No connection between AC grounding wire and DC earth.

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Ghoster

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I was planning on fitting a galvanic isolator to the AC, but on checking the existing circuit I do not believe the grounding wire is connected to the DC ground. According to Nigel Calder there should be a connection, but apparently not in my case. Does anyone have any thoughts on this?
 
The ISO requirement is that the AC protective earth is grounded (that is connected to the water) onboard the craft. If the DC negative is connected to ground (the water), for instance by way of engine block, gearbox, shaft, propeller, then the AC PE can use the same path to ground (hence the advice "connect AC PE to DC negative"). If the DC system is fully insulated (no connection to ground/the water, perhaps because of an unbridged flexible coupling) then the AC earth should be connected to an external grounding plate (or the hull, if metal).
In both cases a galvanic isolator might be a good idea, as the connection of AC protective earth to ground/the water is a source of corrosion, this because there may be a potential difference between the landline PE and the water ground.
This said, if the AC use onboard is light, for instance only for battery charging etc, then many people do not bother with grounding the AC PE onboard, thereby avoiding the risk of corrosion, which may be sensible.
 
This is a point of contention on the forum.

I have a steel hul boat and I do not have a connection between the AC earth and the DC negative.

I do have a connection between the AC art and the steel hull through a galvanic isolator to prevent and galvanic current flow.

On a GRP boat my view is the as most metal partes are naturally isolated from each other ans as the DC system has both a DC positive wire and a return negative wire the DC system is an isolated system so no DC negativ and AC earth is not required.

Others may disagree. It does depends on your exact setup i.e engine/gearbox isolated from the propshaft by a flexible coupling
 
Ghoster said:
I was planning on fitting a galvanic isolator to the AC, but on checking the existing circuit I do not believe the grounding wire is connected to the DC ground. According to Nigel Calder there should be a connection, but apparently not in my case. Does anyone have any thoughts on this?
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As Baba Yaga says the current ISO standard ( 13297) requires the AC earth to be bonded to the DC negative , except when the DC system is fully isolated. An isolated DC system is one in which the engine block etc is not used as the common DC negative return.

However older editions of ISO 13297 allowed the earth not to be bonded to the DC negative provided the whole AC installation was protected by an RCD.

If your AC earth is not bonded to the DC system and anodes etc you do not need a galvanic isolator. If however you are planning to rewire / upgrade your shore power system a GI is highly recommended, or even essential, if the boat remains plugged into the shorepower supply for extended periods . If you only plug in to shorepower occasionally for short periods you probably do not need a GI although it may still be a "good idea"

Check on the engine electrical system before you start grounding things if you do decide on that route in case it is an isolated DC system
 
Im not sure Nigel Calder says that in his maintenace book. I have the same in that AC earth and DC earths are not connected. Ive even gone as far as isolating my prop and shaft from engine (no bonding strap across R&D coupling).
I have his book and the way it read to me was you have both options of which either is not wrong.
 
Cornishwesterly said:
Im not sure Nigel Calder says that in his maintenace book. I have the same in that AC earth and DC earths are not connected. Ive even gone as far as isolating my prop and shaft from engine (no bonding strap across R&D coupling).
I have his book and the way it read to me was you have both options of which either is not wrong.
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IF you do that and require a hull anode to protect the stern gear you have to provide an alternative electrical connection between anode and shaft ( eg an "Electro eliminator") or rely solely on shaft anodes and/or prop anodes.

(The R&D coupling actually uses an internal earthing conductor )
 
VicS said:
IF you do that and require a hull anode to protect the stern gear you have to provide an alternative electrical connection between anode and shaft ( eg an "Electro eliminator") or rely solely on shaft anodes and/or prop anodes.
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Stern gear is connected to pear mounted hull anode (y), no anodes on prop or shaft and for last 20 odd years, no corrosion
 
Cornishwesterly said:
Stern gear is connected to pear mounted hull anode (y), no anodes on prop or shaft and for last 20 odd years, no corrosion
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How is the electrical circuit made between the anode and the stern gear .

Normally anodes are bonded to enegine/ gearbox then the flexible coupling bridged to complete the circuit.
 
VicS said:
IF you do that and require a hull anode to protect the stern gear you have to provide an alternative electrical connection between anode and shaft ( eg an "Electro eliminator") or rely solely on shaft anodes and/or prop anodes.

(The R&D coupling actually uses an internal earthing conductor )
Click to expand...
Really? I ask because I carried out a continuity test between shaft (outboard of R&D) and gearbox and it was open circuit.
 
VicS said:
How is the electrical circuit made between the anode and the stern gear .

Normally anodes are bonded to enegine/ gearbox then the flexible coupling bridged to complete the circuit.
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Rudder post electrically bonded to anode. The anode lasts years.
The engine and prop has no anodes. Im also looking at a recent photo of my R&D (split from gearbox for replacement) and I cant see this internal earthing conductor?
 
Cornishwesterly said:
Really? I ask because I carried out a continuity test between shaft (outboard of R&D) and gearbox and it was open circuit.
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If there is no continuity then an internal earthing connector is not fitted ... it is an "extra", not part of the basic R&D installation
 
Cornishwesterly said:
Rudder post electrically bonded to anode. The anode lasts years.
The engine and prop has no anodes.
Click to expand...

If you have no corrosion problems with the prop there is no need for it to have any cathodic protection. ........ A good quality manganese bronze prop perhaps
 
Ghoster said:
I was planning on fitting a galvanic isolator to the AC, but on checking the existing circuit I do not believe the grounding wire is connected to the DC ground. According to Nigel Calder there should be a connection, but apparently not in my case. Does anyone have any thoughts on this?
Click to expand...

If your AC Earth was connected to the boats anode circuit, the shore power Earth would also be connected to all of the surrounding boats AC Earth circuit and the anodes and underwater gear of any boat that had the AC Earth/anode connection. Your anode could be rapidly depleted, along with other metal gear, if there were problems on other boats. The GI, on your boat, prevents this from happening. If there is no electrical connection between the AC Earth and anode bonding on your boat, there is no point in fitting a galvanic isolator.

The AC Earth/DC negative connection is a red Herring. The connection should be made between the AC Earth and your anode/underwater gear. Very often, the DC negative is also connected to the same circuit, hence it's often stated that the connection should be AC Earth/DC neg.

This connection wasn't normally made on older boats, but it's a requirement now on new boats. If yours isn't wired that way, you are under no obligation to change it and there is no need to fit a GI.
 
PaulRainbow said:
The AC Earth/DC negative connection is a red Herring. The connection should be made between the AC Earth and your anode/underwater gear. Very often, the DC negative is also connected to the same circuit, hence it's often stated that the connection should be AC Earth/DC neg.

This connection wasn't normally made on older boats, but it's a requirement now on new boats. If yours isn't wired that way, you are under no obligation to change it and there is no need to fit a GI.
Click to expand...

Very true, but it was made a requirement for newer boats as it's an important safety feature. It certainly wouldn't hurt to retro-fit it.
 
As I am in the process of partial re-wiring and installing a Galvanic Isolator, can you please direct me to the benefits of AC Earth/DC coupling? why is it safer?

In case of badly engineered AC Shore installation, is that an extra point of safety?

Also, if there is indeed a leak into the AC Ground, will that not fry everything on board? (yet, keep us safe, no doubt).

I have read BS EN ISO 13297-2014 AC Installations and BS EN ISO 10133-2012 DC installations.
 
I've never understood the need to connect AC earth to the sea. The AC circuit already has a dedicated earth wire like any other AC circuit and is also protected by an RCD. The boat is only connected to mains AC when alongside and as such it's connected to AC earth. If I put a battery charger on my car I don't feel the need to drive an earth pole into the adjacent flower bed and connect the AC earth to it. Same applies when on a campsite with a mains power supply. I see no difference just because the boat is floating in water. Perhaps someone who knows could explain.
 
antaris said:
can you please direct me to the benefits of AC Earth/DC coupling? why is it safer?

In case of badly engineered AC Shore installation, is that an extra point of safety?

Also, if there is indeed a leak into the AC Ground, will that not fry everything on board? (yet, keep us safe, no doubt).
Click to expand...

In addition to what is mentioned in post #2, my interpretation (ISOs don't give motivations) is that an additional path to ground (the water) is safer because the shore power PE connection might be compromised somewhere between the boat/quayside and the substation.
If so, this additional path to ground will pass current so that the over current protection will trip, should there be a short line to ground, for instance the metal case of some equipment.
 
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