eathing of a boat

[tckearney]

Hi all.  I have an electrical question or two. 

Firstly on my 2000 B42  is the keel  and/or engine casing/saildrive connected electrically to the negative bus, with the engine not running and battery isolating switch off.     Secondly when in a marina in UK and the mains Live and Neutral are not connected to the boat (I have a two pole isolating switch fitted)  but the earth wire is connected. Will this cause any electrical activity between the keel/anodes and the marina pontoon? 

[JEN-et-ROSS]

when in a marina in UK and the mains Live and Neutral are not connected to the boat (I have a two pole isolating switch fitted)  but the earth wire is connected. Will this cause any electrical activity between the keel/anodes and the marina pontoon? 

Yes, I think it must if the pontoons/marina piles etc are also connected to the earth system of the marina.......Though I'm no expert....Bill

[tiger79]

Firstly on my 2000 B42  is the keel  and/or engine casing/saildrive connected electrically to the negative bus, with the engine not running and battery isolating switch off.     

The saildrive should ideally be electrically isolated, if it isn't there'll be an extra earthing relay on the engine, so as to only connect battery negative to the engine when the engine is running.  If the saildrive is electrically isolated, the engine will normally be connected permanently to battery negative.  The keel will usually not be connected.

Secondly when in a marina in UK and the mains Live and Neutral are not connected to the boat (I have a two pole isolating switch fitted)  but the earth wire is connected. Will this cause any electrical activity between the keel/anodes and the marina pontoon?

Yes, very likely.  The way to avoid this risk is to fit a galvanic isolator.  I also unplug the shorepower connection when I'm not on the boat.

[tckearney]

Thanks for the advice.  Although I do have good electrical knowledge I am struggling to get my head around the current flow from a plastic boat to a pontoon that is sat in the water, effectively at earth.   The keel  is also sat at the same potential, as it’s in the sea.    Good point about the sail drive insulation, thanks I will check that.   I have actually fitted a shore power connection at either end of the boat to save cables along the deck in marinas.  I then had to fit a changeover switch to ensure that only one socket is live at any one time.   The changeover switch has a centre "off" position so is isolated when mains is not being used.  The switch is only two pole though, so live and neutral are isolated but earth is not.   Also wonder what the difference is in Europe where there is no neutral just two live wires L1 & L2

[Yngmar]

Also wonder what the difference is in Europe where there is no neutral just two live wires L1 & L2

That is not the case. Our standard marina sockets (and domestic single-phase) wiring works just like yours, with L, N and PE (protective earth), except at 230V and 50 Hz nominal. The wire colour coding may differ. There is also three-phase power, with L1, L2, L3, N and PE, so there is a neutral available there too, although in marinas you'll not see these much except maybe at superyacht berths, so probably not something you need to worry about. The shorepower connectors are typically IEC 60309.

As for grounding AC earth to the keel, I believe the RCD requirement for this has changed sometime after our boats were built (post 2001 I think), so they won't be grounded if the shorepower installation is original. This avoids any electrolysis issues from AC, although some would argue it is potentially dangerous to people who like to swim in marinas (something I do try to avoid most of the time). Good detail on this can be found here: http://www.smartgauge.co.uk/earthing.html

There will be a large copper rod connecting your mast through the compression post to the keel. This is for lightning protection, so the strike won't punch a hole through your hull but rather exit through the keel instead. It has nothing to do with either AC or DC grounding (although could be abused for this, if your keel isn't well encased in paints).

Marina wiring in general is connected at random and often far more dangerous than boat wiring. Common to have reversed L and N, or N and PE miswired, inappropriate breakers, no RCDs and at times we've seen loose wires dangling from below pontoons into the water. Recommend having a simple socket tester to check for these things and a few connectors made up to uncross the wires. Also your boat should have a RCD or RCBO, which you should test occasionally.

[Jeff Jones]

I am struggling to get my head around the current flow from a plastic boat to a pontoon that is sat in the water, effectively at earth.   The keel  is also sat at the same potential, as it’s in the sea.   

The best way to look at this problem is to ensure your onboard mains (shore power system) is as robust as possible, and will indicate correct Live and Neutral Polarity prior you to switching on your onboard mains switch. Then a means of detecting a leakage of current ie with a RCD or RCBO

My B34 has a indicator to show correct polarity, and a RCD mains incoming circuit breaker. the RCD checks the current in the Live and Neutral conductors is equal, any difference >30mA will trip the RCD (am pretty sure Bavaria fitted this as standard).
Test your RCD every time you connect to a new shore power point.

re: your quote above.
you hit the nail on the head 'effectively at earth' you would think its all at the same electrical potential "Zero resistance" but never is.. NORMALLY THE RESISTANCE IS VERY LOW but resistance never the less.

A fault current will find the path of least resistance to complete a circuit.

It is very rare for things to be perfectly connected and this will cause a very small voltage drop which is caused by resistance, every cable, every joint has a minute resistance - including the sea - normally this never causes a problem apart from very special installation ie a boat being one in my opinion.

To give you a example, a cow stood in a field near a electrical pole or buried cable with a leakage fault, can be electrocuted because for the distance between its front legs and rear legs.. the resistance of the ground caused a voltage drop between its legs - which could be enough to kill the cow... (from a working example in my electrical engineers study days)

"People have stopped breathing when shocked with currents from voltages as low as 49 volts. Usually, it takes about 30 mA of current to cause respiratory paralysis. Currents greater than 75 mA may cause ventricular fibrillation (very rapid, ineffective heartbeat)" https://sciencebasedlife.wordpress.com/.../how-much-voltage-can-you-take-on-before-y...

The main point is, that if you have a RCD or RCBO as yngmar mentioned that device will detect a leakage fault and trip safe, when you have a faulty appliance  onboard that cause a stay current.. so your safer..