Generator question

[gtmoore]

I installed shorepower into our boat last year carefully following the PBO manual and Nigel Calders book and used all the normal recommended bits such as RCD, MCB's etc. I realise there are differing opionions on bonding earth to the 12V negative and I have decided to do this via a galvanic isolator. My question is, if I were to buy a small generator such as one of the Honda 'suitcase' types, is it just a case of plugging the output from this into my shorepower socket or is there something else I need to consider?

Thanks

<hr width=100% size=1>Gavin

 

[MainlySteam]

Gavin, there was a similar question on MoBo a week or so ago. Many plug portable generators into their shore power connection, without any obvious history of problems that I am aware of, but I wouldn't.

Firstly, the fire risk if the generator is petrol fuelled (I think all the small ones are). I would also think an insurer would consider not responding to such a loss.

The other reason, which you will also be able to see in Calder's book is that all sources of ac supply should have their ground and neutral bonded at the point of supply. When you are on shore power, these are bonded on shore and if you have an invertor, they will be bonded at the invertor, however both are not to be bonded at the same time so if there is an invertor then switching between shore power and the invertor should break the bonding at the invertor when switching to shore power (some invertors eg Heart do this automatically).

I am told, and I have taken the trouble to check some myself, that most (if not all) small portable generators do not have their ground and neutral bonded at the generator. So you end up with a floating neutral supply which means a fault to ground will not necessarily trigger a protective device in the circuit. This is not necessarily dangerous but it does pose an additional risk in my opinion when one is talking about the generator feeding a distribution system in the boat rather than just an appliance.

Many users who plug them into the shore connection of their boat report that if they have an ac supply reverse polarity indicator on board that it shows a polarity fault whichever way around they switch the neutral and phase. Ordinarily this would indicate voltage between both the neutral and ground and the phase and ground, and be regarded as a fault condition. I would assume that it is a shocking condition the danger of which is dependant on the voltage and the current generating capacity of the source. If the polarity indicator is a LED, there will normally be a 25 kohm or so resistor in series with it both to limit the current through the LED and as protection against leakage currents - if it glows at full brightness then there is high voltage between the phse or neutral and ground with enough source current capacity to be dangerous.

The likelihood of a problem seems small on the evidence of practice, but I would not do it unless I had an expert electrical opinion given me after inspection of the boats electrical system, and checks made as to the likelihood of high voltage gradients betweeen components in the boat. Again, if there was an electrical fire, ones insurer may decide not to respond.

Another danger, for which I have seen a number of reports of death, is that of carbon monoxide poisoning, and this would be a special risk if the generator was run in the cockpit of a yacht with a closed transom and open companionway. On MoBo's the practice seems to be to run the generator on the rear boarding platform.

Again, I reiterate, many do it without there seeming to be a great risk. I would only do it myself after thorough and professional on the spot advice and checks that the particular set up was safe, and if the generator was diesel fuelled.

I would be very interested in any advice from any experienced marine electricians on the forum (I am not one), as to their interpretation of the electrical risk.

Keep safe

John

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by MainlySteam on 07/08/2003 11:19 (server time).</FONT></P>

 

[hebdena]

Re: Galvanic Isolator & RCD

Not related to the generator issue but, I thought the galvanic isolator was to be fitted between the shore earth and the AC distribution box RCD earth connection. On the issue of linking the AC earth and DC -ve , to and from which points should they be connected if one has an RCD? I have my DC -ve grounded on the anode, should this then be linked back to the earth point on the RCD or to the boat side connection on the galvanic isolator?. Will this then trip the RCD if a bit of the 12V circuit become 240V live and runs to earth via the 12V -ve anode connection?

Andrew

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[andyball]

a modern residual current circuit breaker should trip if the difference between the current flowing in live & neutral is 30mA or more (some are 10mA,which is even safer).

Whether or not the neutral is earthed in the generator housing, any fault that allows mains current to flow through the human body (aside from holding the live in one hand & the neutral in the other) will trip the rcd as soon as you touch it & probably before.

So as long as an rcd is properly used, I'd be quite happy to use a generator as described.

Electrical fire??? fuses/mcb's & cables should be sized correctly to prevent that : if you're gonna have a fire it'll happen whether it's a generator or shorepower.

Petrol fire? Many people have petrol fuelled outboards for their tenders...what's the difference?

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[MainlySteam]

Re: Galvanic Isolator & RCD

The only reason that I am aware of that some codes eg ABYC require the dc negative to be grounded is to provide protection in the case that you have a ac to dc short circuit, in which case the short will fault to ground through the dc negative. In that case the dc negative should be grounded to the same ground as the ac is, normally the engine.

Given that many marine engines these days are built with their electrics and sensors isolated from the dc, it all seems a bit pointless connecting it back together again by grounding the ac and dc to it. In my opinion, and it is only an opinion, for small vessels with reliable wiring it should not be necessary to ground the dc, especially on metal vessels where leakage currents can be a problem. On larger metal vessels there will usually be a residual current monitoring device in the dc to ground link to protect against leakage currents - putting a galvanic isolator in there would serve the same purpose.

John

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by MainlySteam on 07/08/2003 13:24 (server time).</FONT></P>

 

[gtmoore]

Re: Galvanic Isolator & RCD

Andrew
Sorry - I worded that unclearly. The isolator is fitted between the earth lead from the shore power and AC distribution box (I assume that's what you mean by the RCD as an RCD breaker only has Live and Neutral terminals I seem to remember). The bonding of the 12V negative is done between the galvanic isolator and the AC distribution box.

Regarding tripping the RCD, if the 240 live circuit (via the RCD) is supplying the power into the 12V side via a fault wouldn't this cause an imbalance and trip the switch?

I'm not at all qualified in this apart from what I read in books and ask about here so don't take any of this as gospel.



<hr width=100% size=1>Gavin

 

[MainlySteam]

Andy, I thought the same about RCD's until I saw the results of a number of tests conducted in North America for floating neutral systems - they do not reliably work. I have forgotten which way around it is, but I think it is that they have a lower delta setpoint in theirs than you do there as well, not vice versa. I understand that military systems on ships have floating neutrals for the sake of hardness when damaged, but I have only been told that, and if so they are obviously appropriately designed. I hope my feelings came across in my first post in that it is something that I feel wary about, not something I regard as immediately of high risk.

I agree about the outboards, but they are usually in the tender if they are refuelled hot. I would not refuel a hot one on my boat. Also, an insurer may (and I say may) regard an outboard as built for marine use, and so fit for purpose, but not so a portable generator.

John

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[andyball]

interesting....are the tests online anywhere?

Anyone seen tests of uk sold "rcd's" if used w/o an earthed neutral?

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[MainlySteam]

Yes, I have seen some on line. Its 1 in the morning here at the moment, so I'll see if I can find some tomorrow and if I do I'll post the links here for you.

John

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[Jean]

Without a “controlled” link between earth and neutral point, ie floating neutral, a single line earth fault (eg, resulting in a live appliance frame) would not necessarily be a problem and may well go unnoticed for a long time (as no return path for example to give the RCD imbalance needed to operate this protective device). The potential life-threatening situation only then arises if a second earth fault occurs (particularly from the neutral and if it is on the load side of the protective device).

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[clio]

is a"rcd" the same as a ground fault interupter I assume it is, if so in north america
you are supposed to test once a month.they come witha test button, also a log to record test results.very seldom done, however it do'es give your insure'er a way out.

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[MainlySteam]

Yes. RCD is same as GFCI in USA and my understanding is that they are supposed to be tested by pushing the button whether required by regulation or not. I don't have the packaging for a portable one to hand, but I seem to remember that the instructions on those state to test it everytime the device is used.

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[MainlySteam]

Andy, I had a quick search and the only direct reference to tests I could find on the internet was this one <A target="_blank" HREF=http://www.mfsia.org/portable_gen.html>http://www.mfsia.org/portable_gen.html</A> which refers to tests conducted by the Construction Safety Association of Ontario (CSAO) and which briefly refers to the unreliable performance of GFCI's in floating neutral systems part of the tests. Unfortunately the CSAO report itself is only available for a charge, but the description of it on the CSAO site indicates that it is fairly comprehensive. I am sure that it is one of the ones I read and perhaps it was on the internet somewhere, sometime. There are other references to the same report on the internet.

In the end, I think we are talking about floating neutral being a non-complying arrangement against most standards, and one which may implicate the dc services as well if the dc negative is bonded to ac ground. That alone, means in the case of accident one could be liable to criminal charge.

The further unknown is the poor installations of electrical systems on many small vessels, including some commercial ones in survey (I came across an approx 60 foot commercial vessel running skippered charters where the ac was cabled in automotive figure 8, and no ground, and another relatively new one I know of has had two electrical fires within about a year). I have also come across a number of cases in yachts with fires in the ac as the shore connection, and the charger also, may be exposed to wet salty environment. One case was with a friend who had a cockpit locker fire when halfway to Tonga - how it happened I do not know as they had no generator or invertor (I assume that it was a charger fault on the dc side), they got such a fright and had felt themselves at such risk they just had the shore power ripped out.

It would be very interesting to see a full professional exposee of what the risks are (if indeed there are any risks) but I have never come across one yet. I suspect that many who plug portable generators into boats do not even know that it is illegal in most jurisdictions to do the same thing with one in a house (the primary risk being one that does not apply to boats though), and I think my concern is that given the uncertainties it should not be done on a boat unless one is very sure of the installation and the possible outcomes.

John




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[gtmoore]

Thanks to everyone for their comprehensive replies - I think I need to give it some more thought and dig around a bit more. I just thought things would be safer running through my 'proper' installation with its circuit breakers etc rather than having an extension lead trailing from the bow where I intend to site the genny though hatches etc to where I need the power. I also had hoped to use my Sterling battery charger with it which is wired directly into AC (via it's own MCB). I guess I can change this to have a flying lead with plug if need be.

Thanks again for the advice.

<hr width=100% size=1>Gavin

 

[saturn]

small generators do not allow you to bond nuetral/earth because of the way they are designed,unlike larger generators .
these small generators were put on sale for use mainly on site in the construction and engineering industry where no other source of power was available and there were many fatalities using 240 volt equipment so they are now only used with a transformer 110 volt,try to use these onsite with 240 volt kit and you will be offsite before you start work.
i would not use one plugged directly into my shore power as being in powergen industry i know better.
if someone was to use one on a boat it would be better to dedicate a couple of sockets to this use only and if you can find someone willing, take advise on how to protect yourself safely.
imho better is to get a 110volt transformer as most kit on sale in uk will have input for 110volt and use as designed.
good luck.

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