Why connect shorepower earth to boat's ground?

hwmbo said:
As a drift from this thread about galvanic isolators (http://www.ybw.com/forums/showthread.php?353219-Galvanic-Isolator) why would you want/need to connect your shorepower earth to your boat's DC negative ground in the first place?

Surely the shorepower supply will be earthed anyway? Why risk rapidly accelerated galvanic corrosion?
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For electrical safety reasons on board.

Pros and cons much debated on here and elsewhere

Current advice from the experts in the field is that shorepower earth and DC negative should be bonded, except of course with isolated DC systems

The old version of ISO 13297 does allow them not to be bonded provided a whole craft RCD is fitted http://www.earlhill.de/files/ISO_13...Systems_Alternating_current_installations.pdf (see section 4)

There is a new edition of ISO 13297 now. but I dont know what it says although I was lead to believe that the above concession would not be included, bringing the standard in line with others.
 
It's a safety issue. Even though many appliances are double-insulated, earth grounding reduces the risk of lethal shock.

However, it's more complex with a boat as you then get into discussions about connecting 12V -Ve and AC circuit earth and then into whether or not to connect skin fittings to earth as well.

I think you'll find that following are generally accepted:

AC earth back to shorepower: Yes, on safety grounds
AC earth linked to DC earth: Yes, variety of reasons (no doubt someone will expand)
DC earth to Skin fittings: Generally not requ'd with GRP, just keep then completely insulated (wire inside hose can link unintentionally)
 
hwmbo said:
As a drift from this thread about galvanic isolators (http://www.ybw.com/forums/showthread.php?353219-Galvanic-Isolator) why would you want/need to connect your shorepower earth to your boat's DC negative ground in the first place?

Surely the shorepower supply will be earthed anyway? Why risk rapidly accelerated galvanic corrosion?
Click to expand...

The best way to avoid galvanic currents is to use an isolating transformer, then the shorepower earth just earths the transformer and is not connncted to the boat electrics.
 
VicS said:
For electrical safety reasons on board.

Pros and cons much debated on here and elsewhere

Current advice from the experts in the field is that shorepower earth and DC negative should be bonded, except of course with isolated DC systems

The old version of ISO 13297 does allow them not to be bonded provided a whole craft RCD is fitted http://www.earlhill.de/files/ISO_13...Systems_Alternating_current_installations.pdf (see section 4)

There is a new edition of ISO 13297 now. but I dont know what it says although I was lead to believe that the above concession would not be included, bringing the standard in line with others.
Click to expand...

Thanks, VicS. I'm planning on having all negative 24v and 12v wires running back the relevant bus bars. Would this be an isolated DC system?
 
VicS said:
Current advice from the experts in the field is that shorepower earth and DC negative should be bonded, except of course with isolated DC systems
Click to expand...

Do we know what the reasoning is? The only one that springs to my mind is the risk of a loose AC conductor touching an un-insulated DC negative and thus making the whole DC system live at 240v, including things you may touch. In this case, earthing the DC negative would allow either a large enough fault current to trip a breaker (not sure I'd want to rely on that - it's a long and winding path from a random DC light fitting back to the marina's earthing point ashore) or leakage which would trip an RCD (more plausible). But unlike a car, the boat structure is not connected to DC negative, so how likely is it that a stray mains wire will connect with DC minus anyway? And presumably it's equally likely to touch a DC positive wire instead, with a similar result. But we don't see calls for the positive side of the DC system to be protected from this.

What have I missed? I'd like to understand the engineering rather than the paperwork.

(Also, what is an "isolated DC system" in this context?)

Cheers,

Pete
 
Mistroma said:
I think you'll find that following are generally accepted:

AC earth back to shorepower: Yes, on safety grounds
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Isn't this what happens with shorepower anyway? I can't see the difference between a shorepower supply, and just putting an extension lead through a window and plugging in an onboard AC appliance - or have I missed something?
 
Bonding the AC earth ad the DC earth is done not just for the safety of those on board but for the safety of swimmers in the water near your boat. See here for a detailed explanation (this guy is talking about narrowboats but the same applies to yachts).
 
I think there is too much generalization on this topic and there are various materials boat are made from and different types of mains equipment fitted on the boats, so each IMHO needs to be considered separably

On any kind of boat the AC earth back to shore power: Yes, on safety grounds

AC earth linked to DC earth: Yes, variety of reasons (no doubt someone will expand) I do not see the point in this and the only comment I have seen is to that if there is a short between DC and AC wiring it will cause the safety equipment to trip. IMHO the mains and DC wiring should be in separate trunking so that cannot happen. Pls if any one disagrees lets have details details and more details.

Now connecting the AC earth to all the metal fittings IMHO that is only necessary if it is possible for the metal to become live by a short of AC mains to the metal item that is open to be accidently touched by any one on board.

This depends and if the main hull material is conductive i.e metal boats as opposed to GRP hulled boats.

I have a steel boat so my DC negative is not connected to my hull at any point to try to prevent DC current loops. My engine is insulated from my hull for that reason.

My AC earth is connected to my hull through a Galvanic isolator and I have lots of anodes welded to my hull and rudder. My prop shaft is insulated from my engine by a rubber coupling and my prop and shaft protected by shaft anodes.

My AC and DC wiring is also totally separate in separate trunking .

My anodes have lasted almost 4 years and do show signs of a small amount erosion so are working and no underwater paint damage due to Galvanic effects.

To me the most important thing on a GRP boat is to have any metal cased appliances that can be touched be connected to shore power earth and an indicator that shows the status of that earth connection.
 
Tony Cross said:
this guy is talking about narrowboats but the same applies to yachts
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Narrowboats are tin tubs, though, and conductive. Most yachts are GRP, and not conductive. So his idea of treating the boat like an appliance (which I rather like) assumes it's a Class I appliance and needs earthing (nb, to the hull, not the DC negative) whereas my boat is more like a Class II double-insulated appliance.

Pete
 
Tony Cross said:
this guy is talking about narrowboats but the same applies to yachts).
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Most yachts are not the same as narrowboats as they are GRP so non conductive and most of the metal fittings are insulated from each other.

My boat is steel and so most of his comments I agree with except connection the DC negative to the hull. BTW Boat DC systems do not have an earth they only have 2 wires positive and negative.

Th only item he indicated that could be connected to both AC and DC is a battery charger and I would only permanently fix one that included a double would input transformer. All small car type I have are double insulated so no mains earth anyway

I would also like to understand the engineering behind the "standards" recommendations on bonding. I can understand why the anode suppliers recommend bonding all metal fittings on GRP boats as the anodes will erode quicker thus they sell more.
 
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prv said:
Narrowboats are tin tubs, though, and conductive. Most yachts are GRP, and not conductive. So his idea of treating the boat like an appliance (which I rather like) assumes it's a Class I appliance and needs earthing (nb, to the hull, not the DC negative) whereas my boat is more like a Class II double-insulated appliance.

Pete
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Not really, a class II device has no exposed metal parts, there are loads of those on yachts, both inside and outside.
 
Grounding / earth

"I have a steel boat so my DC negative is not connected to my hull at any point to try to prevent DC current loops."


I believe that unless on a metal boat,

the DC negative (12 volt negative ) is connected to the hull

(at the deemed grounding point) stainless steel bolt welded to hull in my case,

then, if there is a short in any of the DC circuits

the circuits breakers cannot protect the circuit.

Both a Coastguard engineering officer and a qualified electrical engineer have verified this on my metal boat.
 
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It is a complex question. I think the general assumption is that in a boat many things will be at a very good earth via the sea water ie sinks stoves even wet floor. A mains earth wire going back to the main switch board can acquire some voltage if another user has a fault situation. This voltage can be a potential difference to the sea hence parts of your boat.
I think also there is an assumption that the negative DC circuit will be at sea (earth ) potential as many motor vehicle components are used with body negative. Isolated negative is a rarity. Hence the general recommendation All negatives and earths connected together.
This of course has led to rapid corrosion problems where the boat earth and electrics are carrying current for the marina or where the galvanic currents are are made possible by this connection. Hence the use of galvanic isolator to provide a safe earth while providing a blockage for small voltage/currents.
The use of an extension cord and small appliances is OK to a limited degree. Double insulated ie plastic devices heaters kettles etc are small risk of active mains connecting to any metal part. But a battery charger even though made of plastic has this connection of negative out to ships electrics. (The mains would not differentiate between + and negative lines). So that insulation of mains to +or- output wires may be your safety or its failure could compromise your safety. (if ships wiring is not earthed to mains earth)
good luck olewill (actually probably no help at all to OP)
 
ianabc said:
then, if there is a short in any of the DC circuits

the circuits breakers cannot protect the circuit.

Both a Coastguard engineering officer and a qualified electrical engineer have verified this on my metal boat.
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Not always true. It depends between what 2 points the short is. I think you and Both a Coastguard engineering officer and a qualified electrical engineer are assuming the short in question is between you positive line and the hull of your boat. IMHO if the wiring installation is done correctly then a short in the DC circuit to the hull would not be possible and the most likely short in the DC circuit is either inside a piece of equipment or in the wiring inside the trunking, in which case the circuit breaker/fuse will trip.


As stated before by prv and myself lets have the engineering basic principals behind the convention.

I have always been one to question convention and to look at things from basic principals and have found convention not always based on known engineering facts but sometimes on assumed facts.

Roger Shaw BSc machanical and electrical engineering MIMechE C Eng (retired).
 
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Rogershaw said:
As stated before by prv and myself lets have the engineering basic principals behind the convention.
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Please note that I asked for (and am still waiting for) the reason to connect DC negative to AC earth. The thread has partially drifted towards connecting various other things, particularly the hull of a metal boat. That I can understand, because a short from any wiring to the hull is much more likely. I am specifically asking about connecting AC earth to DC negative in a GRP boat, and I don't think anyone has yet given a technical reason for it.

Pete
 
The question of the reason to connect AC earth to DC negative also applies IMHO to metal boats. I my mind that is also a question as why should the AC earth be connected to ALL the metal fittings on A GRP boat as moat of them are insulated from each other by the GRP structure.

I again request "lets have the engineering basic principals behind the conventions".
 
prv said:
Do we know what the reasoning is? The only one that springs to my mind is the risk of a loose AC conductor touching an un-insulated DC negative and thus making the whole DC system live at 240v, including things you may touch. In this case, earthing the DC negative would allow either a large enough fault current to trip a breaker (not sure I'd want to rely on that - it's a long and winding path from a random DC light fitting back to the marina's earthing point ashore) or leakage which would trip an RCD (more plausible). But unlike a car, the boat structure is not connected to DC negative, so how likely is it that a stray mains wire will connect with DC minus anyway? And presumably it's equally likely to touch a DC positive wire instead, with a similar result. But we don't see calls for the positive side of the DC system to be protected from this.

What have I missed? I'd like to understand the engineering rather than the paperwork.

(Also, what is an "isolated DC system" in this context?)

Cheers,

Pete
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By isolated DC system I meant a fully insulated two-wire d.c. system in which the d.c. negative is isolated from the ground (earth), i. e. not connected to the water through a metallic hull or the propulsion system, nor earthed through the a.c. protective conductor ( see ISO 10133)
 
VicS said:
By isolated DC system I meant a fully insulated two-wire d.c. system in which the d.c. negative is isolated from the ground (earth), i. e. not connected to the water through a metallic hull or the propulsion system, nor earthed through the a.c. protective conductor ( see ISO 10133)
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Ah - like mine, then. Cheers.

So I don't need to connect it to the shorepower earth, then?

Pete
 
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