Monday, 25 January 2010

Galvanic corrosion

Well we went to have a look at yet another narrowboat yesterday. We were once again quite impressed with all the facilities onboard. During discussions it was pointed out that the boat had a "galvanic corrosion" inhibitor. This set me to thinking. Now, I will be the first to admit that a narrowboat is an important asset worthy of the best corrosion and electrical safety grounding protection available. While protective coatings cover the steel hull and sacrificial anodes can go some way to help protect your boat. What is needed is a sure method to deal with the galvanic corrosion created by the shore power connection and other vessels in close proximity.

Marine galvanic isolation is the principle of isolating electrical systems on board your boat so that an electrical ground loop charge cannot move from one boat to another. Usually because they are sharing the same mains electrical hookup supply. Galvanic isolation is used in situations where two or more electrical points are used to supply power, but their grounds may be at different voltage potentials. It is the potential difference that creates the ground loop. Using an Galvanic isolation is a method of breaking ground loops, by preventing unwanted current from travelling between two boats sharing a common mains power supply. However, there is a much better way then using Diode Galvanic Isolation and it is by way of an Isolation Transformer.

What is a transformer?

An electrical transformer is a device that transfers electrical energy from one circuit to another. This is done through inductively coupled conductors - which are in reality the transformer's physical coils. A varying current (AC alternating current) or voltage in the first or primary coil creates or induces a voltage in the secondary coil. This effect is called mutual induction. By appropriate selection of the ratio of turns, a transformer allows an alternating current (AC) voltage to be "stepped up" by making the output greater than input, or "stepped down" by making output less than input. Transformers need an alternating current to work. They will not work with DC or direct current.

I.E. If we have a transformer with 100 turns on the input or primary winding. With 50 turns on the output or secondary winding the voltage will step down. 100V AC input on the primary would become 50v AC output on the secondary windings. If you turn the transformer round and put 100v AC on the secondary, then the voltage on the primary would be 200v AC.

This sound wonderful but.... there is a catch... An ideal transformer would have no energy losses, and would be 100% efficient. The efficiency of modern transformers is very good, but there are losses. Current flowing through the windings causes resistive heating of the conductors. A small transformer, such as a plug-in power adapter type used for low-power consumer electronics, may be no more than 85% efficient, with a 5-10% loss even when not supplying any power to the device. The no-load loss can be significant when your power budget is small, meaning that even an idle transformer constitutes a drain on an electrical supply. So always disconnect small chargers when not in use.

Also as in all things, size matters. The greater the power requirements the greater the physical size the transformer will be. We measure the power that the transformer is  required to constantly deliver in watts. However you should then add 100% to the power budget to cover requirements for intermittent items.

The closer you are to saturating (using the maximum current the device can deliver) the transformer the hotter the transformer will become.

Constant supply budget.
Battery charger.
Bilge Pump.
Heating Pump.

Intermittent supply budget.
Waste water pump.
Electric kettle.
Electric Iron.

A typical isolation transformer rated at 3600VA at 230v AC will happily deliver all you need from a 16A marina mains supply.

See this link

A isolation transformer is a specially constructed transformer, often with symmetrical windings, 1:1 same number of turns on the input and output of the transformer. I.E. 240v AC input and 240v AC output. An isolation transformer essentially allows the AC power to be taken from a mains power source. And as if by magic, fed into another circuit without physically connecting the two circuits. Thus preventing ground loops from forming. Ground loops are what cause radio interfearance and the damage to the boats hull.  Hopefully only causing actual damage to the sacrificial galvanic anodes.

There is no connection between the shore mains earth and the earth on the boat. This is what protects your boat from galvanic corrosion. The Neutral and Earth are bonded on the output side of the transformer. This recreates exactly the same situation as we normally receive from the national grid and is what allows fuses and RCDs to operate on board in the correct manner.

Finally note that there is a laminated metal core between the primary winding and the secondary winding which is connected to shore-power earth. This is so that any fault current in the primary (for instance from insulation breakdown) is returned to shore-power earth to trip the circuit breaker or RCD rather than electrifying the boat hull.

In effect, the transformer recreates a totally isolated AC supply electrically isolated from everything else.

Normal mains transformers are quite cheap to buy - for some reason Isolation transformers seem to be quite expensive by comparison.  However, there is a solution to save costs. Create a double isolated transformer from two cheap but identical standard ones. Take two 240v to 110v transformers and connect them back to back.

VoilĂ ! an instant cheap isolation transformer. Much cheaper than periodically replacing galvanic anodes.

NOTE. If you fit a mains isolation transformer on your boat it is important that you remember to disconect the Shoreside Mains Earth conductor from being connected to the boats hull.


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