Earthing

[Graham_Wright]

Following on from the query about earthing shore supplies on board; if the regs require shore side earth to be connected to the engine and you want to keep the electrical nasties out of the water, how do you electrically insulate a shaft carrying 40 h.p. or so?

There is one semi-flex coupling that appears to be plastic but otherwise……………?

Graham

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[Birdseye]

Interesting. Are there UK regulations that specifically require earthing of the engine block?

I'm no expert and I certainly am not suggesting that you do what I did - you should take professional advice. But after talking to the man at Sterling and establishing that there was no connection between the negative output of the battery charger and earth, I left the 12v and mains circuits completely un-connected. So all the skin fittings go to battery negative and none are connected to shore side earth. No risk of being the anode for lots of other boats also on the mains.

Mains comes in via an RCD in the boat. So as far as I can see, my only risk is if there is an internal short in the battery charger which makes battery positive live. Very unlikely without tripping the RCD and if it did, then there is no point in the boat that I can find where battery positive isnt effectively insulated against mains.

I would be interested in the views of an electrical engineer on the real risks I am taking. I am no expert so dont copy me. folks.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[john_morris_uk]

Another risk would be if the 'polarity' of the mains coming on board is reversed. (As it sems to be in some French marinas for instance) Neutral should be at or near ground potential, and it depends on the circuit and protective measures in the Sterling Charger (which incidentally we also use) as to the likelyhood of your whole boat wiring floating to mains potential.

Like many people we have a polarity indicator between neutral and ground, and a very short lead to swap the live-neutral over if it is incorrect.

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[Birdseye]

Every time the polarity has ben reversed in France, the RCD has tripped and refused to go back in.

But even then, there would need to be a break between battery charger mains and output. If the neg were to be live, it would be earthed through the seacocks etc.And there is no where I can find where batt pos is not insulated.

As I've already said, I am not recommending anyone else to do this since it is against all professional advice, but I am curuious as to what scenario it could cause harm.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[Graham_Wright]

There was a previous post on the earthing topic referring originally to the requirement for a consumer unit and ring main. One of the responses referred to the regulation requiring the engine block to be earthed. I have a problem understanding the need for an earth on a boat as the RCB protects against mains leakage through the body to earth. Uless you are hanging on to something metallic which is thus earthed (such as the engine I agree) you are unlikely to suffer a shock on a dry grp boat anyway. The RCB however, would perform a valuable task in tripping in the event of a short from mains (either live or neutral) to an earthed appliance case.

The real way out seems to be an isolation transformer which, for 230 to 230 is probably expensive. However, if the shore supply is only doing battery/12v supplies duty, there is an isolation transformer anyway. But then that begs the question of whether you go for a 12v only fridge.

Now I think I've answered my own question as, (apart from the conversion losses), watts is watts no matter at what voltage. So the current draw on the shore supply would not be vastly greater. When it somes to fan heaters and the like, that is a different matter but that's why we've chosen the Med as a cruising ground.

The great benefit of this forum is that it helps the reasoning process and weans you away from the tunnel vision.

Thanks for the response.
Graham

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[Graham_Wright]

Does that mean that the Sterling charger does not use an isolation tranformer?

The thought of a "chopper" convertor or the like horrifes me!

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[Bejasus]

I tried to ask something similar on the last post, but it disappeared without a response, so perhaps I could ask it again.
I have a 40 yr old mobo cruiser on the Broads. There has never been any anodes fitted, nor any signs of corrosion on this boat. It had a petrol engine with a twin battery set up via a 1,2,both,off switch. It also had 2 socckets in the saloon, wired to a 3 pin plug in the cockpit. These were not earthed to anything on the boat. Needless to say, I never used these.
Now I have re-engined/rewired the boat and added on a seperate engine start battery, pos & neg bus bars etc.
I fitted a 16 amp socket and wired it to the 2 x 3 pin sockets on board via an RCD. There is no connection between the mains electricity set up and the 12v setup, whatsoever.
Questions: Have I done something wrong? Should I have some cathodic protection?

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[TrueBlue]

battery charger

Like most modern equipment Sterling battery charges use a Switch Mode power supply to convert 230v AC to 12v DC. An equivalent transformer charger would be much heavier and bulkier.

You probably can't do multi stage charging without modern electronics.

Switch mode kit has been around for years and years and if properly designed (as Charles Sterling says) should not cause problems as his are designed to be fail-safe.

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Stop what you're doing and wait my signal

 

[Graham_Wright]

Re: battery charger

Thanks for that. I thought even switched mode voltage converters had transformers albeit working at high frequency and therefore small.

I will definitely not be going in that direction. The thought of a low voltage connection to either pole of a mains supply is terrifying. I am quite happy to use a transformer even with its weight penalty. Low down it should compensate for my lack of ballast!

Insofar as adjustment of current is concerned, I have an all nicad installation which is much more tolerant of coarse control.



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[john_morris_uk]

Our RCD doesn't normally trip on reverse polarity. After all its designed to trip if there is a difference of more than the specified number of milliamps in the current flowing in the live and neutral (Obviously in a perfect world then these currents are identical)

Perhaps this indicates that there is a leak somewhere in your system.

I was suggesting that in the event of reverse polarity there is another "possible" source of a problem - depending on how well isolated the switch mode charger is.

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[john_morris_uk]

Re: battery charger

You are correct when you say that switch mode supplies still use a transformer. Its just that the size can be much much smaller (and more efficient) when you use higher frequencies. The double benefit is that you can also control the outputs and voltages electronically very easily. The design of the Sterling meets all the regulations and will be designed to 'fail safe' so I have no qualms about using one.

The physics of using a conventional transformer charger means that you need something very very big and heavy to approach the performance of a switch mode design. Adding intelligent four stage charging is also much more difficult.

The point that is relevant to the original question is that yacht wiring shouldn't really be allowed to 'float', because in the unlikely event of failure, it is just possible that lethal voltages "could" be within reach without warning.

This has nothing to do with the added desirability of a galvanic isolator (except to explain why the certified ones are so darned expensive!) I also have no interest in Sterling Products. (and I wish that they'd rewrite their instruction leaflets!)

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[JohnL]

Hi check out the PBO electrical experts web site Merlin Equipment ,sorry dont know full address, galvanic isolators is the page to look at, the red bits may answer your question I did not get a reply to aquestion about earthing either

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[Geordie]

Re: battery charger

Ditto about the instruction leaflets. They are not very professional. They might be accurate for the first design, but Sterling upgrades his designs but not his leaflets. Judging by the way they are written, he does them himself, and what the table of LED's on the front of his chargers mean he only knows.

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[halcyon]

Re: battery charger

Can I make a few comments on normal transformer chargers.

They do not have to be very very big and heavy, the 25 amp charger we used to make was only around 10% larger than the Stirling and was only about 4 / 5 kg heavier than his 25 amp unit. The units we made going back to 1984 used electronic control, controlling voltage to 0.01 volt and current down to 20 ma. The last units were 5 stage, and were getting near the same efficency as a switched mode. Afraid a lot of it is myths but out by switch mode people years ago to justify people buying them.

On the other hand I dnot know to many well designed transformer chargers around, come to that switch mode as well, at the moment. That is not that they do not meet regs atc, but doeing what people actually need in use today, not what they were doing 20 years, and are still having to due to current marine equipment.

Brian

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[Graham_Wright]

Re: battery charger

Give me a conventional charger any time. Every switched mode charger or power source I have ever had has failed and been impossible to economically repair. A transformer/rectifier power source is inherently very simple and faults easy to diagnose.
With Nicads, if you just thump the charge in and monitor the temperature rise you really can't go wrong.

On a spin-off thought;- why go to the trouble of splitting alternator output with all it's attendant volts drop problems? Most engines have room for a second alternator and the redundancy benefits are obvious.

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[halcyon]

Re: battery charger

The advantage of relays for charge splitting, no volt drop, in lot of cases no need for alternator booster's to get round diode drop, thus a lot simpler.

Brian

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[Graham_Wright]

Re: battery charger

Yeah but they're cheap! I guess that's why they're not pushed by the trade!

They also allow disconnecting the charge to avoid startup spikes and low volts to sensitive kit during charging. But you have to remember that alternators shouldn't be left off load or they go pop.

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[Birdseye]

I couldnt understand why the RCD tripped on reverse polarity either, and my first though was the same as yours. But trip it did, even when nothing on the boat was connected to it. And on every occasion (of which there were a few!) that this happened in France. Reversing the polarity by the usual means and there was no tripping problem.

I agree with the comment about isolation relays. Have this system on my Prout and it is very effective at ensuring domestic use does not drain the starter battery. Relays are froma Mini.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[halcyon]

Re: battery charger

As the alternator is always connected to the engine battery with a relay, you never get the problem with the alternator off load.

Brian

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[halcyon]

The RCD trips if the reverse polarity light is after the RCD, power is feed from the neutral to earth line via the neon, therefore there is a inbalance between live and neutral.
If the reverse polarity neon is before the RCD no current flows through the RCD thus no trip.
Fitting reverse polarity indicator neon after RCD will give trip when live and neutral are reversed, and will not light the neon.

Brian


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