Mains electric earthing to battery negative???

[Jaguar 25]

Could someone please advise what are the possible problems if the mains input earth connection into a grp boat is not then connected to the battery negative?

 

[RupertW]

Somebdoy more expert will come along but it's basically the same as using an extension lead from a marina plug - everything around earthing relies on the marina earth being good. That's what I've done (and am doing right now) in my little boat but I have full mains earthing to negative on my other boat, with galvanic isolator, RCD etc.

That way I can be sure that a marina won't affect me if their earth fails and I should be fine avoiding leaks between AC and DC and dezincing even in unknown marinas abroad.

 

[BabaYaga]

As already said it has to do with personal safety in case of an electrical fault in some equipment AND the marina earth lead not functioning as it should.
But as I understand it, the objective is not to connect to battery negative per se, but to create an alternative path to ground (surrounding water) by way of engine block, shaft, keel etc.
If battery negative is isolated from the water, I see no point at all linking the two.
Could be wrong, though.

 

[rogerthebodger]

As I understand it the theory goes that if you have the mains earth and the DC negative connected and if there is a fault that causes the AC live shorts to the DC side due to a cable fault that the RCD or circuit breaker/fuse will disconnect the mains live from the DC side rendering it safe if a person touches somthing on the DC side they will not get a shock.

In my view the mains and DC cabling should be in different trunking/ routing in the boat so a short should be near imposible. But that assumes that the wiring is done correctly using totally separate trunking/routing.

I have a steel boat so my DC negative is not connected to the hull but my mains earth is for the possibility of a mains live to hull short. The DC negative is not connected to eliminate possible galvanic issues with the steel hull.

 

[sailorman]

This subjet is well covered on the YBW, suggest a search :encouragement:. Then move on to what is the best anchor :sleeping:

 

[Hydrozoan]

In my view the mains and DC cabling should be in different trunking/ routing in the boat so a short should be near imposible.

Yes, but it's not just the cabling: AC and DC do come in close proximity in battery chargers, for example, so a fault in one of those could potentially put mains into the DC circuitry.

 

[rogerthebodger]

Yes, but it's not just the cabling: AC and DC do come in close proximity in battery chargers, for example, so a fault in one of those could potentially put mains into the DC circuitry.

Yes but a good quality battery charger should have a normal transformer (isolated windings) which overcome that problem.

 

[BabaYaga]

This subjet is well covered on the YBW, suggest a search :encouragement:. Then move on to what is the best anchor :sleeping:

A search throws up a lot of threads covering the general subject, but perhaps not the answer to the OP:s specific question: What are the potential problems/dangers of not connecting mains earth to DC negative. What could happen? Or is this so well known that it is not worth discussing?

Yes, and I think the Delta is quite alright...

 

[VicS]

Could someone please advise what are the possible problems if the mains input earth connection into a grp boat is not then connected to the battery negative?

Much debated subject!

Difficult actually I find to put together a reasoned explanation of the hazards which may exist if the shorepower earth is not bonded to the dc negative etc..... the "etc" perhaps being more relvant than the "DC negative" because there are boats/engine installations in which the DC system is isolated.

All the standards AFAIK which refer to AC installations require the shorepower earth to be bonded to the DC negative or the boats internal earth system except the now obsolete 2001 edition of ISO 13297.

that says

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).
4.3 For craft with fully insulated d.c. systems (see ISO 10133), the a.c. protective conductor shall be connected
to the hull of a metallic hull craft, the craft external ground (earth) or the craft lightning-protection ground plate, if
fitted.

I am not expecting to find that the concession not to require the connection if there is a "whole craft" RCD is included in the 2012 edition.

The only "benefit" to be derived from not connecting the shorepower earth to the DC negative etc is that it will avoid the galvanic corrosion, in particular rapid rate of loss of anodes which can be associated with shorepower connections.

 

[nigelmercier]

Much debated subject! ...

Indeed. I would put forward the case to keep the mains earth separate from the DC, unless you have a specific reason or bit of equipment that requires it. I'm my view if it were necessary, then shore power chargers would be thus connected.

 

[nimbusgb]

Perhaps the question is better posed as 'What are the potential dangers of connecting shore power earth to the boat DC negative'.

 

[Deleted member 36384]

Indeed. I would put forward the case to keep the mains earth separate from the DC, unless you have a specific reason or bit of equipment that requires it. I'm my view if it were necessary, then shore power chargers would be thus connected.

Sterling's instructions for the Pro Charge Ultra state that earths should all be connected to the boats ground. I would assume Sterling understand what they are talking about to make such a general statement, as I assume you do as well. A quote from the instructions: -

Ground / bonding / earthing . This is
extremely important and often overlooked
there are, in effect, 3 grounds,
1)the earth wire ( A/C input, the ground ) ,
2) the Chassis / bonding ground ( going to
a vehicle body / boats bonding system,
the bolt on the side of the charger )
3)the D/C negative.
In most installations all these will end up
at the same point, ie the A/C power source
should be connect to the boat/vehicle
chassis ( for safety ).The chassis earth will
also go there, and the D/C neg should
also go there, in effect bonding the total
system together ensuring any fault to the
chassis will blow a fuse. This could vary
for steel/aluminium boats.

 

[nigelmercier]

I would assume Sterling understand what they are talking about ...

Really?

 

[superheat6k]

As I understand it the major reason for connecting the ships bonding circuit to the shore earth system, which should include the DC negative, is for protection of swimmers or fallers in by the boat.

If the bonding system is not connected to shore earth and somehow a fault develops say in a battery charger the 240vac could be connected to the bonding circuit, but not back to the normal shore earth.

This issue is worse in fresh water as this is a worse conductor than something salty such as a human being. I also read that high voltage will stratify along the surface, and that could be via the poor sod in the water, whether or not he planned to be there.

Nigel Calder, Charlie Wing and Edwin Sherman along with the American ABYC standards all elaborate further on this subject. I will see if I can find the most informative reference and explanation in their various tomes.

The link from the bonding circuit to shore earth will remain on my boat. Not sure where BS, EN or IEE standards lay this down for the UK / EU.

 

[nigelmercier]

As I understand it the major reason for connecting the ships bonding circuit to the shore earth system, which should include the DC negative, is for protection of swimmers or fallers in by the boat...

On a metal boat, perhaps.

 

[VicS]

The link from the bonding circuit to shore earth will remain on my boat. Not sure where BS, EN or IEE standards lay this down for the UK / EU.

BS EN ISO 13297:2012
Small craft. Electrical systems. Alternating current installations​

In my earlier post I quoted from the previous edition, which can be found on-line

The new edition came into effect approx a year ago but being copyright material is not available for free on line ... yet!

Unfortunately it costs quite a bit to buy so those in the trade who have bought copies are keeping its contents to themselves.

 

[VicS]

On a metal boat, perhaps.

Or GRP boats where the stern gear, anodes, keel , ground plate, lightning conductor, anchor chain might all have a connection to the DC negative.

ITYWF however that the objection is relying solely on an RCD tripping in the event of an obscure fault occurring. They are known to fail to operate.

If the earth bonding is made the idea is that if the RCD does not trip then a MCB will trip ( or a fuse will blow).

 

[SolentSnowgoose]

I think the best way to answer the OP's question is to consider the whole purpose of "bonding" all exposed metal to the earth of the mains system.

The basic reason for this in any installation ( marine or land) is so that in the event of a fault which connects the live circuit to the metal work there will immediately be a circuit back to the ground point of the mains supply transformer such that a fault current will flow and the fuse will blow ( or the breaker trip)

The alternative to this is for the exposed metal ( or the DC wiring) to remain connected to the live AC supply until some other event ( eg a person touching the DC circuit) completes the circuit and the fault current tries to flow via their body with potentially fatal results.

As far as I can see at first analysis it probably makes no difference to the fault current path if the earth is connected to the positive or the negative side of the DC circuit ( 12V DC offset superimposed on the 240V AC ) but there is more likely to be exposed metal (eg the engine block) connected directly to the DC side and in terms of convention the AC neutral has historically been bonded to the earth so the negative seems the logical connection point.

 

[JumbleDuck]

The only "benefit" to be derived from not connecting the shorepower earth to the DC negative etc is that it will avoid the galvanic corrosion, in particular rapid rate of loss of anodes which can be associated with shorepower connections.

A faulty supply (live earth) won't electrify your entire boat if the earths aren't bonded

 

[nigelmercier]

A faulty supply (live earth) won't electrify your entire boat if the earths aren't bonded

While it is unlikely that an earth would ever become "live", there is often a potential between the earth wire and "local" earth. Not enough to give you a jolt, but plenty to bypass a GI.