Why do I need one?
Economical Corrosion prevention
Protect against stray currents Peace of mind
Why now? No one really bothered before!
When two or more boats sit together in the water (or one boat and one jetty!) There is a tendency for a small electrical current to flow between the metal components of the two hulls. This occurs when dissimilar metals i.e. skin fittings, propellers, shafts etc are in close proximity to one another connected effectively by the water: This in itself does not create a problem as the current drawn is usually very small: The amount of current flowing is dependent on the type of metals, the area of the metals, the proximity of the hulls and finally the composition of the water, i.e. the salt content or metallic content of the water.
The action of this small current flow creates a small problem! As all metals have different rates of corrosion a metal at one end of the Galvanic (corrosion) Scale will dissolve faster than one at the other end of the scale. If for example we have a brass skin fitting on one Hull and a stainless propeller shaft on the adjacent boat, the brass fitting will undoubtedly disappear before the prop-shaft! As a second example, a greater problem may exist with a large metal boat moored alongside a small cruiser with elderly skin fittings: It takes no imagination to see who wins that battle! Most boat owners are familiar with “Anodes” (or ”Sacrificial Anodes” to be technical). These are large lumps of metal usually zinc/magnesium etc, at the far end of the Galvanic Scale, clamped to the underwater hull and designed to erode away in preference to your more valuable underwater skin fittings. These anodes are an essential protection to corrosion and should be checked regularly for deterioration: Once they are gone so is your protection!
In reality it may take years to have any major deterioration.
When we connect to shore power (mains power) we connect all our boats together via the earth (green) cable in the shore power leads.
This earth cable is essential for our safety and also ensures correct operation of the shore power and vessels fuses and electrical trips. It is vital that this earth connection remains in circuit at all times (not if the vessel has its own shore power isolation transformer).
Removal can be fatal! In the event of a major electrical defect, lack of proper earth connections can be lethal to not only yourselves but to your immediate neighbours. Unfortunately it becomes obvious that the earth cable now present between adjacent boats makes an excellent conductor between them, thus allowing the easy passage of electrical current twixt the vessels. This in turn increases the rate of deterioration of fittings.
Thus we have the problem! We have created a giant battery! The rate of erosion is affected by several factors:
The amount of salt or other minerals in the water
The areas of metal involved
The types of metals involved
The proximity of the vessels
Construction material of the jetties
The temperature of the water.
Condition of electrical installation on adjoining vessels.
It is not unknown in extreme conditions for skin fittings to deteriorate within a few weeks through Galvanic corrosion. Although this rate of loss is rare, it is obvious there is a problem in need of redress.
If your vessel is under 20 mtrs and fed from a 62 amp or less circuit breaker/rcd then you probably will not have a shore power isolation transformer and fitting back to back diodes in the earth lead is a good idea, see here.
However if you vessel is larger and almost certainly requiring a 3 phase supply you should have a shore power isolation transformer configured as delta / star with the centre tap of the star bonded to the earth rail / hull (if metal). There is no point in carrying the earth from the shore power supply as the earth loop is broken via the transformer. If you use a shore power inverter, consult with the manufacturer to determine if they have isolated the supply via a transformer before they start the inverter.
In my experience as a Marine electrical engineer the best solution is an isolating transformer. Lightning or large power surges can blow the diodes and then you have no safety earth....