the common and ground. As commented prevously GFI and GCFI look for an inbalance between black and white wires and trips.

[boatpoker]

Originally Posted by Three888s

Will try to explain in as few as words as possible. In USA, Ac Colors- supply-Black wire, white wire- common, green wire-ground. Main panel you may have common and ground connected.

NO ! NO ! NO !

You must not have common and ground connected !
You are going to kill someone !

STOP offering electrical advice !

[Wotname]

Originally Posted by boatpoker

NO ! NO ! NO !

You must not have common and ground connected !
You are going to kill someone !

STOP offering electrical advice !

Totally agree!
Common (AC neutral) and ground should only be connected at the SOURCE - your main panel or any other panels are NOT sources.
Unless you have M.E.N. arrangement which I have never seen or a boat - but never say never...

[Three888s]

I should have made my explanation more clear. Power lines come into building ( marina) are connected to a panel. (main panel.) They are not coming from grid to the dock. This is what i meant as main panel. Any panel that is fed power after that is a sub panel.

[goboatingnow]

Personally no EU boat I’ve ever owned has AC protective earth bonded to DC ground. It’s a massive source of impressed corrosion

And you need to properly quote ISO 13297

“4.2 The protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as practicable to the battery (d.c.) negative terminal.
NOTE If an RCD (whole-craft residual current device) or an isolation transformer is installed in the main supply circuit of the a.c. system (see 8.2), the negative ground terminal of the d.c. system need not be connected to the a.c. shore ground (protective conductor).


Since Eu boats always have RCD devices there is never any dc ac interconnection and in my view that’s an extremely good thing

[mitiempo]

Originally Posted by goboatingnow

Personally no EU boat I’ve ever owned has AC protective earth bonded to DC ground. It’s a massive source of impressed corrosion

And you need to properly quote ISO 13297

“4.2 The protective conductor shall be connected to the craft's d.c. negative ground (earth) as close as practicable to the battery (d.c.) negative terminal.
NOTE If an RCD (whole-craft residual current device) or an isolation transformer is installed in the main supply circuit of the a.c. system (see 8.2), the negative ground terminal of the d.c. system need not be connected to the a.c. shore ground (protective conductor).


Since Eu boats always have RCD devices there is never any dc ac interconnection and in my view that’s an extremely good thing

In North America ABYC requires the AC-DC connection even if there is an ELCI (RCD).

[goboatingnow]

Originally Posted by mitiempo

In North America ABYC requires the AC-DC connection even if there is an ELCI (RCD).

Agreed but the point is major standards bodies disagree about the importance. ISO feels that whole boat RCD protection removes the need. As I mentioned I’ve never seen it such a connection in EU boats in the last 20 years.

Personally I don’t want any path for external current into my dc system and “ protective earths “ in marinas are often nothing of the sort.

It should be pointed that ISO standards are legal requirements in the EU. ABYC in the US is a recommendation