Galvanic isolator, should I have bothered?

[Sans Bateau]

I just recently replaced the RCD and the MCB's after a little scare. I replaced some wiring and checked the rest as a precaution.

Whilst on the shorepower campaign, I decided to install a galvanic isolator bonding the DC neg to shore earth. I now read in the current YM, Nigel Calder, says that this is not needed with European boats with RCD's and that the two (DC and AC earth) should be kept separate.

Any views of anyone with experience in this field would be greatly welcome.

 

[Norman_E]

I have only connected the mains earth though the Galvanic Isolator. I think that the DC negative should not be connected to the galvanic isolator, and may make matters worse if it is. The isolator works by blocking very low voltages (exact voltage depends on whether the isolator uses single diodes, or pairs) and allowing higher voltages to pass to earth. By doing so it prevents the earth wire acting as the return leg of a galvanic circuit, whilst still allowing mains voltage to pass safely to earth in the event of an equipment failure. Boat 12 volt circuitry is not connected to the earth except to the extent that your negative side might be earthed to the engine and thence to the propeller shaft, in which case there would be no point that I can see in connecting the same negative feed to the galvanic isolator, and therefore potentially to earth. If 12 volts was to flow through the galvanic isolator for any reason it would close the circuit to earth for the mains, and defeat the object.
One further point. If your 12 volt negative and mains earth are connected to the same terminal you could end up with your 12 volt wiring becoming live at 230 volts if a piece of mains equipment shorted to earth, though hopefully your RCD would limit the duration to a fraction of a second, you might still lose electronic equipment.

 

[VicS]

I am not a reader of YM but if I understand you correctly what Calder says in there is a major change in practice. Hitherto the current practice ((and recommendation) is for the DC negative to be connected to the shorepower earth, on the ships side of the GI. Typically that would be at the battery negative connection to the engine.

One thing for sure, whether or not the DC negative is earthed, if you have a GI then the incoming earth connection goes though that before anything else is connected to it.

A GI protects underwater bits and pieces in particular the anodes from stray currents originating from low voltage sources that can occur when a shore power supply is left connected while as Norman_E says allowing the normal protection that you get from an earth.

 

[Sans Bateau]

I think what he is saying is, if you have an RCD, then it will be rated at 30ma, any current leaking to earth would have to be less than that. Any more and the source of the current would be cut as the RCD would trip. Therefore with such low currents potentialy leaking, there is no need for an RCD. If no RCD then the GI comes into its own.

 

[philip_stevens]

Quoting from Adverc

[ QUOTE ]
INSTALLATION

1. A Galvanic Isolator must only be used in conjunction with an RCD. The isolator is installed in the earth safety circuit and should it fail, there is no protection when connected to shore power.
2. Mount the Galvanic Isolator adjacent to the AC distribution panel.
3. Connect the incoming shore mains earth cable to either terminal on the Galvanic Isolator. Ensure that this connection is the only one made in the shore mains earth line and comes directly form the bulkhead input plug.
4. Connect the other terminal on the Galvanic Isolator to the earth buss of the AC distribution panel and from there to the common DC negative / earth stud on the hull.

[/ QUOTE ]

 

[BlueChip]

I'm no electrician but I do know that since I fitted one about 5 years ago at the end of each season my anodes are hardly eroded, whereas before I fitted the isolator I was replacing well worn anodes every year.

 

[rogerthebodger]

Being as I am in the process of building a steel boat a Galvanic isolator is very important and I have looked into the mains electrical from basic principals.

My view is

1) If the underwater metal work and anodes of your boat are connected to the mains earth you must have a Galvanic isolator else the anodes will waste very quickly.

2) IMHO on a plastic boat the mains earth need not be connected to the underwater metal work but MUST be connected to the earth of any metal cased appliances you have on board. If there is no connection between the mains earth and the underwater fittings / anodes a Galvanic isolator may be not needed. Metal boats are very different. The plastic boat is like a double insulated appliance in its self. you cannot get a shock from the hull and most metal fittings are insulated from each other by the hull/deck its self.

3) Now on a metal boat the earth MUST be connected to the hull as the hull/deck could become live on a plastic boat this is normally not possible.

4) the normal design of a Galvanic isolator required the current carrying capacity to be greater than the fuse/MCB capacity fitted. Now if a RCD is fitted the max current that can flow in the RCD before it trips will be 30mA so why have diodes with a capacity od 70/100/120 amps?

5) One very important point with any mains installation that includes a RCD is to check the phase of the live/neutral is connected correctly and that the neutral and earth are connected together at the supply side of the RCD else the RCD WILL NOT PROTECT YOU.

My boat will have shore, generator and inverter supply switchable and the earth/neutral will be connected together and to the hull at the inverter/generator but not for the shore power. The incoming earth on shore power will only be connected to the hull plus all appliance earth connections.

 

[rogerthebodger]

[ QUOTE ]
A Galvanic Isolator must only be used in conjunction with an RCD.

[/ QUOTE ]

I do not agree with this a Galvanic Isolator can be use with out a RCd but must be rated to carry the current rating of the main fuse / MCD


[ QUOTE ]
The isolator is installed in the earth safety circuit and should it fail,

[/ QUOTE ]

I do agree with this and to ensure the earth is intact a green in my case neon must be fitted between the live and earth line between the RCD and the GI.

I have also fitted a red neon between the neutral and the earth. If the green neon is on and the red is off all is OK. If red on amd green off reversed live / neutral. If both off earth fault exists.

 

[markdj]

The ABYC standards say that

If you don't have an RCD, ie. earth leakage trip then you will need a galvanic isolator and connect the DC and AC earths

If you do have an RCD then you can keep the AC and DC earths separate as any earth leakage will trip the supply.



Extract from ISO13297

4.2 The protective conductor (AC Earth) 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.

4.9 A galvanic isolator or other suitable device may be fitted in the protective conductor to resist imported stray galvanic current flow while permitting the passage of a.c. current, if present. Galvanic isolators shall be designed to
withstand the application of power from a short-circuit test from a source capable of delivering 5 000 A r.m.s. symmetrically to its output test terminals for the time required for the circuit-breaker in the test circuit to trip. After three applications of the short-circuit test, the electrical and mechanical characteristics of the isolator shall be unchanged.




Fully insulated systems are :

3.16
fully insulated two-wire d.c. system 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.

 

[rogerthebodger]

[ QUOTE ]
If you don't have an RCD, ie. earth leakage trip then you will need a galvanic isolator and connect the DC and AC earths

[/ QUOTE ]

I would like to know why as this makes no sense to me.



[ QUOTE ]
fully insulated two-wire d.c. system 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.

[/ QUOTE ]

On a metal hull boat I would never never connect the negative of the DC supply to the hull as this could induce galvanic corrosion in the hull.

 

[lw395]

5) One very important point with any mains installation that includes a RCD is to check the phase of the live/neutral is connected correctly and that the neutral and earth are connected together at the supply side of the RCD else the RCD WILL NOT PROTECT YOU.
This doesn't sound right. N is not an earth.
If you connect the N and E together, then the RCD on the pontoon may/will trip.
An RCD only needs two wires, the whole point being the residual current ie the difference between live and neutral current., which should be zero if there are no unintended branches to the circuit.
Please explain if I'm wrong!
Point two is dead right imho, dunno what the regs say though.

 

[VicS]

[ QUOTE ]
On a metal hull boat I would never never connect the negative of the DC supply to the hull as this could induce galvanic corrosion in the hull.

[/ QUOTE ] I am not quite sure what you have in mind there.

To be pedantic galvanic corrosion is what occurs when two dissimilar metals that are connected to each other are immersed in an electrolyte (eg sea water). Presumably you are therefore suggesting that some electrolytic action occurs driven by the boats DC system.

However that won't happen for two reasons.

Firstly there is nothing completing the circuit back from the seawater to the positive side of the supply and secondly if there were it would not cause corrosion of the steel hull as that would be the cathode and therefore protected by what in essence is the beginning of an impressed current cathodic protection system.

In such a system the positive side of the supply would be connected to electrodes (anodes) which are chemically inert such as platinum coated titanium rods. This diagram illustrates the principle:

 

[markdj]

[ QUOTE ]
If you don't have an RCD, ie. earth leakage trip then you will need a galvanic isolator and connect the DC and AC earths

[/ QUOTE ]
otherwise you could be inducing stray current in the water due to earth leakage.


[ QUOTE ]
On a metal hull boat I would never never connect the negative of the DC supply to the hull as this could induce galvanic corrosion in the hull.

[/ QUOTE ]
I think you will find that unless you have a fully insulated engine, that the engine itself will be the path of earth to the hull. This is not a problem as long as you have your anodes connected, it won't introduce galvanic corrosion.

IF you look at an RCD you will find that the Live and Neutral are connected through it. As the current is AC, any difference in the current of the 2 wires causes the RCD to trip as any difference would suggest current leaking, generally to earth.

 

[rogerthebodger]

[ QUOTE ]
If you connect the N and E together, then the RCD on the pontoon may/will trip.

[/ QUOTE ]

The point I made was that the neutral / earth connection MUST be the supply side of the RCD.

In the case of shore power this will/must be at the shore power pontoon distribution box and NOT on your boat. If you have a generator, inverter or isolating transformer then neutral /earth must be connected at the supply between the supply device and its RCD

 

[rogerthebodger]

As far as I understand Vic if DC ground is connected to the hull of a metal boat it could act as a reverse impressed current causing increased corrosion of the anodes

In a number of articles this has been raised and I have adopted this to be safe than sorry. If I am wrong I don't see a down side unless you have some comment Vic.

Your diagram implies that it could in fact prevent corrosion and not induce it

Your comment about no positive return is true except if there is any current leakage from the positive side of the DC supply and with so much sea water around that could in time be possible.

My engine has and negative connection to the block but the engine is isolated from the hull by rubber mountings and a rubber flexible prop shaft coupling

 

[rogerthebodger]

[ QUOTE ]
otherwise you could be inducing stray current in the water due to earth leakage

[/ QUOTE ]

Hence the importance for a RCD to be included in the installation.


In my case I do have a fully insulated engine.

Yes but for the leakage current to complete the circuit the neutral and earth must be connected on the supply side of the RCD either internally for internal power supplies (generator, inverter) or at the pontoon in the case of shore power.

 

[Sans Bateau]

From the Merlin Powerstore web site, the company who supplied my product:

ZINCGUARD prevents Galvanic Corrosion caused when you install a shorepower system on board your boat.

According to latest BMEA and CE regulations, the AC earth from shorepower MUST be bonded to your anode. Because the boat next door also has the same bonding system, the shorepower earth wire is essentially connecting all underwater metal parts on each boat together. Both vessels are sat in water that acts as an electrolyte causing an electrical cell (rather like a very low power battery). This will cause 'weaker' (or metals with a lower galvanic number) to transfer to 'stronger' metals - which means your sterngear could easily end up
electroplating next door's boat or the pontoon!

ZINCGUARD is fitted in the incoming AC shorepower line. It interupts any Galvanic Circuit but still maintains earth continuity for safety. EARTH BONDING IS ABSOLUTELY ESSENTIAL TO ENSURE CORRECT SAFETY. IT IS ILLEGAL AND DANGEROUS NOT TO BOND AC EARTH TO METAL PARTS AND DC NEGATIVE ON BOARD!

For Steel Hulled Vessels, we recommend an Isolation Transformer instead of a Galvanic Isolator.

So this does say the same as the instructions from the Advec website. Looks like I will keep my DC neg connected to the GI.

However what I have learned from this thread is the importance of ensuring the RCD is on the live, rather than return side of the AC shore power. I believe there are cases reported where the connection on the shore has been reversed, so one could unknowingly have their onboard AC reversed.

 

[rogerthebodger]

ZINCGUARD is in effect a Galvanic isolator.

[ QUOTE ]
EARTH BONDING IS ABSOLUTELY ESSENTIAL TO ENSURE CORRECT SAFETY. IT IS ILLEGAL AND DANGEROUS NOT TO BOND AC EARTH TO METAL PARTS AND DC NEGATIVE ON BOARD!

[/ QUOTE ]

Why ? On a plastic boat all the metal parts are insulated from each other and these underwater are very unlikely to come in contact with a person unless the metal case of your mains appliances are connected to the boat metalwork. On a metal boat this is quite likely so yes the mains earth must be connected which also connects to the anodes.

The most likely danger is a mains live to metal case fault making the case of the appliance live this must be connected to the earth / neutral with an RCD to trip as soon this fault happens thus making the system safe. If you connect the mains earth to all metal fittings on a plastic boat you are creating the possibility that all the metal fittings could become live and dangerous if the RCD is not fitted correctly of there is no earth /neutral connection which is very possible in unknown marina installations.

As I have said before this makes some live, neutral or earth connection check very important.

On my early boat I did not have short power but did have an inverter and generator where the earth and neutral were not connected at the generator but I did have a RCD in circuit. One of my appliances developed a live to metal case fault which made the case live but the RCD did not trip as there was not return path to the neutral of the generator. When I connected the neutral and earth together at the generator with the RCD between this connection and the appliance the RCD then worked when the case of the appliance became live.

 

[lw395]

I have seen it recommended to have a resistance between N and E at a source such as a generator or inverter. This would limit the current to something non-lethal but adequate to trip breakers.
I think there may be more than one approach to this issue, it may be best to follow one book or scheme rather than a mixture of two schemes by getting ideas from the forum and possibly taking sound practise out of context.
Also a lot depends on whether you wish to distribute mains around the boat or just have a battery charger.

 

[Billjratt]

There's a lot of chatter going on here, and some technical competence.
to try and simplify:

GI and RCD work together, but have different tasks.
RCD disconnects the system from the mains in the event of >30ma current flowing in the earth lead, due to a fault or leakage problem. This is the bit that saves lives.
GI does not let ground current flow until the potential difference between the boat and shorepower-ground is greater than 1.2 volts. (this small potential will not produce anything like 30mA, but, if allowed to flow, would cause electrolytic corrosion. Thus it protects the underwater metallic boat fittings which are connected via the common ground in the boat.
In the event of an electrical fault aboard, the current in the ground cable will reach a level higher than 30mA and will trip the RCD.

No GI = possible corrosion.

No RCD = Possible death.

You need both, or, certainly, the RCD! Happily in this country RCDs are fitted in the shorepower distribution system. That is why I cheated and fitted four low current diodes in my boat (they will only need to carry a >30mA spike for a short while.)
The rules are set so that the diodes will carry a fuse-blowing current in the event of an RCD not being fitted, so will be in the 100amp range (and expensive).
Finally, an electrical problem on an neighbour's boat may well set up the potential that you are trying to protect yourself from, rather than problems on your own boat...