Galvanic isolator

[smileypete]

Just to change tack slightly - how could you test to see if a boat's fitted with a GI? Is there simple test using a multimeter on the earth wires from the landline to the earth pin in a socket?

 

A good way to check any GI is not open circuit, is with a 9V alkaline PP3 battery and a smallish 12V bulb, like a side light bulb.

 

Just disconnect the shoreline from the boat, then put battery and bulb in series between the boat's shoreline inlet earth pin (the large one) and some bare metal on the hull. Repeat the test with the batt the other way round to check the GI in both directions.

 

If the lamp lights in both cases, there's continuity between the supply inlet earth pin and the hull, which is good.

 

To check the diodes in the GI haven't shorted, or that a GI is indeed fitted, or help tell if it's correctly wired, repeat the above test with a multimeter, set to 2 volt DC range, connected between the inlet pin and hull. If it reads 1.2V or 2.4V then it shows a GI is fitted and the diodes are OK.

 

 

 

My take on this matter is GI's with cheap components

 

Ages of all tested between 5 and 10 years say

 

Specification of an GI can be found in a document compiled by the ABYC section A-28

 

Would be good to have proper details of which brands they are, the type of boat, and the method of testing. I don't doubt that very cheaply manufactured generic ones have been or may still be available.

 

Chunky diodes are fairly cheap these days, so I'd be very surprised to see the failure of a reasonably decent GI, and any failure would be more likely to be a short circuit.

 

I still think that for the average boater on here with 16A supply, a reasonably decent GI would be perfectly fine. I'd be very interested in any clear information that suggests otherwise.

 

Well there's also space required to take into account, and to a small extent, weight, noise, efficiency, fire risk, and turn-on surge. A GI that works properly and is robust seems a more elegant solution to me.

 

There's also the ABYC approved GIs available, if one is really paranoid, Sterling do one...

 

http://sterling-power.com/products/abyc-zinc-savers-galvanic-isolators

 

cheers, Pete.

~smpt~

Edited June 18, 2015 by smileypete

[GUMPY]

There's also the ABYC approved GIs available, if one is really paranoid, Sterling do one...

 

http://sterling-power.com/products/abyc-zinc-savers-galvanic-isolators

 

cheers, Pete.

~smpt~

£250, you can get a decent isotransformer for that

http://www.airlinktransformers.com/boating_transformers/BT3231/

Edited June 18, 2015 by Loddon

[smileypete]

£250, you can get a decent isotransformer for that

http://www.airlinktransformers.com/boating_transformers/BT3231/

 

Yeah but if you're that concerned about the hypothetical risks, you should keep the iso transformer on the shore side, which isn't always that practical... but if people really want to buy an iso transormer, that's certainly fine by me.

 

cheers, Pete.

~smpt~

Edited June 18, 2015 by smileypete

[Keith M]

In a few days time I am looking at an installation where when the shoreline is connected the GI starts to conduct. This is indicated by a volt meter on the GI.

 

At the moment the only details I have, this has come to light in the last few weeks

 

The mooring is in a busy marina.

 

And most boats are plugged in

 

My take is that there is a small voltage between the earth pin on the shore-cable and what I am calling true earth

 

Keith

[nicknorman]

In a few days time I am looking at an installation where when the shoreline is connected the GI starts to conduct. This is indicated by a volt meter on the GI.

 

At the moment the only details I have, this has come to light in the last few weeks

 

The mooring is in a busy marina.

 

And most boats are plugged in

 

My take is that there is a small voltage between the earth pin on the shore-cable and what I am calling true earth

 

Keith

Yes sounds like that must be the case. Since most other plugged-in marina dwellers will have nothing, or a GI without any indicators, I think the only solution is to get the marina to sort out their supply. Being practical about it you are not going to persuade all the boat owners to fit ITs. Are there any (land-based) regulations concerning the max allowable voltage on the PE conductor compared to "local earth"? I've a feeling there are.

[Keith M]

I am not sure how the marina can sort out this situation

 

Needs a bit of a think

 

I am sure there are regs to cover, I need to get the regs books out

 

Keith

[nicknorman]

I am not sure how the marina can sort out this situation

 

Needs a bit of a think

 

I am sure there are regs to cover, I need to get the regs books out

 

Keith

The marina will probably have a 3 phase supply, so perhaps they just need to rebalance how much load is on each phase?

[by'eck]

I am not sure how the marina can sort out this situation

 

Needs a bit of a think

 

I am sure there are regs to cover, I need to get the regs books out

 

Keith

 

There is a requirement for a local earth to be provided when running a household supply (armoured cable) to a remote garage or workshop typically in the garden. The armoured cable shielding, which carries the PE from home end, must be broken at the remote end where it connects to local consumer unit. PE via a local copper spike then allows the RCD in this to work effectively.

 

Not sure how this would translate to a typical marina installation, but the cable runs are likely to be well in excess of those in a typical home plot. I somehow doubt there are any local earths provided though.

[nicknorman]

There is a requirement for a local earth to be provided when running a household supply (armoured cable) to a remote garage or workshop typically in the garden. The armoured cable shielding, which carries the PE from home end, must be broken at the remote end where it connects to local consumer unit. PE via a local copper spike then allows the RCD in this to work effectively.

 

Not sure how this would translate to a typical marina installation, but the cable runs are likely to be well in excess of those in a typical home plot. I somehow doubt there are any local earths provided though.

Don't think so (your first para). This type of earthing is one way of doing it, but you can also use the supply earth in a PME system. This latter is prohibited for caravan sites but marinas don't get a mention. Edited June 18, 2015 by nicknorman

[jonathanA]

There is a requirement for a local earth to be provided when running a household supply (armoured cable) to a remote garage or workshop typically in the garden. The armoured cable shielding, which carries the PE from home end, must be broken at the remote end where it connects to local consumer unit. PE via a local copper spike then allows the RCD in this to work effectively.

 

Not sure how this would translate to a typical marina installation, but the cable runs are likely to be well in excess of those in a typical home plot. I somehow doubt there are any local earths provided though.

I'm pretty sure nick is right, I think you are specifically prevented from providing a local earth in the latest regs for outhouses/garage. You are right that the armour should not be used to provide the earth

[jonathanA]

In a few days time I am looking at an installation where when the shoreline is connected the GI starts to conduct. This is indicated by a volt meter on the GI.

 

At the moment the only details I have, this has come to light in the last few weeks

 

The mooring is in a busy marina.

 

And most boats are plugged in

 

My take is that there is a small voltage between the earth pin on the shore-cable and what I am calling true earth

 

Keith

Err shouldn't you be looking at the boat electrics to start with...... Edited June 18, 2015 by jonathanA

[Keith M]

This has been a know fact for more than 5 years that as more switch mode power supplies are used

 

Some GI will start to conduct because of a small voltage which is on the earth wire at the power bollard

 

This voltage is because of switch mode power supplies.

 

Good information on the safe-sure site

 

Keith

[nicknorman]

This has been a know fact for more than 5 years that as more switch mode power supplies are used

 

Some GI will start to conduct because of a small voltage which is on the earth wire at the power bollard

 

This voltage is because of switch mode power supplies.

 

Good information on the safe-sure site

 

Keith

True in theory, but the voltage across the GI will be AC and so not show up on a DC voltmeter. Some GIs have capacitors now I believe, to pass this low level AC current. Of course if you turn off the equipment with the SMPS the effect should disappear. In practice I think that in a marginal case, where the DC voltage across the GI is approaching but not quite reaching the diode turn-on voltage, the presence of this SMPS-generated small AC current can tip the GI into conduction on one half-cycle, but the regs only allow for a tiny current to be sent down the earth wire so I think you would have to have an awful lot of such devices before there was a problem.

Err shouldn't you be looking at the boat electrics to start with......

You're right of course, but on the other hand it might be easier to unplug the shore lead and then measure the voltage between the shore earth pin and the hull, that way you will know whether it is a boat fault or a shore fault. That would be easier than spending hours checking out the boat only to find it was a shore fault!

[tree]

hmm.. now this is confusing ..especially as my degree is in the social field not electrics.

 

I am on a river mooring and used to be moored away from my lovely fellow moorers. My choice as I do like to dance on my decking and sing !!

 

Recently two other boats have moved nearer to me and frequently use their landlines, which is not an issue, so thinking that I was being prudent, I bought a GI from a well known and published company. The model I bought is below, but after reading this thread I am now unsure which was the best option.. a GI or the T thingybob

 

GI with 2 Status Monitors,

Fault Indicator

1 LED Light = DC Leakage

2 LED Light = AC Leakage.

[George94]

Well there's also space required to take into account, and to a small extent, weight, noise, efficiency, fire risk, and turn-on surge. A GI that works properly and is robust seems a more elegant solution to me.

 

Indeed. So long as it is also easy to test. And has an owner who knows how to do it.

 

We are not really at odds here. You are talking about a well-made device in the hands of a well-informed owner, whilst I am referring to a different thing - a cheap device in the hands of (dare I say it?) the average owner.

[cuthound]

When I had the house power extended to the garden, to power the boat, the electrician insisted on fitting a copper earth spike adjacent to the electricity post. He said the latest issue if BS 7671 made it mandatory for external supplies not in outbuildings. Hopefully this means there is little or no difference in potential between my maons supply earth and the boat earth.

 

Since retiring I no longer have access to the latest issue of BS7671, although I am aware that the 3rd amendment is imminent. Does anyone know what is to changed?

 

I have recently cut an inspection hole in my panelling , so that I can test my 7 and a half year old Victron GI easily (much easier than having an additional elbow fitted to my arm).

[GUMPY]

Boat and caravan metalwork is not permitted to be connected to the PE conductor on a pme system.

 

On the phone at present so can't post iee article

[by'eck]

Boat and caravan metalwork is not permitted to be connected to the PE conductor on a pme system.

 

On the phone at present so can't post iee article

 

Makes sense as long as there is a local PE. Equipotential zone generally limited to outer walls of home in the case of feeds to oubuildings. Similar situation with caravan/boat I would imagine.

[Psycloud]

This has all got me thinking...

 

At Willowbridge there are loads of green cabinets with card meters in them - would all the cabinets be earthed together or would they have individual earth spikes? Reason I ask is that my boat is on a meter in a cabinet on it's own, and even the cabinet nearest most the boats are on hard standing - so I'm wondering if there would be any common earth that would cause the galvanic corrosion?

[GUMPY]

Last time I was at WB, admitedly 15 years ago the L&N on the bollard I had were reversed!

Somewhere like WB I would not use anything but an IT as it removes all possible yard wiring problems.

 

As for earths it depends on the system installed. Local earths come into play to convert a tnc-s (pme) system to a tt system. A layman couldn't tell what system it is unless its labeled.

Edited June 19, 2015 by Loddon

[ditchcrawler]

 

There is a requirement for a local earth to be provided when running a household supply (armoured cable) to a remote garage or workshop typically in the garden. The armoured cable shielding, which carries the PE from home end, must be broken at the remote end where it connects to local consumer unit. PE via a local copper spike then allows the RCD in this to work effectively.

 

Not sure how this would translate to a typical marina installation, but the cable runs are likely to be well in excess of those in a typical home plot. I somehow doubt there are any local earths provided though.

I am out of touch with latest regs by several years, but reading the above then what is the difference between the earth being supplied by a 6 ft stake in the ground or 60 ft of steel in water?

[GUMPY]

I am out of touch with latest regs by several years, but reading the above then what is the difference between the earth being supplied by a 6 ft stake in the ground or 60 ft of steel in water?

Conductivity

[ditchcrawler]

Conductivity

It would be interesting to measure it. I haven't done that since I was at collage in 1965

[nicknorman]

It would be interesting to measure it. I haven't done that since I was at collage in 1965

It will vary tremendously according to the purity of the water - pure water being a pretty good insulator.

[GUMPY]

I am out of touch with latest regs by several years, but reading the above then what is the difference between the earth being supplied by a 6 ft stake in the ground or 60 ft of steel in water?

The proper way to lay an earth is not a stake but an Earth Mat, they don't come cheap