jjj
Well-Known Member
I was wondering what are the effects of reverse polarity, it there increased corrosion and other nasty things occuring to the engine etc. JJJ
reverse polarity
- Thread starter jjj
- Start date
DaveS
Well-Known Member
A few more clues needed. Reverse polarity of what?
jjj
Well-Known Member
Sorry I was referring to shorepower connections. I was interested in the effects of reverse polarity e.g live to neutral etc. I know the situation is dangerous as the fuse is bypassed but was wondering about the increased chance of adverse metal corrosion. JJJ
SvenglishTommy
Well-Known Member
I've often wondered this as well, if it's an AC current what the hell difference does it make, and how can polarity devices check for it when it is alternating?
I guess it must be something to do with the neutral being grounded but I've never understood why.
I guess it must be something to do with the neutral being grounded but I've never understood why.
Stemar
Well-Known Member
I don't know about corrosion, but there are real dangers to you.
In the UK, but not, unless it's changed, in France, the neutral is earthed, so the live is alternately +220v & -220v. If the polarity is wrong, you can switch something off, but it's still live. For example, the threaded part of a screw fitting lightbulb or fitting could still bite you.
In the UK, but not, unless it's changed, in France, the neutral is earthed, so the live is alternately +220v & -220v. If the polarity is wrong, you can switch something off, but it's still live. For example, the threaded part of a screw fitting lightbulb or fitting could still bite you.
snowleopard
Well-Known Member
The polarity of an AC circuit relates to the voltage of the wires relative to earth. The common standard in new UK installations is called PME which has earth and neutral bonded together at the source. This means that the live line has an average voltage of 240 compared to earth. It is vital that any switches be in the live line. If the polarity is reversed and the switch is in the neutral line, any appliances connected to the supply will still be live even though not functioing. If you get a live appliance wet you're in for a shock.
I think that in the US the earth is midway between live and neutral so either line only has an average of 55 volts from earth and polarity is not a factor.
An alternative arrangement for shore power is to use an isolation transformer which separates you from the shore supply. Ship's earth may be connected to a central tapping on the output side of the transformer so that both lines have only 120v difference from earth so are safer but then you should ideally have double pole switches that disconnect both live and neutral.
I think that in the US the earth is midway between live and neutral so either line only has an average of 55 volts from earth and polarity is not a factor.
An alternative arrangement for shore power is to use an isolation transformer which separates you from the shore supply. Ship's earth may be connected to a central tapping on the output side of the transformer so that both lines have only 120v difference from earth so are safer but then you should ideally have double pole switches that disconnect both live and neutral.
VicS
Well-Known Member
[ QUOTE ]
The polarity of an AC circuit relates to the voltage of the wires relative to earth
[/ QUOTE ] What we need is a word other than polarity when talking about AC circuits. I always put it in inverted commas just to show that I am not using the word correctly
The polarity of an AC circuit relates to the voltage of the wires relative to earth
[/ QUOTE ] What we need is a word other than polarity when talking about AC circuits. I always put it in inverted commas just to show that I am not using the word correctly
Sea Devil
Well-Known Member
[ QUOTE ]
I think that in the US the earth is midway between live and neutral so either line only has an average of 55 volts from earth and polarity is not a factor.
[/ QUOTE ]
Actually that is not quite right I think... this is way out of my area of expertise but it appears American polarity is very important... I had a marina mains lead which I wired round the 'wrong' way (no switches involved) into a 110 battery charger and blew the fuse.. The American tech who was rewiring my boat following a lightning strike made it very clear that having the correct polarity for many 110v appliances was essential!
Michael
I think that in the US the earth is midway between live and neutral so either line only has an average of 55 volts from earth and polarity is not a factor.
[/ QUOTE ]
Actually that is not quite right I think... this is way out of my area of expertise but it appears American polarity is very important... I had a marina mains lead which I wired round the 'wrong' way (no switches involved) into a 110 battery charger and blew the fuse.. The American tech who was rewiring my boat following a lightning strike made it very clear that having the correct polarity for many 110v appliances was essential!
Michael
MedMan
Well-Known Member
[ QUOTE ]
how can polarity devices check for it when it is alternating?
[/ QUOTE ] It doen't matter that it is alternating. Don't make the mistake of thinking in terms of plus and minus - that's fine for your 12v DC supply, but not for understanding AC. Think in terms of live and neutral and remember that neutral is bonded to earth, first at the power station and again, locally. What is important is that there is a potential difference between live and earth but (provided your installation and that of the marina is sound) not between neutral and earth.
The polarity checker on my mains panel has a small LED connected between the neutral pole of the main switch and earth. If the supply polarity is correct the neutral pole will be neutral and at the same potential as earth so the LED does not light. If the supply potential is reversed the neutral pole will be live and there will be a potential difference between it and earth. A current will therefore flow and the LED warning light will come on.
how can polarity devices check for it when it is alternating?
[/ QUOTE ] It doen't matter that it is alternating. Don't make the mistake of thinking in terms of plus and minus - that's fine for your 12v DC supply, but not for understanding AC. Think in terms of live and neutral and remember that neutral is bonded to earth, first at the power station and again, locally. What is important is that there is a potential difference between live and earth but (provided your installation and that of the marina is sound) not between neutral and earth.
The polarity checker on my mains panel has a small LED connected between the neutral pole of the main switch and earth. If the supply polarity is correct the neutral pole will be neutral and at the same potential as earth so the LED does not light. If the supply potential is reversed the neutral pole will be live and there will be a potential difference between it and earth. A current will therefore flow and the LED warning light will come on.
William_H
Well-Known Member
Coming back to the original question. No it should not cause any more corrosion problems if the active and neutral are reversed. It is as stated a safety concern. If your only device used is a double insulated device with no earth connection then there is little concern if it is on the end of an extension cord. It is with devices like battery chargers hot water heaters and devices with a lot of metal exposed that you get more danger. Of course if you have a proper mains power installation with RCDs and ships earth connected to amins earth it is vital that the neutral is correctly connected. olewill
dmayes
Well-Known Member
People seem to have missed the fact that a lot of equipment will have any metal such as a chassis connected to earth and therefore the neutral. Picking up a device that has the wrong connection may connect the "new" live chassis to earth through your arms. New electrical items are "double insulated" to prevent this but it is far better to be safe than sorry. Our 240 volts is just the correct amount to kill you if run across the two arms as it makes the heart try to beat at 50 times a second!!!
VicS
Well-Known Member
[ QUOTE ]
a lot of equipment will have any metal such as a chassis connected to earth and therefore the neutral. Picking up a device that has the wrong connection may connect the "new" live chassis to earth through your arms.
[/ QUOTE ] This is only partially true. When we refer to "reversed polarity" we we mean having the live and neutral swapped over. That will not make the chasis live.
What will make the chasis live is swapping the live and earth over. That is a much more serious and dangerous situation.
a lot of equipment will have any metal such as a chassis connected to earth and therefore the neutral. Picking up a device that has the wrong connection may connect the "new" live chassis to earth through your arms.
[/ QUOTE ] This is only partially true. When we refer to "reversed polarity" we we mean having the live and neutral swapped over. That will not make the chasis live.
What will make the chasis live is swapping the live and earth over. That is a much more serious and dangerous situation.
jjj
Well-Known Member
Thanks for the information much appreciated.jjj
Birdseye
Well-Known Member
I would be less confident that there could be no effect. Electrolytic corrosion usually results from dissimilar metals in an electrolyte but it can just as easily come from similar metals at different voltages in an electrolyte. It is hard to think off the top of a situation where a floating neutral (ie a neutral not bonded to earth) in some shipboard equipment could cause this to happen, but its certainly more likely to happen when the boat systems are presented with a situation they might not have been designed for in the first place. So in my view the best possible approach is to keep the two systems - mains and 12 v - seperate
On my own boat I have always religiously kept the 12v and the limited mains system seperate with the only link being a battery charger which the manufacturer assures me has no connection between 12v neg and mains neutral or earth.
On my own boat I have always religiously kept the 12v and the limited mains system seperate with the only link being a battery charger which the manufacturer assures me has no connection between 12v neg and mains neutral or earth.
VicS
Well-Known Member
The Official stand is that the 12v system, usualy the -ve side, should be connected to the 240v system earth The danger in not doing so arises when a fault connects it to the 240v live. The rcd will not trip if the 12v system insulation is good and the fuse will not blow. Then you touch the 12v system and electrocute youself (well hopefully the RCd will then trip and save you)
You avoid most electrolysis problems by fitting a galvanic isolator in the shore power earth connection but low level AC leakage may need a capacitor across the G.I. if problems persist.
You avoid most electrolysis problems by fitting a galvanic isolator in the shore power earth connection but low level AC leakage may need a capacitor across the G.I. if problems persist.
Ships_Cat
Well-Known Member
It depends what you regard as the "official stand". That recommendation is a hangover from the past in very slow moving codes and standards such as ABYC which tend to lag in the past.
It is quite ok to have the 12v and 230/110v systems entirely separate according to the classification society rules I am aware of (including Lloyd's) and have boats built that way including my own. Is especially important to separate them on metal boats and as I think Birdseye is also saying is best solution for all boats.
As always, the boat should be wired correctly, but from what I see on these forums many seem to regard an amateur conceived hash up as good enuff - bad in either situation.
John
It is quite ok to have the 12v and 230/110v systems entirely separate according to the classification society rules I am aware of (including Lloyd's) and have boats built that way including my own. Is especially important to separate them on metal boats and as I think Birdseye is also saying is best solution for all boats.
As always, the boat should be wired correctly, but from what I see on these forums many seem to regard an amateur conceived hash up as good enuff - bad in either situation.
John
VicS
Well-Known Member
I know many people prefer to keep 240v and 12v separated but that's the first time I have heard that it is "offically" sanctioned. As you say it takes a long time for codes of practice etc to catch up with actual practice.
I would have thought that a steel boat was the one place earthing the DC would have been advisable. Perhaps I am wrong. Best then not to have any equipment that is connected to both systems such as battery chargers! At least not cheap and nasty ones.
It must be difficult to completely separate them. If the engine block is connected to DC -ve then even if the prop shaft etc does not make a circuit to earth via the water then some sort of circuit must surely exist via the cooling water even if indirecty cooled.
That means that there must be insulated -ve connections to starter, alternator and all alarms and senssors. All proper marine equipment, no starters or alternators from the car breakers!
I would have thought that a steel boat was the one place earthing the DC would have been advisable. Perhaps I am wrong. Best then not to have any equipment that is connected to both systems such as battery chargers! At least not cheap and nasty ones.
It must be difficult to completely separate them. If the engine block is connected to DC -ve then even if the prop shaft etc does not make a circuit to earth via the water then some sort of circuit must surely exist via the cooling water even if indirecty cooled.
That means that there must be insulated -ve connections to starter, alternator and all alarms and senssors. All proper marine equipment, no starters or alternators from the car breakers!
Ships_Cat
Well-Known Member
Regarding engines, many yacht auxiliary engines (and for MoBo's certainly those of larger size 500 kW upwards that I have experienced - I have little experience with smaller MoBo engines but suspect no difference) are either, or are available with all electrical components including alternator and starter chassis isolated from DC. My own little yacht engine is a common 10 year old Volvo and is so isolated.
On those engines, because a very high resistance does exist (the isolation is not an infinite resistance) for reasons along the lines you suggest, with a digital multimeter one will actually measure 12v between the battery positive and the block, but the resistance is so high very little current will flow eg one cannot light a low resistance 12 volt lamp between them. There are also obvious advantages in minimising the risk of arcing if a conductor comes loose at the alternator or starter and touches the surrounding metal.
If one replaces an alternator or adds a high capacity one then it must also be isolated if the integrity is to be maintained. Such alternators are commonly available no problems at all, but as you say unlikely to turn one up at the car wreckers /forums/images/graemlins/smile.gif.
Separation of DC -ve from AC ground is in fact more important on a metal boat as it removes the possibility of DC currents returning through the hull and possible resulting electrolysis.
For metal boats rules normally (always?) require double pole breakers (ie -ve and +ve are broken) but I am unsure if this requirement applies to plastic boats. For our own boat we used all 2 pole breakers and is desirable in any boat.
Marine battery chargers must use isolating transformers and the DC negative must be isolated from the chassis. So ac earth and dc negative should not be found to be connected in the charger (I think I recall another forunite's recent post where he mentioned that the electrician or supplier had assured him this was so for his charger). ABYC actually also requires this I believe, but then goes on to say that the chassis is to be connected to the boats common ground point which if followed means that the -ve and ac earth are connected there, but at the charger itself they are not connected. I have not looked much at non marine chargers but maybe DC -ve is connected to the charger chassis in those but in that case they should not be used for marine purposes and don't comply with ABYC even.
This, of course, means that if one bonds the underwater metal fittings (which is another whole discussion /forums/images/graemlins/smile.gif) they will normally become your ac earth (in a non metal boat) and therefore in an isolated DC system they are also not commoned with the DC -ve.
It is interesting because Calder in his "Boatowners Mechanical, etc... Manual" seems to ignore DC isolated engines completely and does not mention the separation of DC and AC (not that I have ever noticed and I have both the first and current editions). Maybe it is because he is American and feels the need to not drift away from ABYC, but putting that to one side I am familiar with commercial MoBo's built in USA that certainly have them isolated but are built to Lloyds or whatever.
John
On those engines, because a very high resistance does exist (the isolation is not an infinite resistance) for reasons along the lines you suggest, with a digital multimeter one will actually measure 12v between the battery positive and the block, but the resistance is so high very little current will flow eg one cannot light a low resistance 12 volt lamp between them. There are also obvious advantages in minimising the risk of arcing if a conductor comes loose at the alternator or starter and touches the surrounding metal.
If one replaces an alternator or adds a high capacity one then it must also be isolated if the integrity is to be maintained. Such alternators are commonly available no problems at all, but as you say unlikely to turn one up at the car wreckers /forums/images/graemlins/smile.gif.
Separation of DC -ve from AC ground is in fact more important on a metal boat as it removes the possibility of DC currents returning through the hull and possible resulting electrolysis.
For metal boats rules normally (always?) require double pole breakers (ie -ve and +ve are broken) but I am unsure if this requirement applies to plastic boats. For our own boat we used all 2 pole breakers and is desirable in any boat.
Marine battery chargers must use isolating transformers and the DC negative must be isolated from the chassis. So ac earth and dc negative should not be found to be connected in the charger (I think I recall another forunite's recent post where he mentioned that the electrician or supplier had assured him this was so for his charger). ABYC actually also requires this I believe, but then goes on to say that the chassis is to be connected to the boats common ground point which if followed means that the -ve and ac earth are connected there, but at the charger itself they are not connected. I have not looked much at non marine chargers but maybe DC -ve is connected to the charger chassis in those but in that case they should not be used for marine purposes and don't comply with ABYC even.
This, of course, means that if one bonds the underwater metal fittings (which is another whole discussion /forums/images/graemlins/smile.gif) they will normally become your ac earth (in a non metal boat) and therefore in an isolated DC system they are also not commoned with the DC -ve.
It is interesting because Calder in his "Boatowners Mechanical, etc... Manual" seems to ignore DC isolated engines completely and does not mention the separation of DC and AC (not that I have ever noticed and I have both the first and current editions). Maybe it is because he is American and feels the need to not drift away from ABYC, but putting that to one side I am familiar with commercial MoBo's built in USA that certainly have them isolated but are built to Lloyds or whatever.
John