JMcKillop
Well-Known Member
Could some of you knowledable guys give me advice as to where the 230V output socket earth cable should be attached to within the generator. (This is a onboard generator). Does it connect to the battery negative point? 
Generator earthing advice
- Thread starter JMcKillop
- Start date
superheat6k
Well-Known Member
If your negative is bonded to the engine and other earthing connectors then yes, but you must be careful not to inadvertently create a conducting earth conductor.
This is also where a misunderstanding about what the neutral is and does arises.
In any power source the neutral is connected directly to earth, BUT ONLY AT THE SOURCE This can be at the local HV > LV transformer where the star point of a 3 phase system is usually earthed.
This means that the neutral from the generator should be earthed, BUT IT MUST BE DISCONNECTED COMPLETELY from the rest of the electrical system when the generator is not in use.
The earth system of the boat should also be connected to the metal case of the generator. In turn the generator should be bonded to the engine/s and other bonding circuit components. Now there is some disagreement as to whether or not the Negative should be connected into the bonding circuit, but for most engines it is. This digresses into issues with earth leakage and stray current corrosion. This is not what was asked here.
It is important you must not ever have two neutral earthing points, which is why the switch that isolates the generator should have full pole separation 'Break before Make' for both live and neutral, but not the earth - this remains permanently connected. At the same time it must not be possible to have an external power supply connected to the generator supply unless and only when complex paralleling equipment is provided, and unless you have a super yacht this is highly unlikely to be the case.
For this reason the On (Shore) - Off - On (Genny) switch is essential. Such a switch provides absolute Break before Make safety for two supplies that must never ever meet. Should they meet not only will there be two neutrals to earth, possibly creating a conducting earth system, but also that the phases on the live connection will most likely meet out of phase, however watching the generator rip itself off its bedplate when it attempts to correct the phase difference would be quite interesting.
So to answer your actual question simply connect the 230v socket earth to the generator metal case AND the rest of the bonding / and earthing circuits (note they are not the same thing). When alongside with a generator running, the shore earth connection should be maintained to prevent the boats local ground rising in potential relative to the external true earth. Otherwise someone on the pontoon who is grounded to the shore earth through his feet could get a nasty shock when he tried to board and grabs hold of the metal bonded guardrail.
This is also where a misunderstanding about what the neutral is and does arises.
In any power source the neutral is connected directly to earth, BUT ONLY AT THE SOURCE This can be at the local HV > LV transformer where the star point of a 3 phase system is usually earthed.
This means that the neutral from the generator should be earthed, BUT IT MUST BE DISCONNECTED COMPLETELY from the rest of the electrical system when the generator is not in use.
The earth system of the boat should also be connected to the metal case of the generator. In turn the generator should be bonded to the engine/s and other bonding circuit components. Now there is some disagreement as to whether or not the Negative should be connected into the bonding circuit, but for most engines it is. This digresses into issues with earth leakage and stray current corrosion. This is not what was asked here.
It is important you must not ever have two neutral earthing points, which is why the switch that isolates the generator should have full pole separation 'Break before Make' for both live and neutral, but not the earth - this remains permanently connected. At the same time it must not be possible to have an external power supply connected to the generator supply unless and only when complex paralleling equipment is provided, and unless you have a super yacht this is highly unlikely to be the case.
For this reason the On (Shore) - Off - On (Genny) switch is essential. Such a switch provides absolute Break before Make safety for two supplies that must never ever meet. Should they meet not only will there be two neutrals to earth, possibly creating a conducting earth system, but also that the phases on the live connection will most likely meet out of phase, however watching the generator rip itself off its bedplate when it attempts to correct the phase difference would be quite interesting.
So to answer your actual question simply connect the 230v socket earth to the generator metal case AND the rest of the bonding / and earthing circuits (note they are not the same thing). When alongside with a generator running, the shore earth connection should be maintained to prevent the boats local ground rising in potential relative to the external true earth. Otherwise someone on the pontoon who is grounded to the shore earth through his feet could get a nasty shock when he tried to board and grabs hold of the metal bonded guardrail.
JMcKillop
Well-Known Member
Thanks @superheat6k for you technical answer. Mr Google has thown up a lot of technical information that goes way above my head
. Let me try and explain my installation. There is a generator/shore power change over switch but not sure if it is break before make. (Will have to check this out). On the 230V distribution board, there is a 30mA RCB and MCBs installed. There is also a Safeshore galvanic isolator fitted onboard. The generator engine is bonded to the battery negative but obviously isolated from the casing by the rubber engine mounts. Currently there is no bonding/earth connection to the casing. Could you please try and explain, "AND the rest of the bonding / and earthing circuits (note they are not the same thing)". Sorry for the lack of terminolgy understanding, must be an age thing :redface-new:.
superheat6k
Well-Known Member
I have just seen your thread, but its a bit late for the technical / understandable answers. I will respond for you tomorrow when I'm a bit fresher though. I will also PM you some reference manuals you might find of use.Thanks @superheat6k for you technical answer. Mr Google has thown up a lot of technical information that goes way above my head
superheat6k
Well-Known Member
The earthing and bonding circuits are linked but perform different functions.
The bonding circuit links underwater metallic fittings including the anodes, indeed an anode won't protect anything other than its own mounting bolts if it is not electrically connected to the metal parts intended to be protected. In so doing it forms a simple galvanic cell, similar to a battery, whereby the anode forms the positive and the metals being protected form the negatives. If you examine a table of metal nobility the least noble metals are more positive relative to the more noble metals. Apart from magnesium, zinc is the lowest on the scale.
By connecting the bonding circuit to the boats DC negative this encourages the bonding to remain at the lowest (most negative) potential or voltage. The voltages and currents flowing within the galvanic bonding circuit are tiny. This why larger cables are used because the natural resistance of thinner wires can impede the low current flow, which would remove the protection provided by the anodes.
The bonding circuit connection to negative is generally at the engine block. The bonding circuit should not be used as a negative return for any boat's electrical equipment, because the voltage drops occurring will easily exceed the low level galvanic voltages and can cause drastic anode wear by making the anode too positive. Bilge pump float switches are a common cause of galvanic problems due to stray DC leakage into the bonding circuit.
The Protective Earth circuit provides a different function entirely, and the earth circuit is bonded also to the engine block and hence DC negative and the bonding circuit. The earth circuit function is to tie all electrical equipment to ground earth, so if any high voltage equipment develops a fault, the fault current is led safely to earth, otherwise exposed metallic parts could become directly connected to lethal voltages.
As for the bonding circuit, the earth circuit is not to be used as a routine conducting part of the circuit, it should only conduct when handling a fault current. Such a fault current should be immediately detected by the RCD device, which would then disconnect the power at its entry source.
However, for the Protective Earth to work when a fault situation connects the Live feed to the earth circuit somewhere, the earth must complete an electrical circuit back to the source neutral. This is achieved by connecting the neutral conductor to earth at its source - at the HV substation transformer for the shore supply, at the neutral connection point at a boat's generator. This why when not in use the isolation switch must break both live and neutral connections.
The RCD device works on the principle of a balance scale - in a normal situation the current flowing in the live and neutral must be precisely the same. If a fault leakage to earth return occurs then this creates an immediate imbalance in the live and neutral flowing through the RCD, because some of the current is now flowing back to the source through the earth circuit, which is instantly detected and the RCD trips.
As mentioned above the earthing and bonding circuits are electrically connected. Galvanic Isolators are provided to stop the shore earth circuit acting as an unwanted extension of the boat's own bonding circuit. The Boat's anodes are intended to protect the boat's own underwater metalwork only. If the bonding circuit is inadvertently extended via the earth connection to the shore earth it will then become electrically connected to any other underwater metal in the vicinity, and your anodes will then be trying to protect a far wider range of metallic objects. It will also be susceptible to any leakage faults in other boat's bonding circuits to which it is then connected.
The galvanic isolator uses the voltage drop across a two pairs of parallel and opposite polarity connected diodes. The forward breakdown voltage of the diodes is higher than the very low level galvanic bonding circuit voltages so effectively blocks them, but high voltage AC earth faults will pass straight through safely to ground. Be aware that extended connection of an AC fault current even if grounded will destroy anodes in short order, so properly functioning RCDs are essential not only for safety reasons.
For your Break before Make if it has a central Off position then you will definitely have a break before make, but this must be the case anyway. The rubber mounts are for vibration elimination only, ago I would install an earth strap between the generator and the casing. The earth should not be switched, but permanently connected.
As an aside to the foregoing regarding connection of PE and bonding circuits there are many who prefer not to this. This normally due to poor experiences of the effect of leakage faults occurring and destroying underwater metalwork, such as Outdrive legs. On this subject I am both with ISO and the US insistence that these circuits must be connected. Do not ever go swimming near a boat who does not tie the bonding circuit to the earth circuit, especially in fresh water.
The bonding circuit links underwater metallic fittings including the anodes, indeed an anode won't protect anything other than its own mounting bolts if it is not electrically connected to the metal parts intended to be protected. In so doing it forms a simple galvanic cell, similar to a battery, whereby the anode forms the positive and the metals being protected form the negatives. If you examine a table of metal nobility the least noble metals are more positive relative to the more noble metals. Apart from magnesium, zinc is the lowest on the scale.
By connecting the bonding circuit to the boats DC negative this encourages the bonding to remain at the lowest (most negative) potential or voltage. The voltages and currents flowing within the galvanic bonding circuit are tiny. This why larger cables are used because the natural resistance of thinner wires can impede the low current flow, which would remove the protection provided by the anodes.
The bonding circuit connection to negative is generally at the engine block. The bonding circuit should not be used as a negative return for any boat's electrical equipment, because the voltage drops occurring will easily exceed the low level galvanic voltages and can cause drastic anode wear by making the anode too positive. Bilge pump float switches are a common cause of galvanic problems due to stray DC leakage into the bonding circuit.
The Protective Earth circuit provides a different function entirely, and the earth circuit is bonded also to the engine block and hence DC negative and the bonding circuit. The earth circuit function is to tie all electrical equipment to ground earth, so if any high voltage equipment develops a fault, the fault current is led safely to earth, otherwise exposed metallic parts could become directly connected to lethal voltages.
As for the bonding circuit, the earth circuit is not to be used as a routine conducting part of the circuit, it should only conduct when handling a fault current. Such a fault current should be immediately detected by the RCD device, which would then disconnect the power at its entry source.
However, for the Protective Earth to work when a fault situation connects the Live feed to the earth circuit somewhere, the earth must complete an electrical circuit back to the source neutral. This is achieved by connecting the neutral conductor to earth at its source - at the HV substation transformer for the shore supply, at the neutral connection point at a boat's generator. This why when not in use the isolation switch must break both live and neutral connections.
The RCD device works on the principle of a balance scale - in a normal situation the current flowing in the live and neutral must be precisely the same. If a fault leakage to earth return occurs then this creates an immediate imbalance in the live and neutral flowing through the RCD, because some of the current is now flowing back to the source through the earth circuit, which is instantly detected and the RCD trips.
As mentioned above the earthing and bonding circuits are electrically connected. Galvanic Isolators are provided to stop the shore earth circuit acting as an unwanted extension of the boat's own bonding circuit. The Boat's anodes are intended to protect the boat's own underwater metalwork only. If the bonding circuit is inadvertently extended via the earth connection to the shore earth it will then become electrically connected to any other underwater metal in the vicinity, and your anodes will then be trying to protect a far wider range of metallic objects. It will also be susceptible to any leakage faults in other boat's bonding circuits to which it is then connected.
The galvanic isolator uses the voltage drop across a two pairs of parallel and opposite polarity connected diodes. The forward breakdown voltage of the diodes is higher than the very low level galvanic bonding circuit voltages so effectively blocks them, but high voltage AC earth faults will pass straight through safely to ground. Be aware that extended connection of an AC fault current even if grounded will destroy anodes in short order, so properly functioning RCDs are essential not only for safety reasons.
For your Break before Make if it has a central Off position then you will definitely have a break before make, but this must be the case anyway. The rubber mounts are for vibration elimination only, ago I would install an earth strap between the generator and the casing. The earth should not be switched, but permanently connected.
As an aside to the foregoing regarding connection of PE and bonding circuits there are many who prefer not to this. This normally due to poor experiences of the effect of leakage faults occurring and destroying underwater metalwork, such as Outdrive legs. On this subject I am both with ISO and the US insistence that these circuits must be connected. Do not ever go swimming near a boat who does not tie the bonding circuit to the earth circuit, especially in fresh water.