240v Earthing on Boat

[Bob Fleming]

Hi all,

re my earlier question re Adverc Galavanic Isolators - thanks to all who
contributed to the thread, I learnt a lot of stuff there, all to the good.

As these things always go, I thought of another question that there must be
a simple answer to but I just cant see it ...

OK, so we established that on the 240v circuit a good earth path back ashore
is important (potentially life saving) and that it would appear that the
most reliable, but expensive, option was an isolating transformer. For a
16amp supply it turns out to be a humungous piece of kit from what I can
find out. Anyway, to the next question ...

When I am cruising and using the inverter to deliver 240 volts onboard
(Mastervolt Dakar Combi 1500/50) I am not connected to the shore power so
have no shore earth, so... hwo can I use 240 with no earth when I am
cruising but need one when I am on shore line alongside? Obvious thought is
if it is safe when cruising, why is it not safe to do without an earth when
"alongside". If the earth path when cruising is to the water, then why cant
this be the same at the marina. I guess the fitted RCD wouldnt work for one
thing, would it?

There must be an obvious answer but I am just not seeing it!

Cheers,

Bob,
nb Tugby.

[Peter A Forbes]

Post by Bob Fleming
Hi all,
re my earlier question re Adverc Galavanic Isolators - thanks to all who
contributed to the thread, I learnt a lot of stuff there, all to the good.
As these things always go, I thought of another question that there must be
a simple answer to but I just cant see it ...
OK, so we established that on the 240v circuit a good earth path back ashore
is important (potentially life saving) and that it would appear that the
most reliable, but expensive, option was an isolating transformer. For a
16amp supply it turns out to be a humungous piece of kit from what I can
find out. Anyway, to the next question ...
When I am cruising and using the inverter to deliver 240 volts onboard
(Mastervolt Dakar Combi 1500/50) I am not connected to the shore power so
have no shore earth, so... hwo can I use 240 with no earth when I am
cruising but need one when I am on shore line alongside? Obvious thought is
if it is safe when cruising, why is it not safe to do without an earth when
"alongside". If the earth path when cruising is to the water, then why cant
this be the same at the marina. I guess the fitted RCD wouldnt work for one
thing, would it?
There must be an obvious answer but I am just not seeing it!
Cheers,
Bob,
nb Tugby.

There are several questions in there, but as far as the shore/boat supply goes,
you always have a line and neutral with shore-based supplies, while an isolated
supply through a transformer or an inverter supply are not referenced to
anything unless you make them that way.

The reason for a good earth is purely to enable any safety device such as a
fuse/MCB to blow/trip if you get a line fault down to the casing of the device,
so the casing can't become live itself.

On inverters and isolating transformers you either earth one leg of the output
or earth a centre-tapped point. You still need protection against faults though,
even if an RCD won't like a centre-tapped supply running through it.

Peter

[Allan Jones]

Post by Bob Fleming
Hi all,
re my earlier question re Adverc Galavanic Isolators - thanks to all who
contributed to the thread, I learnt a lot of stuff there, all to the good.
As these things always go, I thought of another question that there must be
a simple answer to but I just cant see it ...
OK, so we established that on the 240v circuit a good earth path back ashore
is important (potentially life saving) and that it would appear that the
most reliable, but expensive, option was an isolating transformer. For a
16amp supply it turns out to be a humungous piece of kit from what I can
find out. Anyway, to the next question ...
When I am cruising and using the inverter to deliver 240 volts onboard
(Mastervolt Dakar Combi 1500/50) I am not connected to the shore power so
have no shore earth, so... hwo can I use 240 with no earth when I am
cruising but need one when I am on shore line alongside? Obvious thought is
if it is safe when cruising, why is it not safe to do without an earth when
"alongside". If the earth path when cruising is to the water, then why cant
this be the same at the marina. I guess the fitted RCD wouldnt work for one
thing, would it?
There must be an obvious answer but I am just not seeing it!
Cheers,
Bob,
nb Tugby.

Bob,

The shore Neutral supply is earthed at the sub-station, which means that the
Live wire is at a potential of 240v ac relative to earth. If you stand on
wet ground, and you touch just the live wire, you will get a painful (and
possibly lethal) surprise. The presence or lack of an earth is not relevant
but an RCD would probably save your life because it measures the difference
between the current that flows out from the Live, and the current that
returns from the Neutral. Any difference is the current that has flowed from
the Live, through you (still painfully) to earth, and thence directly back
to the sub-station without use of the Neutral connection.

To repeat, the RCD does not need an earth (there was an older version,
called an ELCB, that did need an earth, but let's ignore that one; if I
found one today I'd throw it away). The earth is more about making sure that
potentially lethal conditions do not arise (would you like to bet your life
on an RCD that has never previously been used? Are you happy to allow enough
current to flow through your body, to operate the trip-relay in the RCD?) If
you have any equipment with a metal casing, and it develops a fault such
that the live supply touches the metal, a lack of earth would mean that when
you touched it you would get a shock. The same could apply if the cable were
damaged or pierced, etc etc. The earth ensures that the fault condition
would immediately blow the fuse instead.

On a boat, the same applies when using a shore line. The hull can act as a
very effective earth connection.

When you use an inverter or generator, however, the Neutral is not
necessarily earthed (there are divided opinions as to whether or not it
should be). A single fault would have no effect; if the Live supply touched
the metalwork of the boat, it would simply cause the Neutral supply to adopt
a position 240v away from that, and there would be no hazard EXCEPT that
this condition could remain un-noticed for years until another fault
developed, such as a piece of faulty equipment whose Neutral had shorted to
the casing. That equipment would work fine on shore, but when you brought it
on board it would be potentially lethal as the (now live) Neutral supply
from the inverter takes the casing to a potential 240v ac different from the
hull of the boat. For that reason a properly earthed system is essential on
the boat, even if the inverter has only a 2-terminal (live and neutral)
output; you MUST use 3-core cable with the earth properly bonded to the
boat's hull (usually the battery negative connection will do, but check that
the battery isolator switch has been correctly installed and doesn't
disconnect your safety earthing).

[Neil Rothwell]

On Thu, 22 Jan 2004 12:11:02 +0000 (UTC), "Allan Jones"
<***@btinternet.com> wrote:

<snip - great description of earthing>

To throw another question into the discussion:

I'm sure we agree internal 240V AC boat equipment should be earthed to steel
hulls and this hull earth should be connected to the shore earth via a suitable
galvanic isolator (diodes or transformer).

Question - should the battery negative also be deliberately connected to the
hull earth ?

Reading the classic texts such as Nigel Calder opinion seems very divided and
vague on this. I'm favouring not making this connection but going for the
isolated approach, for simplicity more than any other reason, i.e. I don't have
to start worrying about stray earth loop DC currents causing accelerated
corrosion.

Have to make sure I don't have any non-deliberate earth paths of course such as
through engine mounts, propshaft, battery charger etc.

Cheers

Neil

[Graham Brooker]

Usually the starter motor negative, various sensors on the engine and the
alternator take their negative connection via the engine casing. You
therefore end up with battery negative grounded to the hull via the engine
mounts.

Do any engine manufacturers avoid using the engine casing as a common
negative - one man in my marina claims his starter motor has an insulated
negative feed connection and his old air cooled engine has no sensors at
all.

Graham Brooker

Post by Neil Rothwell
On Thu, 22 Jan 2004 12:11:02 +0000 (UTC), "Allan Jones"
<snip - great description of earthing>
I'm sure we agree internal 240V AC boat equipment should be earthed to steel
hulls and this hull earth should be connected to the shore earth via a suitable
galvanic isolator (diodes or transformer).
Question - should the battery negative also be deliberately connected to the
hull earth ?
Reading the classic texts such as Nigel Calder opinion seems very divided and
vague on this. I'm favouring not making this connection but going for the
isolated approach, for simplicity more than any other reason, i.e. I don't have
to start worrying about stray earth loop DC currents causing accelerated
corrosion.
Have to make sure I don't have any non-deliberate earth paths of course such as
through engine mounts, propshaft, battery charger etc.
Cheers
Neil

[timleech]

On Fri, 23 Jan 2004 11:01:43 +0000 (UTC), "Graham Brooker"

Post by Graham Brooker
Usually the starter motor negative, various sensors on the engine and the
alternator take their negative connection via the engine casing. You
therefore end up with battery negative grounded to the hull via the engine
mounts.
Do any engine manufacturers avoid using the engine casing as a common
negative - one man in my marina claims his starter motor has an insulated
negative feed connection and his old air cooled engine has no sensors at
all.

Listers certainly used to go to some lengths to ensure no permanent
earth connection, though they did use a standard starter motor which
was connected via two solenoids, so that the body was connected to the
battery only during starting.
'proper' marine alternators and sender units don't use the engine
block as an earth return, but they tend to be quite a bit more
expensive than the standard automotive type.

Cheers
Tim

Tim Leech
Dutton Dry-Dock

Traditional & Modern canal craft repairs

[Adrian Stott]

On Thu, 22 Jan 2004 13:19:56 +0000, Neil Rothwell

Post by Neil Rothwell
On Thu, 22 Jan 2004 12:11:02 +0000 (UTC), "Allan Jones"
<snip - great description of earthing>
I'm sure we agree internal 240V AC boat equipment should be earthed to steel
hulls and this hull earth should be connected to the shore earth via a suitable
galvanic isolator (diodes or transformer).
Question - should the battery negative also be deliberately connected to the
hull earth ?
Reading the classic texts such as Nigel Calder opinion seems very divided and
vague on this. I'm favouring not making this connection but going for the
isolated approach, for simplicity more than any other reason, i.e. I don't have
to start worrying about stray earth loop DC currents causing accelerated
corrosion.

I just rechecked Calder. He is quite definite that there should be a
single ground point for both AC and DC on board.

Post by Neil Rothwell
Have to make sure I don't have any non-deliberate earth paths of course such as
through engine mounts, propshaft, battery charger etc.

Calder also discusses this at some length. He suggests ways to ground
independently devices like the alternator and the starter, and also
mentions the possibility of a stray ground route through the
instrusments (via the sensor loads). However, I think that in
practice most people accept that there will be a route through the
engine block, and just make very sure there is a large grounding strap
from the block to the single ground point.

Adrian

[Neil Rothwell]

Post by Adrian Stott
Calder also discusses this at some length. He suggests ways to ground
independently devices like the alternator and the starter, and also
mentions the possibility of a stray ground route through the
instrusments (via the sensor loads). However, I think that in
practice most people accept that there will be a route through the
engine block, and just make very sure there is a large grounding strap
from the block to the single ground point.

Looks like I need to re-read :)

We have flexible rubber engine mounts and a flexible propshaft coupling so no
path through those. just thought of another path though - through the throttle
and gearbox control cables up to the lever controls (all metal) which are
fastened to the hull.

[Nick Atty]

On Fri, 23 Jan 2004 16:49:38 +0000, Neil Rothwell

Post by Neil Rothwell
We have flexible rubber engine mounts and a flexible propshaft coupling so no
path through those. just thought of another path though - through the throttle
and gearbox control cables up to the lever controls (all metal) which are
fastened to the hull.

I once saw a boat where some sort of fault (I think a poorly thought out
ordering of BMS, batteries and earth points) meant that the full
charging current went through the bowden cables of the throttle control.

As these were bundled with the electric wiring to the control panel it
was rather a mess.

[Tony Brooks]

"Allan Jones" <***@btinternet.com> wrote in message news:buoekm$m8j$***@sparta.btinternet.com...
snip

Post by Allan Jones
on board it would be potentially lethal as the (now live) Neutral supply
from the inverter takes the casing to a potential 240v ac different from the
hull of the boat. For that reason a properly earthed system is essential on
the boat, even if the inverter has only a 2-terminal (live and neutral)
output; you MUST use 3-core cable with the earth properly bonded to the
boat's hull (usually the battery negative connection will do, but check that
the battery isolator switch has been correctly installed and doesn't
disconnect your safety earthing).

Please, lets think a little before advising that teh battery negative
might/usually/could make an acceptable 240v earth point.

On a rigid mounted engine in a steel boat, it proably would be OK, because
the engine (battery negative) would be earthed to the hull via the mounting
bolts and engine beds.

On grp or wood, it would not be OK. because if there was no earth path and a
fault occoured you would be putting 240V onto the 12v cables and equipment
that may not be insulated for a continuous 240v.

On flexibly mounted engines in steel hulls I would not be happy in relying
on the control cables and exhaust to (perhaps) earth the engine under 240v
fault conditions.

I would suggest that to be absolutely safe the mains earth should be totaly
seperated from the 12 negative (except on a steel hull it might not be) and
I woudl also suggest that any lightening conductor earths should all so be
to another seperate point - otherwise you could end up with a lightening
strike flowing through all your electrics via a poor earth.

How & what you attach such earths to is another matter. Being lazy, I would
attach the 240V one to the seainlet on GRP or Wood, or any readily visible
and checkable bolt to the hull on steel. I trust that a narrow boat, being a
pretty good Faraday cage would protect me from lightening so ignore it.

Tony Brooks

[Pete C]

On Mon, 26 Jan 2004 11:58:09 -0000, "Tony Brooks"

Post by Tony Brooks
How & what you attach such earths to is another matter. Being lazy, I would
attach the 240V one to the seainlet on GRP or Wood

What if a 12v cable shorts to 240v earth on the boat, you'll end up
with the inlet at 12v and the prop at 0v, hmmm.....

Any wiring scheme has to consider and prevent fault conditions which
might cause electrocution, fire or electrolysis.

cheers,
Pete.